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  • select from multiple tables with different columns

    - by Qaiser Iftikhar
    Say I got this sql schema. Table Job: id,title, type, is_enabled Table JobFileCopy: job_id,from_path,to_path Table JobFileDelete: job_id, file_path Table JobStartProcess: job_id, file_path, arguments, working_directory There are many other tables with varying number of columns and they all got foreign key job_id which is linked to id in table Job. My questions: Is this the right approach? I don't have requirement to delete anything at anytime. I will require to select and insert mostly. Secondly, what is the best approach to get the list of jobs with relevant details from all the different tables in a single database hit? e.g I would like to select top 20 jobs with details, their details can be in any table (depends on column type in table Job) which I don't know until runtime. Thanks in advance. Regards,

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  • Looking into CSS3 Multiple backgrounds

    - by nikolaosk
    In this post I will be looking into a great feature in CSS3 called multiple backgrounds.With CSS3 ourselves as web designers we can specify multiple background images for box elements, using nothing more than a simple comma-separated list. This is a very nice feature that can be useful in many websites.In this hands-on example I will be using Expression Web 4.0.This application is not a free application. You can use any HTML editor you like.You can use Visual Studio 2012 Express edition. You can download it here.Before I go on with the actual demo I will use the (http://www.caniuse.com) to see the support for CSS 3 Multiple backgrounds from the latest versions of modern browsers.Please have a look in this link All modern browsers support this feature. I am typing this very simple HTML 5 markup with an internal CSS style.<!DOCTYPE html><html lang="en">  <head>    <title>Sold items</title>        <style>                #box{        border:1px solid #afafaf;        width:260px;        height:290px;        background-image:url(shirt.png), url(sold.jpg);        background-position: center bottom, right top;        background-repeat: no-repeat, no-repeat;    </style>    </head>  <body>    <header>                <h1>CSS3 Multiple backgrounds</h1>    </header>           <div id="box">              </div>        <footer>        <p>All Rights Reserved</p>      </footer>     </body>  </html>  Let me explain what I do here, multiple background images are specified using a comma-separated list of values for the background-image property.A comma separated list is also used for the other background properties such as background-repeat, background-position .So in the next three lines of CSS code         background-image:url(shirt.png), url(sold.jpg);        background-position: center bottom, right top;        background-repeat: no-repeat, no-repeat; we have 2 images placed in the div element. The first is placed center bottom in the div element and the second is placed at right top position inside the div element.Both images do not get repeated.I view the page in IE 10 and all the latest versions of Opera,Chrome and Firefox.Have a look at the picture below. Hope it helps!!!!

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  • Django: Admin with multiple sites & languages

    - by lazerscience
    Hi everybody! I'm supposed to build some Django apps, that allow you to administer multiple sites through one backend. The contrib.sites framework is quite perfect for my purposes. I can run multiple instances of manage.py with different settings for each site; but how should django's admin deal with different settings for different sites, eg. if they have different sets of languages, a different (default) language? So there are some problem s to face if you have to work on objects coming from different sites in one admin... I think settings.ADMIN_FOR is supposed to be quite helpful for cases like this, but theres hardly any documentation about it and I think it's not really used in the actual Django version (?). So any ideas/solutions are welcome and much appreciated! Thanks a lot...

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  • Inheritance Mapping Strategies with Entity Framework Code First CTP5 Part 1: Table per Hierarchy (TPH)

    - by mortezam
    A simple strategy for mapping classes to database tables might be “one table for every entity persistent class.” This approach sounds simple enough and, indeed, works well until we encounter inheritance. Inheritance is such a visible structural mismatch between the object-oriented and relational worlds because object-oriented systems model both “is a” and “has a” relationships. SQL-based models provide only "has a" relationships between entities; SQL database management systems don’t support type inheritance—and even when it’s available, it’s usually proprietary or incomplete. There are three different approaches to representing an inheritance hierarchy: Table per Hierarchy (TPH): Enable polymorphism by denormalizing the SQL schema, and utilize a type discriminator column that holds type information. Table per Type (TPT): Represent "is a" (inheritance) relationships as "has a" (foreign key) relationships. Table per Concrete class (TPC): Discard polymorphism and inheritance relationships completely from the SQL schema.I will explain each of these strategies in a series of posts and this one is dedicated to TPH. In this series we'll deeply dig into each of these strategies and will learn about "why" to choose them as well as "how" to implement them. Hopefully it will give you a better idea about which strategy to choose in a particular scenario. Inheritance Mapping with Entity Framework Code FirstAll of the inheritance mapping strategies that we discuss in this series will be implemented by EF Code First CTP5. The CTP5 build of the new EF Code First library has been released by ADO.NET team earlier this month. EF Code-First enables a pretty powerful code-centric development workflow for working with data. I’m a big fan of the EF Code First approach, and I’m pretty excited about a lot of productivity and power that it brings. When it comes to inheritance mapping, not only Code First fully supports all the strategies but also gives you ultimate flexibility to work with domain models that involves inheritance. The fluent API for inheritance mapping in CTP5 has been improved a lot and now it's more intuitive and concise in compare to CTP4. A Note For Those Who Follow Other Entity Framework ApproachesIf you are following EF's "Database First" or "Model First" approaches, I still recommend to read this series since although the implementation is Code First specific but the explanations around each of the strategies is perfectly applied to all approaches be it Code First or others. A Note For Those Who are New to Entity Framework and Code-FirstIf you choose to learn EF you've chosen well. If you choose to learn EF with Code First you've done even better. To get started, you can find a great walkthrough by Scott Guthrie here and another one by ADO.NET team here. In this post, I assume you already setup your machine to do Code First development and also that you are familiar with Code First fundamentals and basic concepts. You might also want to check out my other posts on EF Code First like Complex Types and Shared Primary Key Associations. A Top Down Development ScenarioThese posts take a top-down approach; it assumes that you’re starting with a domain model and trying to derive a new SQL schema. Therefore, we start with an existing domain model, implement it in C# and then let Code First create the database schema for us. However, the mapping strategies described are just as relevant if you’re working bottom up, starting with existing database tables. I’ll show some tricks along the way that help you dealing with nonperfect table layouts. Let’s start with the mapping of entity inheritance. -- The Domain ModelIn our domain model, we have a BillingDetail base class which is abstract (note the italic font on the UML class diagram below). We do allow various billing types and represent them as subclasses of BillingDetail class. As for now, we support CreditCard and BankAccount: Implement the Object Model with Code First As always, we start with the POCO classes. Note that in our DbContext, I only define one DbSet for the base class which is BillingDetail. Code First will find the other classes in the hierarchy based on Reachability Convention. public abstract class BillingDetail  {     public int BillingDetailId { get; set; }     public string Owner { get; set; }             public string Number { get; set; } } public class BankAccount : BillingDetail {     public string BankName { get; set; }     public string Swift { get; set; } } public class CreditCard : BillingDetail {     public int CardType { get; set; }                     public string ExpiryMonth { get; set; }     public string ExpiryYear { get; set; } } public class InheritanceMappingContext : DbContext {     public DbSet<BillingDetail> BillingDetails { get; set; } } This object model is all that is needed to enable inheritance with Code First. If you put this in your application you would be able to immediately start working with the database and do CRUD operations. Before going into details about how EF Code First maps this object model to the database, we need to learn about one of the core concepts of inheritance mapping: polymorphic and non-polymorphic queries. Polymorphic Queries LINQ to Entities and EntitySQL, as object-oriented query languages, both support polymorphic queries—that is, queries for instances of a class and all instances of its subclasses, respectively. For example, consider the following query: IQueryable<BillingDetail> linqQuery = from b in context.BillingDetails select b; List<BillingDetail> billingDetails = linqQuery.ToList(); Or the same query in EntitySQL: string eSqlQuery = @"SELECT VAlUE b FROM BillingDetails AS b"; ObjectQuery<BillingDetail> objectQuery = ((IObjectContextAdapter)context).ObjectContext                                                                          .CreateQuery<BillingDetail>(eSqlQuery); List<BillingDetail> billingDetails = objectQuery.ToList(); linqQuery and eSqlQuery are both polymorphic and return a list of objects of the type BillingDetail, which is an abstract class but the actual concrete objects in the list are of the subtypes of BillingDetail: CreditCard and BankAccount. Non-polymorphic QueriesAll LINQ to Entities and EntitySQL queries are polymorphic which return not only instances of the specific entity class to which it refers, but all subclasses of that class as well. On the other hand, Non-polymorphic queries are queries whose polymorphism is restricted and only returns instances of a particular subclass. In LINQ to Entities, this can be specified by using OfType<T>() Method. For example, the following query returns only instances of BankAccount: IQueryable<BankAccount> query = from b in context.BillingDetails.OfType<BankAccount>() select b; EntitySQL has OFTYPE operator that does the same thing: string eSqlQuery = @"SELECT VAlUE b FROM OFTYPE(BillingDetails, Model.BankAccount) AS b"; In fact, the above query with OFTYPE operator is a short form of the following query expression that uses TREAT and IS OF operators: string eSqlQuery = @"SELECT VAlUE TREAT(b as Model.BankAccount)                       FROM BillingDetails AS b                       WHERE b IS OF(Model.BankAccount)"; (Note that in the above query, Model.BankAccount is the fully qualified name for BankAccount class. You need to change "Model" with your own namespace name.) Table per Class Hierarchy (TPH)An entire class hierarchy can be mapped to a single table. This table includes columns for all properties of all classes in the hierarchy. The concrete subclass represented by a particular row is identified by the value of a type discriminator column. You don’t have to do anything special in Code First to enable TPH. It's the default inheritance mapping strategy: This mapping strategy is a winner in terms of both performance and simplicity. It’s the best-performing way to represent polymorphism—both polymorphic and nonpolymorphic queries perform well—and it’s even easy to implement by hand. Ad-hoc reporting is possible without complex joins or unions. Schema evolution is straightforward. Discriminator Column As you can see in the DB schema above, Code First has to add a special column to distinguish between persistent classes: the discriminator. This isn’t a property of the persistent class in our object model; it’s used internally by EF Code First. By default, the column name is "Discriminator", and its type is string. The values defaults to the persistent class names —in this case, “BankAccount” or “CreditCard”. EF Code First automatically sets and retrieves the discriminator values. TPH Requires Properties in SubClasses to be Nullable in the Database TPH has one major problem: Columns for properties declared by subclasses will be nullable in the database. For example, Code First created an (INT, NULL) column to map CardType property in CreditCard class. However, in a typical mapping scenario, Code First always creates an (INT, NOT NULL) column in the database for an int property in persistent class. But in this case, since BankAccount instance won’t have a CardType property, the CardType field must be NULL for that row so Code First creates an (INT, NULL) instead. If your subclasses each define several non-nullable properties, the loss of NOT NULL constraints may be a serious problem from the point of view of data integrity. TPH Violates the Third Normal FormAnother important issue is normalization. We’ve created functional dependencies between nonkey columns, violating the third normal form. Basically, the value of Discriminator column determines the corresponding values of the columns that belong to the subclasses (e.g. BankName) but Discriminator is not part of the primary key for the table. As always, denormalization for performance can be misleading, because it sacrifices long-term stability, maintainability, and the integrity of data for immediate gains that may be also achieved by proper optimization of the SQL execution plans (in other words, ask your DBA). Generated SQL QueryLet's take a look at the SQL statements that EF Code First sends to the database when we write queries in LINQ to Entities or EntitySQL. For example, the polymorphic query for BillingDetails that you saw, generates the following SQL statement: SELECT  [Extent1].[Discriminator] AS [Discriminator],  [Extent1].[BillingDetailId] AS [BillingDetailId],  [Extent1].[Owner] AS [Owner],  [Extent1].[Number] AS [Number],  [Extent1].[BankName] AS [BankName],  [Extent1].[Swift] AS [Swift],  [Extent1].[CardType] AS [CardType],  [Extent1].[ExpiryMonth] AS [ExpiryMonth],  [Extent1].[ExpiryYear] AS [ExpiryYear] FROM [dbo].[BillingDetails] AS [Extent1] WHERE [Extent1].[Discriminator] IN ('BankAccount','CreditCard') Or the non-polymorphic query for the BankAccount subclass generates this SQL statement: SELECT  [Extent1].[BillingDetailId] AS [BillingDetailId],  [Extent1].[Owner] AS [Owner],  [Extent1].[Number] AS [Number],  [Extent1].[BankName] AS [BankName],  [Extent1].[Swift] AS [Swift] FROM [dbo].[BillingDetails] AS [Extent1] WHERE [Extent1].[Discriminator] = 'BankAccount' Note how Code First adds a restriction on the discriminator column and also how it only selects those columns that belong to BankAccount entity. Change Discriminator Column Data Type and Values With Fluent API Sometimes, especially in legacy schemas, you need to override the conventions for the discriminator column so that Code First can work with the schema. The following fluent API code will change the discriminator column name to "BillingDetailType" and the values to "BA" and "CC" for BankAccount and CreditCard respectively: protected override void OnModelCreating(System.Data.Entity.ModelConfiguration.ModelBuilder modelBuilder) {     modelBuilder.Entity<BillingDetail>()                 .Map<BankAccount>(m => m.Requires("BillingDetailType").HasValue("BA"))                 .Map<CreditCard>(m => m.Requires("BillingDetailType").HasValue("CC")); } Also, changing the data type of discriminator column is interesting. In the above code, we passed strings to HasValue method but this method has been defined to accepts a type of object: public void HasValue(object value); Therefore, if for example we pass a value of type int to it then Code First not only use our desired values (i.e. 1 & 2) in the discriminator column but also changes the column type to be (INT, NOT NULL): modelBuilder.Entity<BillingDetail>()             .Map<BankAccount>(m => m.Requires("BillingDetailType").HasValue(1))             .Map<CreditCard>(m => m.Requires("BillingDetailType").HasValue(2)); SummaryIn this post we learned about Table per Hierarchy as the default mapping strategy in Code First. The disadvantages of the TPH strategy may be too serious for your design—after all, denormalized schemas can become a major burden in the long run. Your DBA may not like it at all. In the next post, we will learn about Table per Type (TPT) strategy that doesn’t expose you to this problem. References ADO.NET team blog Java Persistence with Hibernate book a { text-decoration: none; } a:visited { color: Blue; } .title { padding-bottom: 5px; font-family: Segoe UI; font-size: 11pt; font-weight: bold; padding-top: 15px; } .code, .typeName { font-family: consolas; } .typeName { color: #2b91af; } .padTop5 { padding-top: 5px; } .padTop10 { padding-top: 10px; } p.MsoNormal { margin-top: 0in; margin-right: 0in; margin-bottom: 10.0pt; margin-left: 0in; line-height: 115%; font-size: 11.0pt; font-family: "Calibri" , "sans-serif"; }

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  • Can nested attributes be used in combination with inheritance?

    - by FoxDemon
    I have the following classes: Project Person Person Developer Person Manager In the Project model I have added the following statements: has_and_belongs_to_many :people accepts_nested_attributes_for :people And of course the appropriate statements in the class Person. How can I add an Developer to a Project through the nested_attributes method? The following does not work: @p.people_attributes = [{:name => "Epic Beard Man", :type => "Developer"}] @p.people => [#<Person id: nil, name: "Epic Beard Man", type: nil>] As you can see the type attributes is set to nil instead of Developer.

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  • Is there any way to achieve multiple inheritance in php?

    - by Starx
    Lets say I have a parent class class parent { } ..... This parent has three sub class class child1 { } class child2 { } class child3 { } and these child classes have further smaller parts like class child1subpar1 { } class child1subpar2 { public function foo() { echo "hi"; } } class child2subpar1 { } class child2subpar2 { } Now, how to sum this whole up like class child1 extends child1subpar1, child1subpar2 { } class child2 extends child2subpar1, childsubpar1 { } class parent extends child1,child2,child3 { } I need to execute the methods in its inherited classes and do something like this $objparent = new parent; $objparent - foo();

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  • Merging multiple top-level domains into a single domain

    - by user23089
    My client had multiple top-level-domains. Each one represented an insurance program within a specific vertical. For all the sites at these alternate domains, there was a 30/70 mix of duplicate vs. original content. Some of the alternate domains ranked very well for their target keyphrase groups, where others were absent in results pages. We advised the client to merge multiple domains into their existing main domain, for usability and SEO reasons. We recently ran the merger. Here was our process: On the main domain, transfer the content such that it matches 1-for-1 content on the various alternate domains Setup Google Webmaster Tools on the main domain Push the new content on the main domain live and submit a corresponding sitemap to Google Establish 301 redirects on the alternate domains, such that each alternate domain URL points to its respective page on the main domain We did this 12 days ago, and pages (previously on the alternate domains) that had ranked well on Google have now plummeted or are entirely non-existent. Did we do the right thing by merging multiple top-level domains into a single domain? Is this initial dip in rankings normal? How soon should we expect to see it return to its normal rankings?

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  • JavaScript - Inheritance in Constructors

    - by j0ker
    For a JavaScript project we want to introduce object inheritance to decrease code duplication. However, I cannot quite get it working the way I want and need some help. We use the module pattern. Suppose there is a super element: a.namespace('a.elements.Element'); a.elements.Element = (function() { // public API -- constructor Element = function(properties) { this.id = properties.id; }; // public API -- prototype Element.prototype = { getID: function() { return this.id; } }; return Element; }()); And an element inheriting from this super element: a.namespace('a.elements.SubElement'); a.elements.SubElement = (function() { // public API -- constructor SubElement = function(properties) { // inheritance happens here // ??? this.color = properties.color; this.bogus = this.id + 1; }; // public API -- prototype SubElement.prototype = { getColor: function() { return this.color; } }; return SubElement; }()); You will notice that I'm not quite sure how to implement the inheritance itself. In the constructor I have to be able to pass the parameter to the super object constructor and create a super element that is then used to create the inherited one. I need a (comfortable) possibility to access the properties of the super object within the constructor of the new object. Ideally I could operate on the super object as if it was part of the new object. I also want to be able to create a new SubElement and call getID() on it. What I want to accomplish seems like the traditional class based inheritance. However, I'd like to do it using prototypal inheritance since that's the JavaScript way. Is that even doable? Thanks in advance! EDIT: Fixed usage of private variables as suggested in the comments. EDIT2: Another change of the code: It's important that id is accessible from the constructor of SubElement.

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  • SQL SERVER – Not Possible – Delete From Multiple Table – Update Multiple Table in Single Statement

    - by pinaldave
    There are two questions which I get every single day multiple times. In my gmail, I have created standard canned reply for them. Let us see the questions here. I want to delete from multiple table in a single statement how will I do it? I want to update multiple table in a single statement how will I do it? The answer is – No, You cannot and you should not. SQL Server does not support deleting or updating from two tables in a single update. If you want to delete or update two different tables – you may want to write two different delete or update statements for it. This method has many issues – from the consistency of the data to SQL syntax. Now here is the real reason for this blog post – yesterday I was asked this question again and I replied my canned answer saying it is not possible and it should not be any way implemented that day. In the response to my reply I was pointed out to my own blog post where user suggested that I had previously mentioned this is possible and with demo example. Let us go over my conversation – you may find it interesting. Let us call the user DJ. DJ: Pinal, can we delete multiple table in a single statement or with single delete statement? Pinal: No, you cannot and you should not. DJ: Oh okey, if that is the case, why do you suggest to do that? Pinal: (baffled) I am not suggesting that. I am rather suggesting that it is not possible and it should not be possible. DJ: Hmm… but in that case why did you blog about it earlier? Pinal: (What?) No, I did not. I am pretty confident. DJ: Well, I am confident as well. You did. Pinal: In that case, it is my word against your word. Isn’t it? DJ: I have proof. Do you want to see it that you suggest it is possible? Pinal: Yes, I will be delighted too. (After 10 Minutes) DJ: Here are not one but two of your blog posts which talks about it - SQL SERVER – Curious Case of Disappearing Rows – ON UPDATE CASCADE and ON DELETE CASCADE – Part 1 of 2 SQL SERVER – Curious Case of Disappearing Rows – ON UPDATE CASCADE and ON DELETE CASCADE – T-SQL Example – Part 2 of 2 Pinal: Oh! DJ: I know I was correct. Pinal: Well, oh man, I did not mean there what you mean here. DJ: I did not understand can you explain it further. Pinal: Here we go. The example in the other blog is the example of the cascading delete or cascading update. I think you may want to understand the concept of the foreign keys and cascading update/delete. The concept of cascading exists to maintain data integrity. If there primary keys get deleted the update or delete reflects on the foreign key table to maintain the key integrity and data consistency. SQL Server follows ANSI Entry SQL with regard to referential integrity between PrimaryKey and ForeignKey columns which requires the inserting, updating, and deleting of data in related tables to be restricted to values that preserve the integrity. This is all together different concept than deleting multiple values in a single statement. When I hear that someone wants to delete or update multiple table in a single statement what I assume is something very similar to following. DELETE/UPDATE Table 1 (cols) Table 2 (cols) VALUES … which is not valid statement/syntax as well it is not ASNI standards as well. I guess, after this discussion with DJ, I realize I need to do a blog post so I can add the link to this blog post in my canned answer. Well, it was a fun conversation with DJ and I hope it the message is very clear now. Reference: Pinal Dave (http://blog.SQLAuthority.com) Filed under: PostADay, SQL, SQL Authority, SQL Joins, SQL Query, SQL Server, SQL Tips and Tricks, T SQL, Technology

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  • C#: Handling Notifications: inheritance, events, or delegates?

    - by James Michael Hare
    Often times as developers we have to design a class where we get notification when certain things happen. In older object-oriented code this would often be implemented by overriding methods -- with events, delegates, and interfaces, however, we have far more elegant options. So, when should you use each of these methods and what are their strengths and weaknesses? Now, for the purposes of this article when I say notification, I'm just talking about ways for a class to let a user know that something has occurred. This can be through any programmatic means such as inheritance, events, delegates, etc. So let's build some context. I'm sitting here thinking about a provider neutral messaging layer for the place I work, and I got to the point where I needed to design the message subscriber which will receive messages from the message bus. Basically, what we want is to be able to create a message listener and have it be called whenever a new message arrives. Now, back before the flood we would have done this via inheritance and an abstract class: 1:  2: // using inheritance - omitting argument null checks and halt logic 3: public abstract class MessageListener 4: { 5: private ISubscriber _subscriber; 6: private bool _isHalted = false; 7: private Thread _messageThread; 8:  9: // assign the subscriber and start the messaging loop 10: public MessageListener(ISubscriber subscriber) 11: { 12: _subscriber = subscriber; 13: _messageThread = new Thread(MessageLoop); 14: _messageThread.Start(); 15: } 16:  17: // user will override this to process their messages 18: protected abstract void OnMessageReceived(Message msg); 19:  20: // handle the looping in the thread 21: private void MessageLoop() 22: { 23: while(!_isHalted) 24: { 25: // as long as processing, wait 1 second for message 26: Message msg = _subscriber.Receive(TimeSpan.FromSeconds(1)); 27: if(msg != null) 28: { 29: OnMessageReceived(msg); 30: } 31: } 32: } 33: ... 34: } It seems so odd to write this kind of code now. Does it feel odd to you? Maybe it's just because I've gotten so used to delegation that I really don't like the feel of this. To me it is akin to saying that if I want to drive my car I need to derive a new instance of it just to put myself in the driver's seat. And yet, unquestionably, five years ago I would have probably written the code as you see above. To me, inheritance is a flawed approach for notifications due to several reasons: Inheritance is one of the HIGHEST forms of coupling. You can't seal the listener class because it depends on sub-classing to work. Because C# does not allow multiple-inheritance, I've spent my one inheritance implementing this class. Every time you need to listen to a bus, you have to derive a class which leads to lots of trivial sub-classes. The act of consuming a message should be a separate responsibility than the act of listening for a message (SRP). Inheritance is such a strong statement (this IS-A that) that it should only be used in building type hierarchies and not for overriding use-specific behaviors and notifications. Chances are, if a class needs to be inherited to be used, it most likely is not designed as well as it could be in today's modern programming languages. So lets look at the other tools available to us for getting notified instead. Here's a few other choices to consider. Have the listener expose a MessageReceived event. Have the listener accept a new IMessageHandler interface instance. Have the listener accept an Action<Message> delegate. Really, all of these are different forms of delegation. Now, .NET events are a bit heavier than the other types of delegates in terms of run-time execution, but they are a great way to allow others using your class to subscribe to your events: 1: // using event - ommiting argument null checks and halt logic 2: public sealed class MessageListener 3: { 4: private ISubscriber _subscriber; 5: private bool _isHalted = false; 6: private Thread _messageThread; 7:  8: // assign the subscriber and start the messaging loop 9: public MessageListener(ISubscriber subscriber) 10: { 11: _subscriber = subscriber; 12: _messageThread = new Thread(MessageLoop); 13: _messageThread.Start(); 14: } 15:  16: // user will override this to process their messages 17: public event Action<Message> MessageReceived; 18:  19: // handle the looping in the thread 20: private void MessageLoop() 21: { 22: while(!_isHalted) 23: { 24: // as long as processing, wait 1 second for message 25: Message msg = _subscriber.Receive(TimeSpan.FromSeconds(1)); 26: if(msg != null && MessageReceived != null) 27: { 28: MessageReceived(msg); 29: } 30: } 31: } 32: } Note, now we can seal the class to avoid changes and the user just needs to provide a message handling method: 1: theListener.MessageReceived += CustomReceiveMethod; However, personally I don't think events hold up as well in this case because events are largely optional. To me, what is the point of a listener if you create one with no event listeners? So in my mind, use events when handling the notification is optional. So how about the delegation via interface? I personally like this method quite a bit. Basically what it does is similar to inheritance method mentioned first, but better because it makes it easy to split the part of the class that doesn't change (the base listener behavior) from the part that does change (the user-specified action after receiving a message). So assuming we had an interface like: 1: public interface IMessageHandler 2: { 3: void OnMessageReceived(Message receivedMessage); 4: } Our listener would look like this: 1: // using delegation via interface - omitting argument null checks and halt logic 2: public sealed class MessageListener 3: { 4: private ISubscriber _subscriber; 5: private IMessageHandler _handler; 6: private bool _isHalted = false; 7: private Thread _messageThread; 8:  9: // assign the subscriber and start the messaging loop 10: public MessageListener(ISubscriber subscriber, IMessageHandler handler) 11: { 12: _subscriber = subscriber; 13: _handler = handler; 14: _messageThread = new Thread(MessageLoop); 15: _messageThread.Start(); 16: } 17:  18: // handle the looping in the thread 19: private void MessageLoop() 20: { 21: while(!_isHalted) 22: { 23: // as long as processing, wait 1 second for message 24: Message msg = _subscriber.Receive(TimeSpan.FromSeconds(1)); 25: if(msg != null) 26: { 27: _handler.OnMessageReceived(msg); 28: } 29: } 30: } 31: } And they would call it by creating a class that implements IMessageHandler and pass that instance into the constructor of the listener. I like that this alleviates the issues of inheritance and essentially forces you to provide a handler (as opposed to events) on construction. Well, this is good, but personally I think we could go one step further. While I like this better than events or inheritance, it still forces you to implement a specific method name. What if that name collides? Furthermore if you have lots of these you end up either with large classes inheriting multiple interfaces to implement one method, or lots of small classes. Also, if you had one class that wanted to manage messages from two different subscribers differently, it wouldn't be able to because the interface can't be overloaded. This brings me to using delegates directly. In general, every time I think about creating an interface for something, and if that interface contains only one method, I start thinking a delegate is a better approach. Now, that said delegates don't accomplish everything an interface can. Obviously having the interface allows you to refer to the classes that implement the interface which can be very handy. In this case, though, really all you want is a method to handle the messages. So let's look at a method delegate: 1: // using delegation via delegate - omitting argument null checks and halt logic 2: public sealed class MessageListener 3: { 4: private ISubscriber _subscriber; 5: private Action<Message> _handler; 6: private bool _isHalted = false; 7: private Thread _messageThread; 8:  9: // assign the subscriber and start the messaging loop 10: public MessageListener(ISubscriber subscriber, Action<Message> handler) 11: { 12: _subscriber = subscriber; 13: _handler = handler; 14: _messageThread = new Thread(MessageLoop); 15: _messageThread.Start(); 16: } 17:  18: // handle the looping in the thread 19: private void MessageLoop() 20: { 21: while(!_isHalted) 22: { 23: // as long as processing, wait 1 second for message 24: Message msg = _subscriber.Receive(TimeSpan.FromSeconds(1)); 25: if(msg != null) 26: { 27: _handler(msg); 28: } 29: } 30: } 31: } Here the MessageListener now takes an Action<Message>.  For those of you unfamiliar with the pre-defined delegate types in .NET, that is a method with the signature: void SomeMethodName(Message). The great thing about delegates is it gives you a lot of power. You could create an anonymous delegate, a lambda, or specify any other method as long as it satisfies the Action<Message> signature. This way, you don't need to define an arbitrary helper class or name the method a specific thing. Incidentally, we could combine both the interface and delegate approach to allow maximum flexibility. Doing this, the user could either pass in a delegate, or specify a delegate interface: 1: // using delegation - give users choice of interface or delegate 2: public sealed class MessageListener 3: { 4: private ISubscriber _subscriber; 5: private Action<Message> _handler; 6: private bool _isHalted = false; 7: private Thread _messageThread; 8:  9: // assign the subscriber and start the messaging loop 10: public MessageListener(ISubscriber subscriber, Action<Message> handler) 11: { 12: _subscriber = subscriber; 13: _handler = handler; 14: _messageThread = new Thread(MessageLoop); 15: _messageThread.Start(); 16: } 17:  18: // passes the interface method as a delegate using method group 19: public MessageListener(ISubscriber subscriber, IMessageHandler handler) 20: : this(subscriber, handler.OnMessageReceived) 21: { 22: } 23:  24: // handle the looping in the thread 25: private void MessageLoop() 26: { 27: while(!_isHalted) 28: { 29: // as long as processing, wait 1 second for message 30: Message msg = _subscriber.Receive(TimeSpan.FromSeconds(1)); 31: if(msg != null) 32: { 33: _handler(msg); 34: } 35: } 36: } 37: } } This is the method I tend to prefer because it allows the user of the class to choose which method works best for them. You may be curious about the actual performance of these different methods. 1: Enter iterations: 2: 1000000 3:  4: Inheritance took 4 ms. 5: Events took 7 ms. 6: Interface delegation took 4 ms. 7: Lambda delegate took 5 ms. Before you get too caught up in the numbers, however, keep in mind that this is performance over over 1,000,000 iterations. Since they are all < 10 ms which boils down to fractions of a micro-second per iteration so really any of them are a fine choice performance wise. As such, I think the choice of what to do really boils down to what you're trying to do. Here's my guidelines: Inheritance should be used only when defining a collection of related types with implementation specific behaviors, it should not be used as a hook for users to add their own functionality. Events should be used when subscription is optional or multi-cast is desired. Interface delegation should be used when you wish to refer to implementing classes by the interface type or if the type requires several methods to be implemented. Delegate method delegation should be used when you only need to provide one method and do not need to refer to implementers by the interface name.

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  • Rails: Single Table Inheritance and models subdirectories

    - by Chris
    I have a card-game application which makes use of Single Table Inheritance. I have a class Card, and a database table cards with column type, and a number of subclasses of Card (including class Foo < Card and class Bar < Card, for the sake of argument). As it happens, Foo is a card from the original printing of the game, which Bar is a card from an expansion. In an attempt to rationalise my models, I have created a directory structure like so: app/ + models/ + card.rb + base_game/ + foo.rb + expansion/ + bar.rb And modified environment.rb to contain: Rails::Initializer.run do |config| config.load_paths += Dir["#{RAILS_ROOT}/app/models/**"] end However, when my reads a card from the database, Rails throws the following exception: ActiveRecord::SubclassNotFound (The single-table inheritance mechanism failed to locate the subclass: 'Foo'. This error is raised because the column 'type' is reserved for storing the class in case of inheritance. Please rename this column if you didn't intend it to be used for storing the inheritance class or overwrite Card.inheritance_column to use another column for that information.) Is it possible to make this work, or am I doomed to a flat directory structure?

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  • C++ private inheritance and static members/types

    - by WearyMonkey
    I am trying to stop a class from being able to convert its 'this' pointer into a pointer of one of its interfaces. I do this by using private inheritance via a middle proxy class. The problem is that I find private inheritance makes all public static members and types of the base class inaccessible to all classes under the inheriting class in the hierarchy. class Base { public: enum Enum { value }; }; class Middle : private Base { }; class Child : public Middle { public: void Method() { Base::Enum e = Base::value; // doesn't compile BAD! Base* base = this; // doesn't compile GOOD! } }; I've tried this in both VS2008 (the required version) and VS2010, neither work. Can anyone think of a workaround? Or a different approach to stopping the conversion? Also I am curios of the behavior, is it just a side effect of the compiler implementation, or is it by design? If by design, then why? I always thought of private inheritance to mean that nobody knows Middle inherits from Base. However, the exhibited behavior implies private inheritance means a lot more than that, in-fact Child has less access to Base than any namespace not in the class hierarchy!

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  • Multiple sites redirected to one main site

    - by mattgcon
    I have a client who insists of having multiple website domains all being redirected to one main website domain. It is getting out of hand and his server has become conveluted and riddled with garbage because of it, not to mention confusing at times. Each of these domains that he is setting up has no content, they simply redirect the user to the main website domain. Is this practice of having multiple domains pointing to one main website common? And does anyone know where I can get information to give to this client to let him know this is a bad practice if it is a bad practice?

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  • structures, inheritance and definition

    - by Meloun
    Hi, i need to help with structures, inheritance and definition. //define struct struct tStruct1{ int a; }; //definition tStruct1 struct1{1}; and inheritance struct tStruct2:tStruct1{ int b; }; How can I define it in declaration line? tStruct2 struct2{ ????? }; One more question, how can i use inheritance for structures defined with typedef struct?

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  • Backbone inheritance - deep copying

    - by Ed .
    I've seen this question regarding inheritance in Backbone: Backbone.js view inheritance. Useful but doesn't answer my question. The problem I'm experiencing is this: Say I have a class Panel (model in this example); var Panel = Backbone.Model.extend({ defaults : { name : 'my-panel' } }); And then an AdvancedPanel; var AdvancedPanel = Panel.extend({ defaults : { label : 'Click to edit' } }); The following doesn't work: var advancedPanel = new AdvancedPanel(); alert(advancedPanel.get('name')); // Undefined :( JSFiddle here: http://jsfiddle.net/hWmnb/ I guess I can see that I can achieve this myself through some custom extend function that creates a deep copy of the prototype, but this seems like a common thing that people might want from Backbone inheritance, is there a standard way of doing it?

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  • Propel Single Table Inheritance Issue

    - by lo_fye
    I have a table called "talk", which is defined as abstract in my schema.xml file. It generates 4 objects (1 per classkey): Comment, Rating, Review, Checkin It also generates TalkPeer, but I couldn't get it to generate the other 4 peers (CommentPeer, RatingPeer, ReviewPeer, CheckinPeer), so I created them by hand, and made them inherit from TalkPeer.php, which inherits from BaseTalkPeer. I then implemented getOMClass() in each of those peers. The problem is that when I do queries using the 4 peers, they return all 4 types of objects. That is, ReviewPeer will return Visits, Ratings, Comments, AND Reviews. Example: $c = new Criteria(); $c->add(RatingPeer::VALUE, 5, Criteria::GREATER_THAN); $positive_ratings = RatingPeer::doSelect($c); This returns all comments, ratings, reviews, & checkins that have a value 5. ReviewPeer should only return Review objects, and can't figure out how to do this. Do I actually have to go through and change all my criteria to manually specify the classkey? That seems a little pointless, since the Peer name already distinct. I don't want to have to customize each Peer. I should be able to customize JUST the TalkPeer, since they all inherit from it... I just can't figure out how. I tried changing doSelectStmt just in TalkPeer so that it automatically adds the CLASSKEY restriction to the Criteria. It almost works, but I get a: Fatal error: Cannot instantiate abstract class Talk in /models/om/BaseTalkPeer.php on line 503. Line 503 is in BaseTalkPeer::populateObjects(), and is the 3rd line below: $cls = TalkPeer::getOMClass($row, 0); $cls = substr('.'.$cls, strrpos('.'.$cls, '.') + 1); $obj = new $cls(); The docs talked about overriding BaseTalkPeer::populateObject(). I have a feeling that's my problem, but even after reading the source code, I still couldn't figure out how to get it to work. Here is what I tried in TalkPeer::doSelectStmt: public static function doSelectStmt(Criteria $criteria, PropelPDO $con = null) { $keys = array('models.Visit'=>1,'models.Comment'=>2,'models.Rating'=>3,'models.Review'=>4); $class_name = self::getOMClass(); if(isset($keys[$class_name])) { //Talk itself is not a returnable type, so we must check $class_key = $keys[$class_name]; $criteria->add(TalkPeer::CLASS_KEY, $class_key); } return parent::doSelectStmt($criteria, $con = null); } Here is an example of my getOMClass method from ReviewPeer: public static function getOMClass() { return self::CLASSNAME_4; //aka 'talk.Review'; } Here is the relevant bit of my schema: <table name="talk" idMethod="native" abstract="true"> <column name="talk_pk" type="INTEGER" required="true" autoIncrement="true" primaryKey="true" /> <column name="class_key" type="INTEGER" required="true" default="" inheritance="single"> <inheritance key="1" class="Visit" extends="models.Talk" /> <inheritance key="2" class="Comment" extends="models.Talk" /> <inheritance key="3" class="Rating" extends="models.Talk" /> <inheritance key="4" class="Review" extends="models.Rating" /> </column> </table> P.S. - No, I can't upgrade from 1.3 to 1.4. There's just too much code that would need to be re-tested

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  • Use a Free Utility to Create Multiple Virtual Desktops in Windows

    - by Lori Kaufman
    If you’ve used Linux, you’re probably familiar with the virtual desktop feature. It provides a convenient way to organize programs and folders open on your desktop. You can switch among multiple desktops and have different programs and folders open on each one. However, virtual desktops is a feature missing in Windows. There are many third-party options for adding virtual desktops to Windows, including one called Dexpot, which we have covered previously. Dexpot is free, but only for private use. Companies, public institutions, non-profit organizations, and even freelancers and self-employed people must buy the program. We found another virtual desktop tool that is completely free for everyone to use, called mDesktop. It’s a lightweight, open source program that allows you to switch among multiple desktops using hot keys and specify open programs or folders to be active on all desktops. You can use mDesktop to group related programs or to work on different projects on separate desktops. mDesktop is portable and does not need to be installed. Simply extract the .zip file you downloaded (see the link at the end of this article) and double-click the mDesktop.exe file. How To Boot Your Android Phone or Tablet Into Safe Mode HTG Explains: Does Your Android Phone Need an Antivirus? How To Use USB Drives With the Nexus 7 and Other Android Devices

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  • Gnome 3 - Multiple Video Cards - Xinerama -- Forced Fallback Mode

    - by Alvin
    Just installed a 2nd nvidia video card -- previously had gnome 3 working perfectly with 2 monitors on a a single video card using twinview tried a number of things thus far twinview on 1 card + xinerama no xinerama no twinview various manual xorg.conf hacks based on random forums (couple references below) xinerama no twinview with and without Extensions Composite The last one is what I'm using now -- it results in a forced fallback mode with Composite Disable set at the end of xorg.conf via nvidia-settings Section "Extensions" Option "Composite" "Disable" EndSection when I disabled that last snippet it boots to gnome 3 full with the left monitor on a black screen and the middle monitor as primary but non-responsive switching to console mode Ctrl+Alt+F1 and then switching back I get 3 black screens with a mouse that can move around but nothing to interact with issue seems related to OpenGL and the multiple video cards -- I can boot into Unity without issue though my Glx-Dock shows up with the black background as barely shows in the screenshot below indicating the OpenGL is not initiated has anyone had any luck with getting Xinerama to work with Multiple NVidia Video Cards with OpenGL support? Found this in the logs while looking a bit further [ 23.208] (II) NVIDIA(1): Setting mode "nvidia-auto-select+0+0" [ 23.254] (WW) NVIDIA(1): The GPU driving screen 1 is incompatible with the rest of the [ 23.254] (WW) NVIDIA(1): GPUs composing the desktop. OpenGL rendering will be [ 23.254] (WW) NVIDIA(1): disabled on screen 1. [ 23.277] (==) NVIDIA(1): Disabling shared memory pixmaps [ 23.277] (==) NVIDIA(1): Backing store disabled [ 23.277] (==) NVIDIA(1): Silken mouse enabled [ 23.277] (==) NVIDIA(1): DPMS enabled According to this page at the NVidia User Docs http://us.download.nvidia.com/XFree86/Linux-x86/173.14.09/README/chapter-14.html I may be out of luck =( Starting this question with the hopes that others may be able to help debug and perhaps gain answers over time as I really want to get the full gnome 3 back.

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  • How to refactor my design, if it seems to require multiple inheritance?

    - by Omega
    Recently I made a question about Java classes implementing methods from two sources (kinda like multiple inheritance). However, it was pointed out that this sort of need may be a sign of a design flaw. Hence, it is probably better to address my current design rather than trying to simulate multiple inheritance. Before tackling the actual problem, some background info about a particular mechanic in this framework: It is a simple game development framework. Several components allocate some memory (like pixel data), and it is necessary to get rid of it as soon as you don't need it. Sprites are an example of this. Anyway, I decided to implement something ala Manual-Reference-Counting from Objective-C. Certain classes, like Sprites, contain an internal counter, which is increased when you call retain(), and decreased on release(). Thus the Resource abstract class was created. Any subclass of this will obtain the retain() and release() implementations for free. When its count hits 0 (nobody is using this class), it will call the destroy() method. The subclass needs only to implement destroy(). This is because I don't want to rely on the Garbage Collector to get rid of unused pixel data. Game objects are all subclasses of the Node class - which is the main construction block, as it provides info such as position, size, rotation, etc. See, two classes are used often in my game. Sprites and Labels. Ah... but wait. Sprites contain pixel data, remember? And as such, they need to extend Resource. But this, of course, can't be done. Sprites ARE nodes, hence they must subclass Node. But heck, they are resources too. Why not making Resource an interface? Because I'd have to re-implement retain() and release(). I am avoiding this in virtue of not writing the same code over and over (remember that there are multiple classes that need this memory-management system). Why not composition? Because I'd still have to implement methods in Sprite (and similar classes) that essentially call the methods of Resource. I'd still be writing the same code over and over! What is your advice in this situation, then?

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  • Best Usage of Multiple Computers For a Developer

    - by whaley
    I have two Macbook Pros - both are comparable in hardware. One is a 17" and the other a 15". The 17" has a slightly swifter CPU clock speed, but beyond that the differences are completely negligible. I tried a setup a while back where I had the 17" hooked up to an external monitor in the middle of my desk with the 15" laptop immediately to the right of it, and was using teleport to control the 15" from my 17". All development, terminal usage, etc. etc. was being done on the 17" and the 15" was primarily used for email / IM / IRC... or anything secondary to what I was working on. I have a MobileMe account so preferences were synced, but otherwise I didn't really use anything else to keep the computers in sync (I use dropbox/git but probably not optimally). For reasons I can't put my finger on, this setup never felt quite right. A few things that irked me was the 15" was way under-utlized and the 17" was overutilized having 2 laptops and a 21" monitor all on one desk actually took up lots of desk space and it felt like I had too much to look at. I reverted back to just using the 17" and the external monitor and keeping the 15" around the house (and using it very sparingly). For those of you who are using multiple laptops (or just multiple machines for that matter), I'd like to see setups that work for you for when you have 2 or more machines that gives you optimal productivity and why. I'd like to give this one more shot but with a different approach than my previous - which was using the 15" as a machine for secondary things (communication, reading documentation, etc. etc).

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  • What is the exact problem with multiple inheritance?

    - by Totophil
    I can see people asking all the time whether multiple inheritance should be included into the next version of C# or Java and C++ folks, who are fortunate enough to have this ability, say that this is like giving someone a rope to eventually hang themselves. What’s the matter with the multiple inheritance? Are there any concrete samples?

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  • improve javascript prototypal inheritance

    - by Julio
    I'm using a classical javascript prototypal inheritance, like this: function Foo() {} Naknek.prototype = { //Do something }; var Foo = window.Foo = new Foo(); I want to know how I can improve this and why I can't use this model: var Foo = window.Foo = new function() { }; Foo.prototype = { //Do something }; Why this code doesn't work? In my head this is more logical than the classical prototypal inheritance.

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