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  • Rails routing/polymorphism issue - how to model the following

    - by 46and2
    Hi all, I have an app where a 'user' belong to a 'client' or a 'vendor' (and client and vendor has_many users). In the admin namespace, I want to administer these users - so an admin would choose a client or a vendor, then nav to that client's or vendor's users. My question is, short of making the user model polymorphic, how could I model/route this? Here is what I have in terms of routing: map.namespace :admin do |admin| admin.resources :clients admin.resources :vendors end I know I could do something like: map.namespace :admin do |admin| admin.resources :clients do |client| client.resources :users admin.resources :vendors do |vendor| vendor.resources :users end end But the above would definitely need me to treat the User as polymorphic. I'm just wondering what you would recommend or what my options are. Thanks.

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  • Best way to order by columns in relationships?

    - by Timmy
    im using sqlalchemy, and i have a few polymorphic tables, and i want to sort by a column in one of the relationship. i have tables a,b,c,d, with relationship a to b, b to c, c to d. a to b is one-to-many b to c and c to d are one-to-one, but polymorphic. given an item in table a, i will have a list of items b, c, d (all one to one). how do i use sqlalchemy to sort them by a column in table d?

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  • How do you pass self to class_eval in ruby?

    - by klochner
    I'm working on a metaprogramming task, where I'm trying to use a single method to define a polymorphic association in the calling class, while also defining the association in the target class. I need to pass in the name of the calling class to get the association right. Here's a snippet that should get the idea across: class SomeClass < ActiveRecord::Base has_many :join_models, :dependent=:destroy end class JoinModel < ActiveRecord::Base belongs_to :some_class belongs_to :entity, :polymorphic=true end module Foo module ClassMethods def acts_as_entity has_many :join_models, :as=:entity, :dependent=:destroy has_many :some_classes, :through=:join_models klass = self.name.tableize SomeClass.class_eval "has_many :#{klass}, :through=:join_models" end end end I'd like to eliminate the klass= line, but don't know how else to pass a reference to self from the calling class into class_eval. any suggestions?

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  • Learning PostgreSql: polymorphism

    - by Alexander Kuznetsov
    Functions in PL/PgSql are polymorphic, which is very different from T-SQL. Demonstrating polymorphism For example, the second CREATE FUNCTION in the following script does not replace the first function - it creates a second one: CREATE OR REPLACE FUNCTION public .GetQuoteOfTheDay ( someNumber INTEGER ) RETURNS VARCHAR AS $body$ BEGIN RETURN 'Say my name.' ; END ; $body$ LANGUAGE plpgsql ; CREATE OR REPLACE FUNCTION public .GetQuoteOfTheDay ( someNumber REAL ) RETURNS VARCHAR AS $body$ BEGIN RETURN...(read more)

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  • Useful links

    - by Madhan ayyasamy
    Difference between STI vs Polymorphic associationsSTI vs PolymorphicDifference between habtm vs has_many :throughhabtm vs throughCapistrano Guide linkCapistrano guideRails application without database stuffclass Car < ActiveRecord::Baseself.abstract = trueendAnother link: rails without databaseNamed scope useful linkNamed scopeDifference between http and https verbhttp vs httpsRails 2.3 useful guide websiterails 2.3 guide

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  • 10 Best Programming Podcast 2010 Edition

    - by mbcrump
    This list is in no particular order. Just the 10 best programming podcast that I have found so far. Stack Overflow Podcast -  Jeff Atwood (of codinghorror.com) and Joel Spolsky (of joelonsoftware.com) discuss the development of their new programming community, StackOverflow.com. [This Podcast hasn’t been updated in a while, but its always great to hear more from Jeff Atwood] Hanselminutes - Hanselminutes is a weekly audio talk show with noted web developer and technologist Scott Hanselman and hosted by Carl Franklin. Scott discusses utilities and tools, gives practical how-to advice, and discusses ASP.NET or Windows issues and workarounds. [This Podcast has recently started talking about random topics like diabetes, plane travel and geek relationship tips.  I am not sure if Scott is trying to move to a more mainstream audience or not] Herding Code - A weekly discussion featuring K. Scott Allen (odetocode.com), Kevin Dente, Scott Koon (lazycoder.com), and Jon Galloway. [Great all all-around podcast that I would recommend to all] Deep Fried Bytes - Deep Fried Bytes is an audio talk show with a Southern flavor hosted by technologists and developers Keith Elder and Chris Woodruff. The show discusses a wide range of topics including application development, operating systems and technology in general. Anything is fair game if it plugs into the wall or takes a battery. [This is one that just keeps getting better] Dot Net Rocks - .NET Rocks! is an Internet Audio Talk Show for Microsoft .NET Developers. [One of the first and usually very high quality content] Connected Show - Connected Show Podcast! A podcast covering new Microsoft technology for the developer community. The show is hosted by Dmitry Lyalin and Peter Laudati. [This and Polymorphic are one of my favorite podcast – Dmitry is a great host and would recommend this to all] Polymorphic Podcast - Object oriented development, architecture and best practices in .NET [Craig is a ASP.NET MVP and a great presenter. His podcast is great and it could only be better if he recorded it more often] ASP.NET Podcast - Wallace B. (Wally) McClure presents interviews and short technical talks on .NET Technologies. [Has great information on ASP.NET of course as well as iPhone Dev] Ruby on Rails Podcast - News and interviews about the Ruby language and the Rails website framework. [Even though I am not a Ruby programmer, I’ve found this podcast very interesting] Software Engineering Radio - Software Engineering Radio is a podcast targeted at the professional software developer. The goal is to be a lasting educational resource, not a newscast. Every ten days, a new episode is published that covers all topics software engineering. Episodes are either tutorials on a specific topic, or an interview with a well-known character from the software engineering world. All SE Radio episodes are original content ? we do not record conferences or talks given in other venues. Each episode comprises two speakers to ensure a lively listening experience. SE Radio is an independent and non-commercial organization. [Another excellent podcast – I would recommend any programmer add this to his/her drive home] If I have missed something, please feel free to email me and it might make the 2011 list. =)

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  • Cast errors with IXmlSerializable

    - by Nathan
    I am trying to use the IXmlSerializable interface to deserialize an Object (I am using it because I need specific control over what gets deserialized and what does not. See my previous question for more information). However, I'm stumped as to the error message I get. Below is the error message I get (I have changed some names for clarity): An unhandled exception of type 'System.InvalidCastException' occurred in App.exe Additional information: Unable to cast object of type 'System.Xml.XmlNode[]' to type 'MyObject'. MyObject has all the correct methods defined for the interface, and regardless of what I put in the method body for ReadXml() I still get this error. It doesn't matter if it has my implementation code or if it's just blank. I did some googling and found an error that looks similar to this involving polymorphic objects that implement IXmlSerializable. However, my class does not inherit from any others (besides Object). I suspected this may be an issue because I never reference XmlNode any other time in my code. Microsoft describes a solution to the polymorphism error: https://connect.microsoft.com/VisualStudio/feedback/details/422577/incorrect-deserialization-of-polymorphic-type-that-implements-ixmlserializable?wa=wsignin1.0#details The code the error occurs at is as follows. The object to be read back in is an ArrayList of "MyObjects" IO::FileStream ^fs = gcnew IO::FileStream(filename, IO::FileMode::Open); array<System::Type^>^ extraTypes = gcnew array<System::Type^>(1); extraTypes[0] = MyObject::typeid; XmlSerializer ^xmlser = gcnew XmlSerializer(ArrayList::typeid, extraTypes); System::Object ^obj; obj = xmlser->Deserialize(fs); fs->Close(); ArrayList ^al = safe_cast<ArrayList^>(obj); MyObject ^objs; for each(objs in al) //Error occurs here { //do some processing } Thanks for reading and for any help.

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  • Is a "factory" method the right pattern?

    - by jdt141
    Hey all - So I'm working to improve an existing implementation. I have a number of polymorphic classes that are all composed into a higher level container class. The problem I'm dealing with at the moment is that the higher level container class, well, sucks. It looks something like this, which I really don't have a problem with (as the polymorphic classes in the container should be public). My real issue is the constructor... /* * class1 and class 2 derive from the same superclass */ class Container { public: boost::shared_ptr<ComposedClass1> class1; boost::shared_ptr<ComposedClass2> class2; private: ... } /* * Constructor - builds the objects that we need in this container. */ Container::Container(some params) { class1.reset(new ComposedClass1(...)); class2.reset(new ComposedClass2(...)); } What I really need is to make this container class more re-usable. By hard-coding up the member objects and instantiating them, it basically isn't and can only be used once. A factory is one way to build what I need (potentially by supplying a list of objects and their specific types to be created?) Other ways to get around this problem? Seems like someone should have solved it before... Thanks!

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  • Rails - Scalable calculation model

    - by H O
    I currently have a calculation structure in my rails app that has models metric, operand and operation_type. Presently, the metric model has many operands, and can perform calculations based on the operation_type (e.g. sum, multiply, etc.), and each operand is defined as being right or left (i.e. so that if the operation is division, the numerator and denominator can be identified). Presently, an operand is always an attribute of some model, e.g. @customer.sales.selling_price.sum. In order to make this scalable, in need to allow an operand to be either an attribute of some kind, or the results of a previous operation, i.e. an operand can be a metric. I have included a diagram of how my models currently look: Can anyone assist me with the most elegant way of allowing an operand to be an actual operand, or another metric? Thanks! EDIT: It seems based on the only answer so far that perhaps polymorphic associations are the way to go on this, but the answer is so brief I have no idea how they could be used in this way - can anyone elaborate? EDIT 2: OK, I think I'm getting somewhere - essentially i presently have a metric, which has_many operands, and an operand has_many metrics. I need a polymorphic self join, where a metric can also have many metrics - do I need to call this something else, perhaps calculated_metrics, so that the metric model can use itself? That would leave me with a situation where a metric has_many operands, and a metric has many calculated_metrics.

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  • how can i set up a uniqueness constraint in mysql for columns that can be null?

    - by user299689
    I know that in MySQL, UNIQUE constraits don't treat NULL values as equal. So if I have a unique constraint on ColumnX, then two separate rows can have values of NULL for ColumnX and this wouldn't violate the constraint. How can I work around this? I can't just set the value to an arbitrary constant that I can flag, because ColumnX in my case is actually a foreign key to another table. What are my options here? Please note that this table also has an "id" column that is its primary key. Since I'm using Ruby on Rails, it's important to keep this id column as the primary key. Note 2: In reality, my unique key encompasses many columns, and some of them have to be null, because they are foreign keys, and only one of them should be non-null. What I'm actually trying to do is to "simulate" a polymorphic relationship in a way that keep referential integrity in the db, but using the technique outlined in the first part of the question asked here: http://stackoverflow.com/questions/922184/why-can-you-not-have-a-foreign-key-in-a-polymorphic-association

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  • Resources for improving your comprehension of recursion?

    - by Andrew M
    I know what recursion is (when a patten reoccurs within itself, typically a function that calls itself on one of its lines, after a breakout conditional... right?), and I can understand recursive functions if I study them closely. My problem is, when I see new examples, I'm always initially confused. If I see a loop, or a mapping, zipping, nesting, polymorphic calling, and so on, I know what's going just by looking at it. When I see recursive code, my thought process is usually 'wtf is this?' followed by 'oh it's recursive' followed by 'I guess it must work, if they say it does.' So do you have any tips/plans/resources for building up your skills in this area? Recursion is kind of a wierd concept so I'm thinking the way to tackle it may be equally wierd and inobvious.

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  • What is the better design decision approach?

    - by palm snow
    I have two classes (MyFoo1 and MyFoo2) that share some common functionality. So far it does not seem like I need any polymorphic inheritence but at this point I am considering the following options: Have the common functionality in a utility class. Both of these classes call these methods from that utility class. Have an abstract class and implement common methods in that abstract class. Then derive MyFoo1 and MyFoo2 from that abstract class. Any suggestion on what would be a better design decision?

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  • Polymorphism, Autoboxing, and Implicit Conversions

    - by dbyrne
    Would you consider autoboxing in Java to be a form of polymorphism? Put another way, do you think autoboxing extends the polymorphic capabilities of Java? What about implicit conversions in Scala? My opinion is that they are both examples of polymorphism. Both features allow values of different data types to be handled in a uniform manner. My colleague disagrees with me. Who is right?

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  • Is factory method proper design for my problem?

    - by metdos
    Hello Everyone, here is my problem and I'm considering to use factory method in C++, what are your opinions ? There are a Base Class and a lot of Subclasses. I need to transfer objects on network via TCP. I will create objects in first side, and using this object I will create a byte array TCP message, and send it to other side. On the other side I will decompose TCP message, I will create object and I will add this object to a polymorphic queue.

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  • Entity Framework Polymorphism

    - by Chubbs
    Hey guys, I wanted some help with trying to implement a simple polymorphic relationship using Entity Framework. An example of a relationship I would like to implement: Comment table ItemType ('Video', 'User') ItemID Body Video has many Comments User has many Comments No idea the best way to do this, I come from a Ruby on Rails way of thinking.

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  • GHC.Generics and Type Families

    - by jberryman
    This is a question related to my module here, and is simplified a bit. It's also related to this previous question, in which I oversimplified my problem and didn't get the answer I was looking for. I hope this isn't too specific, and please change the title if you can think if a better one. Background My module uses a concurrent chan, split into a read side and write side. I use a special class with an associated type synonym to support polymorphic channel "joins": {-# LANGUAGE TypeFamilies #-} class Sources s where type Joined s newJoinedChan :: IO (s, Messages (Joined s)) -- NOT EXPORTED --output and input sides of channel: data Messages a -- NOT EXPORTED data Mailbox a instance Sources (Mailbox a) where type Joined (Mailbox a) = a newJoinedChan = undefined instance (Sources a, Sources b)=> Sources (a,b) where type Joined (a,b) = (Joined a, Joined b) newJoinedChan = undefined -- and so on for tuples of 3,4,5... The code above allows us to do this kind of thing: example = do (mb , msgsA) <- newJoinedChan ((mb1, mb2), msgsB) <- newJoinedChan --say that: msgsA, msgsB :: Messages (Int,Int) --and: mb :: Mailbox (Int,Int) -- mb1,mb2 :: Mailbox Int We have a recursive action called a Behavior that we can run on the messages we pull out of the "read" end of the channel: newtype Behavior a = Behavior (a -> IO (Behavior a)) runBehaviorOn :: Behavior a -> Messages a -> IO () -- NOT EXPORTED This would allow us to run a Behavior (Int,Int) on either of msgsA or msgsB, where in the second case both Ints in the tuple it receives actually came through separate Mailboxes. This is all tied together for the user in the exposed spawn function spawn :: (Sources s) => Behavior (Joined s) -> IO s ...which calls newJoinedChan and runBehaviorOn, and returns the input Sources. What I'd like to do I'd like users to be able to create a Behavior of arbitrary product type (not just tuples) , so for instance we could run a Behavior (Pair Int Int) on the example Messages above. I'd like to do this with GHC.Generics while still having a polymorphic Sources, but can't manage to make it work. spawn :: (Sources s, Generic (Joined s), Rep (Joined s) ~ ??) => Behavior (Joined s) -> IO s The parts of the above example that are actually exposed in the API are the fst of the newJoinedChan action, and Behaviors, so an acceptable solution can modify one or all of runBehaviorOn or the snd of newJoinedChan. I'll also be extending the API above to support sums (not implemented yet) like Behavior (Either a b) so I hoped GHC.Generics would work for me. Questions Is there a way I can extend the API above to support arbitrary Generic a=> Behavior a? If not using GHC's Generics, are there other ways I can get the API I want with minimal end-user pain (i.e. they just have to add a deriving clause to their type)?

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  • C++ game designing & polymorphism question

    - by Kotti
    Hi! I'm trying to implement some sort of 'just-for-me' game engine and the problem's plot goes the following way: Suppose I have some abstract interface for a renderable entity, e.g. IRenderable. And it's declared the following way: interface IRenderable { // (...) // Suppose that Backend is some abstract backend used // for rendering, and it's implementation is not important virtual void Render(Backend& backend) = 0; }; What I'm doing right now is something like declaring different classes like class Ball : public IRenderable { virtual void Render(Backend& backend) { // Rendering implementation, that is specific for // the Ball object // (...) } }; And then everything looks fine. I can easily do something like std::vector<IRenderable*> items, push some items like new Ball() in this vector and then make a call similiar to foreach (IRenderable* in items) { item->Render(backend); } Ok, I guess it is the 'polymorphic' way, but what if I want to have different types of objects in my game and an ability to manipulate their state, where every object can be manipulated via it's own interface? I could do something like struct GameState { Ball ball; Bonus bonus; // (...) }; and then easily change objects state via their own methods, like ball.Move(...) or bonus.Activate(...), where Move(...) is specific for only Ball and Activate(...) - for only Bonus instances. But in this case I lose the opportunity to write foreach IRenderable* simply because I store these balls and bonuses as instances of their derived, not base classes. And in this case the rendering procedure turns into a mess like ball.Render(backend); bonus.Render(backend); // (...) and it is bad because we actually lose our polymorphism this way (no actual need for making Render function virtual, etc. The other approach means invoking downcasting via dynamic_cast or something with typeid to determine the type of object you want to manipulate and this looks even worse to me and this also breaks this 'polymorphic' idea. So, my question is - is there some kind of (probably) alternative approach to what I want to do or can my current pattern be somehow modified so that I would actually store IRenderable* for my game objects (so that I can invoke virtual Render method on each of them) while preserving the ability to easily change the state of these objects? Maybe I'm doing something absolutely wrong from the beginning, if so, please point it out :) Thanks in advance!

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  • Haskell: list of elements with class restriction

    - by user1760586
    here's my question: this works perfectly: type Asdf = [Integer] type ListOfAsdf = [Asdf] Now I want to do the same but with the Integral class restriction: type Asdf2 a = (Integral a) => [a] type ListOfAsdf2 = (Integral a) => [Asdf2 a] I got this error: Illegal polymorphic or qualified type: Asdf2 a Perhaps you intended to use -XImpredicativeTypes In the type synonym declaration for `ListOfAsdf2' I have tried a lot of things but I am still not able to create a type with a class restriction as described above. Thanks in advance!!! =) Dak

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  • Are there any data-binding solution that works in C++ and GWT and supports structures polymorphism?

    - by user116854
    I expect it should share a common description, like XmlSchema or IDL and should generate classes for target language. I found Thrift and it's really nice solution, but it doesn't support structures polymorphism. I would like to have collections of base class objects, where I could place instances of subclasses, serialize this and deserialize at the opposite side. Some mechanism of polymorphic behavior support, like Visitor, would be a perfect. Does anybody know something suitable for these requirements?

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  • How can I view the source code for a particular `predict` function?

    - by merlin2011
    Based on the documentation, predict is a polymorphic function in R and a different function is actually called depending on what is passed as the first argument. However, the documentation does not give any information about the names of the functions that predict actually invokes for any particular class. Normally, one could type the name of a function to get its source, but this does not work with predict. If I want to view the source code for the predict function when invoked on objects of the type glmnet, what is the easiest way?

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  • Tips on Managing Podcast Subscriptions

    - by Ben Griswold
    I listen to a silly number of technical podcasts. I listen to enough of them that it is literally impossible to keep up. I nearly gave up and started dropping feeds from my subscription list when I heard Craig Shoemaker talk about his Polymorphic Podcast fast feed. The idea is he provides the same content at a higher speed so you can listen to his complete show in 3/4th the time. I tried it out with his recent jQuery Secrets with Dave Ward interview and I was shocked with the feed quality. It was a super clear, understandable conversation which only took a fraction of the time commitment. I experimented a bit and played the normal recording at 2x speed on my iPhone and the quality was once again just fine. But now I'm saving half of the time. I'm curious as to how you might manage your podcast subscriptions. Can you offer any tips or advice on how to get the best bang for your buck when it comes to technical podcast listening?

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  • Practical mysql schema advice for eCommerce store - Products & Attributes

    - by Gravy
    I am currently planning my first eCommerce application (mySQL & Laravel Framework). I have various products, which all have different attributes. Describing products very simply, Some will have a manufacturer, some will not, some will have a diameter, others will have a width, height, depth and others will have a volume. Option 1: Create a master products table, and separate tables for specific product types (polymorphic relations). That way, I will not have any unnecessary null fields in the products table. Option 2: Create a products table, with all possible fields despite the fact that there will be a lot of null rows Option 3: Normalise so that each attribute type has it's own table. Option 4: Create an attributes table, as well as an attribute_values table with the value being varchar regardless of the actual data-type. The products table would have a many:many relationship with the attributes table. Option 5: Common attributes to all or most products put in the products table, and specific attributes to a particular category of product attached to the categories table. My thoughts are that I would like to be able to allow easy product filtering by these attributes and sorting. I would also want the frontend to be fast, less concern over the performance of the inserting and updating of product records. Im a bit overwhelmed with the vast implementation options, and cannot find a suitable answer in terms of the best method of approach. Could somebody point me in the right direction? In an ideal world, I would like to offer the following kind of functionality - http://www.glassesdirect.co.uk/products/ to my eCommerce store. As can be seen, in the sidebar, you can select an attribute the glasses to filter them. e.g. male / female or plastic / metal / titanium etc... Alternatively, should I just dump the mySql relational database idea and learn mongodb?

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  • Is duck typing a subset of polymorphism

    - by Raynos
    From Polymorphism on WIkipedia In computer science, polymorphism is a programming language feature that allows values of different data types to be handled using a uniform interface. From duck typing on Wikipedia In computer programming with object-oriented programming languages, duck typing is a style of dynamic typing in which an object's current set of methods and properties determines the valid semantics, rather than its inheritance from a particular class or implementation of a specific interface. My interpretation is that based on duck typing, the objects methods/properties determine the valid semantics. Meaning that the objects current shape determines the interface it upholds. From polymorphism you can say a function is polymorphic if it accepts multiple different data types as long as they uphold an interface. So if a function can duck type, it can accept multiple different data types and operate on them as long as those data types have the correct methods/properties and thus uphold the interface. (Usage of the term interface is meant not as a code construct but more as a descriptive, documenting construct) What is the correct relationship between ducktyping and polymorphism ? If a language can duck type, does it mean it can do polymorphism ?

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  • CodeCritics.com: A no nonsense place for coders to critique code and raise awareness of standards and "good coding standards" [closed]

    - by Visionary Software Solutions
    StackOverflow has been a boon for increasing programming knowledge by allowing developers to ask for help and knowledge related to programming. Oftentimes these questions boil down to: This code is broken, fix it I don't know how to do this Is this the best approach (hard question to answer on StackExchange, but democratic) Oftentimes, however, these questions are discussed at a very high level. "I use web services with a proxy client to ..." But, as Grady Booch is fond of saying "the Truth is raw, naked, running code". Those high level descriptions can be accomplished in any ways. Programming is an Art, and there are an infinite number of different ways to do things. But some are better than others. A site devoted to Q&A can help increase knowledge...a site devoted to critique of code can help elevate standards and result in higher quality knowledge. By upvoting the most elegant ways to solve a short, concise problem statement, or just looking at a piece of code and saying "this is ugly, how can we fix it?" we can increase community participation in discussions about the substantive details of an approach: "is my commenting clear? "Is this 3 nested for-loops with a continue that breaks in a special case a good way of building an object?" "Does this extremely generic and polymorphic inheritance hierarchy have issues?") Code is an art/craft and science/engineering artifact. Doesn't it deserve the same type of review treatment as a painting and an experiment? For praising those that provide that moment of zen when looking at exceptionally good code that makes you believe in a better tomorrow, and panning those whose offal is so offensive that were you to meet them on the job you'd say "YOU! GET OUT!!!" Hence, CodeCritics. A collaborative critiquing platform in the style of StackOverflow focused solely on critiquing code that can act as a collaborative code review and assist in the discovery of Design Patterns.

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