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• Installing 64-bit Ubuntu Server 12.04 LTS, on a VM with VMWare Player, on a 64-bit Windows 7 PC

  - by WannaBeAGeek

  I'm trying to create a VM, using VMWare Player, with an ISO image of Ubuntu Server 12.04 (LTS). The machine I'm doing the installation on has an Intel(R) Core(TM) i5 CPU, and runs 64-bit Windows 7 I managed to create the VM (gave username, password, configured network etc), but I can't install Ubuntu Server. First I get this alert : Binary translation is incompatible with long mode on this platform. Disabling long mode. Without long mode support, the virtual machine will not be able to run 64-bit code. For more details see http://vmware.com/info?id=152. When I click OK, I get another alert : This virtual machine is configured for 64-bit guest operating systems. However, 64-bit operation is not possible. This host supports Intel VT-x, but Intel VT-x is disabled. Intel VT-x might be disabled if it has been disabled in the BIOS/firmware settings or the host has not been power-cycled since changing this setting. (1) Verify that the BIOS/firmware settings enable Intel VT-x and disable 'trusted execution.' (2) Power-cycle the host if either of these BIOS/firmware settings have been changed. (3) Power-cycle the host if you have not done so since installing VMware Player. (4) Update the host's BIOS/firmware to the latest version. For more detailed information, see http://vmware.com/info?id=152. Then, when I click OK, my VM exists, and I get back to the VMWare Player home screen. I don't know much about hardware and virtualisation, so there might be some necessary info I'm not giving. Please don't hesitate to let me know what is missing in my post, for finding solutions. Thanks :)

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• Windows based development environment: HyperV, VMWare, or VirtualBox on development machine?

  - by bleepzter

  I am a software engineer with a little bit of an informal "support" functionality... I am trying to figure out what is the best possible approach to employing virtualization technologies into our development process. Since the code we develop is server-centric, testing it often requires a VM with specific software requirements. I used to use VM Ware player (free version) to run my VM's until both of my laptops started exhibiting issues with corrupted windows 7 services and dying hard drives. All leads pointed to VMWare, which by the way seems to be a solid product if you pay for the Workstation edition ($300). On a side note, I have always been a fan of the Windows Server product line. I think it makes for one of the best development environments out there - it is highly scalable, highly reliable, and very efficient. So to be fair I replaced the drives of the laptops and installed Windows Server 2008R2, VS2010 Ultimate SP1, SQL Server 2008R2, TFS Server 2010 and all other tools and API's needed do do my work properly. So now I am stuck with a bunch of VMWare VMs. I don't want to repeat of what happened before, and I certainly don't want to bog down my machine with an inefficient hypervisor or services that are not needed. Futhermore the VMDK hard-disk format used by VMWare is not compatible with the VHD format of Hyper V. It is my understanding that converting from one format to the other can only happen by Microsoft System Center Virtual Machine which I have downloaded from MSDN and ready to install. I guess the question at this point is: Does SCVM run as another service in Windows? Is it a memory hog? What is a better virtualization technology - Hyper-V or Virtual Box in terms of efficiency ease of use and most importantly - memory footprint? (Keep in mind the development environment already has a ton of services running such as TFS Server, SQL Server, IIS, etc...) How would you advise to proceed at this point so that the VMs are still used in the test process? Thanks Martin

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• VirtualBox with Ubuntu Server guest can't ping outside

  - by Danidan

  Here's my situation: an Ubuntu 12.04 Host running VirtualBox; two guest VMs running Ubuntu Server 12.04 home network, so my Host pc has a wireless connection to the router of my ISP. My problem is in one of the virtual machines: it has 3 NICs, one in NAT mode and the others in Host Only mode. My purpose is to use eth0 (NAT) for Internet access and eth1, eth2 (Host Only) for management of internal virtual network (eth1 uses a VBoxNet with this IP 192.168.69.254). Whenever I try to $ping 8.8.8.8 I get Destination Host Unreachable. While if I $ping 192.168.69.10, that is the IP of the other VM, it works. I can't also ping my Host nor my router My /etc/network/interfaces file is: auto lo iface lo inet loopback auto eth0 iface eth0 inet dhcp auto eth1 iface eth1 inet static address 192.168.69.1 netmask 255.255.255.0 auto eth2 iface ifconfig $IFACE 0.0.0.0 up up ip link set $IFACE promisc on down ip link set $IFASE promisc off down ifconfig $IFACE down $route -n returns: Destination Gateway Genmask Flags Metric Ref Use Iface 0.0.0.0 10.0.2.2 0.0.0.0 UG 100 0 0 eth0 10.0.2.0 0.0.0.0 255.255.255.0 U 0 0 0 eth0 192.168.69.0 0.0.0.0 255.255.255.0 U 0 0 0 eth1 192.168.100.0 0.0.0.0 255.255.255.0 U 0 0 0 virbr0 Forgetting for now what eth2 needs to do and its setup, why I can't go outside the Host box? What can I do to help you helping me? :-)

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• Emails sent from Coldfusion using the same SMTP/Exchange server works from one machine but fails for another

  - by Peter Herdenborg

  First, apologies if this question is too vague or has too little information to really be answerable. I am not normally working with these issues, and I don't have full access to the environment. However, the hosting provider seems to have a hard time tracking down the issue, so I am hoping that someone can at least provide me with some qualified guesses about the most likely problem. Here goes: A client I work for has a hosted IT environment, based on virtual machines running Windows 2008 R2 Standard. Our website, based on Coldfusion 9 was recently migrated from one virtual machine to another, and though Coldfusion is configured in the exact same way, using the same SMTP server, i.e. the client's Exchange server hosted in the same environment and in the same AD as both web servers, sending emails to external recipients is no longer working. It is still working fine when testing from the old machine. This is what I've learnt so far (all emails are sent using a valid from-address on the client's domain): Emails sent to other recipients on the same domain are delivered without any problem. Emails sent to external recipients on other domains are never delivered. When sending emails to both internal and external recipients, no emails are delivered. When receiving one of these emails to an internal address, the sender is now indicated as "[email protected]", while when sent from the old machine, it used to say just "sender". This seems to me that it could hint that the Exchange machine "recognizes" the old web server while it is a stranger to the new. In Coldfusion's mail log, all messages appear to be successfully delivered to the SMTP server. Any ideas what settings to look at, what log entries to search for or how to compare the old web server with the new one will be highly appreciated.

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• Is there a way to use VirtualBox without using it's resource registry?

  - by Catskul

  Summary VirtualBox seems to want everything to be "registered" which makes it much more annoying to work with on the command line. I'm attempting to create an automated script which will create, move, start, stop, and destroy virtual machines and virtual disks. Requiring registration will complicate the task for the following reasons. leaves state information around that can cause unpredicted edgecases causing scripts to fail. creates potential name space collisions for multiple process creating VMs with the same name moving/copying resources on the same machine is more complicated because references in the registry need to be updated copying resources (disk + vm combination) to another machine require reconfiguration once they reach their target machine, and require the transfer of extra meta data to do the reconfiguration. If something unexpectedly fails, and an unregister thus fails to happen, left over configuration information can cause problems in subsequent runs. Use Case My specific use case is for a continuous integration server which creates and destroys VMs and Disk images potentially with the same name, and would require more logic to deal with the registry's statefulness. Imaginary Example It seems that I should just be able to for example (using some imaginary and/or incorrect commands): mkdir foobar customdiskimg_script ./foo/foo.vdi vboxmanage createvm --name "foo" --ostype Linux --basefolder ./foo/foo.xml vboxmanage storagectl ./foo/foo.xml --name foo --add ide vboxmanage storageattach --storagectl foo --medium ./foo/foo.vdi ./foo/foo.xml vboxmanage startvm ./foo/foo.xml TLDR Is there a way to use virtualbox without "registering" harddisks and VMs?

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• linux: per-process monitor, every 10 minutes, with history access

  - by Inbar Rose

  I really didn't know a better way to ask my question, hence you get a horribly named question. I will explain what i want to do, maybe that will help you help me. I would like to have my linux machine continuously monitor (every 10 minutes) all the processes on my machine. The information from each process that I require is the name, CPU usage, allocated (virtual) memory, and resident (ram) memory. If these periodic reports were to be looked at, they would look something like this: PROCESS CPU RAM VIRTUAL name1 % MB MB name2 % MB MB ...etc..etc These reports should be stored in such a way that I can access them at a later date by giving a date/time scope (range). For instance, if I want to see the history of my processes from 12:00:00 1.12.12 till 12:00:00 2.12.12 I can - and it should give me the history of the processes for every 10 minutes between those date/time borders. The format of the return is not important, that will be handled by a script anyway and can be modified into anything I need. I have looked into a few things so far, but have not found something that clearly meets my needs. Among the things i searched: sar, free(1), top(1).. and a few other things. It should be a simple issue, i can already see all this information by simply looking at my htop, but i need only a tool that will gather the desired fields for me for each processes every 10 minutes, and then also let me extract slices of that data based on date/time scopes (ranges). note: I have limited experience with linux, so please give detailed information. note2: The desired output will be something like this (after receiving the desired range) CPU USAGE BY PROCESS: proc_nameA 1,2,2,2,2,2...... numbers represent % usage every 10 minutes... proc_nameB 4,3,3,6,1,2...... The same idea with the other information.

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• Ubuntu 12.04 VirtualBox on powerful W7 quite slow

  - by wnstnsmth

  I own a Thinkpad T420s with 8GB RAM, 160 GB SSD and a quite fast i7 processor. Summa summarum a very fast computer that works perfectly. Now, I am not very impressed by the performance of my Ubuntu 12.04 virtual machine running on VirtualBox 4.1.18. I assume that Virtual Machines are always a bit slower than the guest system, still I think it should be more performant given the hardware settings I give it: 4096 MB RAM 1 CPU without CPU limitation (I would like to give it more but then it does not seem to work - I am not experienced in this maybe somebody could give me advice on this too) Activated PAE/NX, VT-x/AMD-V and Nested Paging 96 MB Graphics Memory (no 2D or 3D acceleration) ~ 14 GB disk space, currently about 7 GB are used Maybe I misconfigured something, could you give me a hint please? Thanks! Edit: What I mean by slow is that for example switching tabs in the browser (whether FF or Chrome) only goes with a 0.5s delay or something, as well as switching application windows and/or double-clicking applications in the dock to get all open windows.. opening Aptana takes about a minute whereas opening something like Photoshop on the guest system takes 5 seconds

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• VM automatic provisioning advice

  - by jdgregson

  In my lab we have 24 workstations, each with five technician-maintained virtual machines set up in VMware Workstation. These provide a lot of management overhead, as we have to update them as well as the host operating systems every three months (the start of the next quarter), which adds up to 144 systems to update instead of just 24. Whenever we need to reimage the hosts, the VMs add another 130GB to each image, which is over 3TB of extra data to send over the network, and a lot more time to apply each image, and then we still have to boot all 120 VMs and assign them a unique IP Address and host names. We would like to get the VMs off the hosts and onto a server, but after looking around for a few days, I still don't know where to begin looking for a solution. There may be a better way to do this, but in my mind, the ideal solution would be to replace the VMs on the host machines with five Thin Client operating systems, each configured to connect to a server and be sent or connected to a unique virtual machine. We can't have 120 VMs running on the server all the time, so the server would have to create a copy of the VM from a template whenever a student tries to boot one, and destroy the VM after the student is finished with it. If there is another client application that has to be installed on the hosts that would be fine, the only reason I'd like to keep them in VMware Workstation is because students already know to look there for the VMs when they need to use them. What, if any, virtualization software will allow this? Is there some other solution I'm not seeing?

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• Unable to connect to Linux (Virtual OS-vmware) through Putty on Windows

  - by RBA

  Hi, I want to access my Linux box (Virtual OS) through Putty on Windows using Run command: putty -ssh -P 22 192.168.171.130,,, but it is returning an error message, not able to connect. But few days back I was able to connect it today. But not now. Why?? Windows IP Configuration Host Name . . . . . . . . . . . . : rba7791fd466 Primary Dns Suffix . . . . . . . : Node Type . . . . . . . . . . . . : Unknown IP Routing Enabled. . . . . . . . : No WINS Proxy Enabled. . . . . . . . : No Ethernet adapter VMware Network Adapter VMnet1: Connection-specific DNS Suffix . : Description . . . . . . . . . . . : VMware Virtual Ethernet Adapter for VMnet1 Physical Address. . . . . . . . . : 00-50-56-C0-00-01 Dhcp Enabled. . . . . . . . . . . : No IP Address. . . . . . . . . . . . : 192.168.234.1 Subnet Mask . . . . . . . . . . . : 255.255.255.0 Default Gateway . . . . . . . . . : Ethernet adapter Wireless Network Connection: Connection-specific DNS Suffix . : Description . . . . . . . . . . . : Dell Wireless 1395 WLAN Mini-Card Physical Address. . . . . . . . . : 00-24-2B-60-A0-88 Dhcp Enabled. . . . . . . . . . . : Yes Autoconfiguration Enabled . . . . : Yes IP Address. . . . . . . . . . . . : 10.0.0.2 Subnet Mask . . . . . . . . . . . : 255.255.255.0 Default Gateway . . . . . . . . . : 10.0.0.1 DHCP Server . . . . . . . . . . . : 10.0.0.1 DNS Servers . . . . . . . . . . . : 10.0.0.1 Lease Obtained. . . . . . . . . . : Friday, August 28, 2009 4:11:09 AM Lease Expires . . . . . . . . . . : Saturday, August 29, 2009 4:11:09 AM Ubuntu Configuration eth0 inet addr:192.168.171.130

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• How to get the path of a derived class from an inherited method?

  - by Jacco

  How to get the path of the current class, from an inherited method? I have the following: <?php // file: /parentDir/class.php class Parent { protected function getDir() { return dirname(__FILE__); } } ?> and <?php // file: /childDir/class.php class Child extends Parent { public function __construct() { echo $this->getDir(); } } $tmp = new Child(); // output: '/parentDir' ?> The __FILE__ constant always points to the source-file of the file it is in, regardless of inheritance. I would like to get the name of the path for the derived class. Is there any elegant way of doing this? I could do something along the lines of $this->getDir(__FILE__); but that would mean that I have to repeat myself quite often. I'm looking for a method that puts all the logic in the parent class, if possible. Update: Accepted solution (by Palantir): <?php // file: /parentDir/class.php class Parent { protected function getDir() { $reflector = new ReflectionClass(get_class($this)); return dirname($reflector->getFileName()); } } ?>

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• ActiveRecord table inheritence using set_table_names

  - by Jinyoung Kim

  Hi, I'm using ActiveRecord in Ruby on Rails. I have a table named documents(Document class) and I want to have another table data_documents(DataDocument) class which is effectively the same except for having different table name. In other words, I want two tables with the same behavior except for table name. class DataDocument < Document #set_table_name "data_documents" self.table_name = "data_documents" end My solution was to use class inheritance as above, yet this resulted in inconsistent SQL statement for create operation where there are both 'documents' table and 'data_documents' table. Can you figure out why and how I can make it work? >> DataDocument.create(:did=>"dd") ActiveRecord::StatementInvalid: Mysql::Error: Unknown column 'data_documents.did' in 'where clause': SELECT `documents`.id FROM `documents` WHERE (`data_documents`.`did` = BINARY 'dd') LIMIT 1 from /Users/lifidea/.gem/ruby/1.8/gems/activerecord-2.3.2/lib/active_record/connection_adapters/abstract_adapter.rb:212:in `log' from /Users/lifidea/.gem/ruby/1.8/gems/activerecord-2.3.2/lib/active_record/connection_adapters/mysql_adapter.rb:320:in `execute' from /Users/lifidea/.gem/ruby/1.8/gems/activerecord-2.3.2/lib/active_record/connection_adapters/mysql_adapter.rb:595:in `select' from /Users/lifidea/.gem/ruby/1.8/gems/activerecord-2.3.2/lib/active_record/connection_adapters/abstract/database_statements.rb:7:in `select_all_without_query_cache' from /Users/lifidea/.gem/ruby/1.8/gems/activerecord-2.3.2/lib/active_record/connection_adapters/abstract/query_cache.rb:62:in `select_all'

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• Accessing Static Methods on a Generic class in c#

  - by mrlane

  Hello, I have the following situation in code, which I suspect may be a bit dodgey: I have a class: abstract class DataAccessBase<T> : IDataAccess where T : AnotherAbstractClass This class DataAccessBase also has a static factory method which creates instances of derived classes of itself using an enum value in a which statement to decide which derived type to create: static IDataAccess CreateInstance(TypeToCreateEnum) Now, the types derived from DataAccessBase<T> are themselves NOT generic, they specify a type for T: class PoLcZoneData : DataAccessBase<PoLcZone> // PoLcZone is derived from AnotherAbstractClass So far I am not sure if this is pushing the limits of good use of generics, but what I am really concerned about is how to access the static CreateInstance() method in the first place: The way I am doing this at the moment is to simply pass any type T where T : AnotherAbstractClass. In particular I am passing AnotherAbstractClass itself. This allows compilation just fine, but it does seem to me that passing any type to a generic class just to get at the statics is a bit dodgey. I have actually simplified the situation somewhat as DataAccessBase<T> is the lower level in the inheritance chain, but the static factory methods exists in a middle tier with classes such as PoLcZoneData being the most derived on the only level that is not generic. What are peoples thoughts on this arrangement?

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• EF4 and multiple abstract levels

  - by Cedric

  I need to use inheritance with EF4 and the TPH model created from DB. I created a new projet to test simples classes. There is my class model: There is my table in SQL SERVER 2008 : VEHICLE ID : int PK Owner : varchar(50) Consumption : float FirstCirculationDate : date Type : varchar(50) Discriminator : varchar(10) I added a condition in my EDMX on the Discriminator field to differentiate the Scooter, Car, Motorbike and Bike entities. MotorizedVehicle and Vehicle are Abstract. But when I compile, this error appears : Error 3032: Problem in mapping fragments starting at lines 78, 85:EntityTypes EF4InheritanceModel.Scooter, EF4InheritanceModel.Motorbike, EF4InheritanceModel.Car, EF4InheritanceModel.Bike are being mapped to the same rows in table Vehicle. Mapping conditions can be used to distinguish the rows that these types are mapped to. Edit : To Ladislav : I try it and error change to become it for all of my entities : Error 3034: Problem in mapping fragments starting at lines 72, 86:An entity is mapped to different rows within the same table. Ensure these two mapping fragments do not map two groups of entities with overlapping keys to two distinct groups of rows. To Henk (with Ladislay suggestion) : There are all of mappings details : What's wrong ? Thanks

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• Best approach for a multi-tab ASP.NET AJAX control?

  - by NovaJoe

  Looking for some implementation advice: I have a page that has a 3-tab ajaxToolkit:TabContainer. The purpose of the page is to expose a calculator that has two basic inputs: geo-location and date. The three tabs are labeled "City and State", "Postal Code", and "GPS Coordinates". The layout of each tab container is the same for each tab, with the exception of the location section; the location section changes because each type of location has different inputs. For example, to specify city/state, there will be three fields: city, country, and state (country and state will use cascading drop-down lists). But Postal code requires only one field (which will validate via regular expression for allowed countries). See the example design mockup: So, what I WOULD LIKE to do (in order to minimize duplicate code), is to have a common control that contains the layout and structure of the calculator without specifying anything about the location section. Then, I'd like to be able to pull in each of the unique location controls based on what tab is selected. The tab structure exists at the page level, not in a control. Any advice? I was looking at templated controls (see MSDN article here), but I'm not convinced that it's the right solution. If I HAVE to create three separate controls with similar layouts and common elements, then that's what I have to do. But REALLY, I'd prefer a more elegant, inheritance-based solution. Any advice would be greatly appreciated. Thanks.

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• How do I so a select input for a STI column in a Rails model?

  - by James A. Rosen

  I have a model with single-table inheritance on the type column: class Pet < ActiveRecord::Base TYPES = [Dog, Cat, Hamster] validates_presence_of :name end I want to offer a <select> dropdown on the new and edit pages: <% form_for @model do |f| %> <%= f.label :name %> <%= f.text_input :name %> <%= f.label :type %> <%= f.select :type, Pet::TYPES.map { |t| [t.human_name, t.to_s] } %> <% end %> That gives me the following error: ActionView::TemplateError (wrong argument type String (expected Module)) I read a suggestion to use an alias for the field #type since Ruby considers that a reserved word that's the same as #class. I tried both class Pet < ActiveRecord::Base ... alias_attribute :klass, :type end and class Pet < ActiveRecord::Base ... def klass self.type end def klass=(k) self.type = k end end Neither worked. Any suggestions? Oddly, it works fine on my machine (MRI 1.8.6 on RVM), but fails on the staging server (MRI 1.8.7 not on RVM).

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• Getting the name of a child class in the parent class (static context)

  - by Benoit Myard

  Hi everybody, I'm building an ORM library with reuse and simplicity in mind; everything goes fine except that I got stuck by a stupid inheritance limitation. Please consider the code below: class BaseModel { /* * Return an instance of a Model from the database. */ static public function get (/* varargs */) { // 1. Notice we want an instance of User $class = get_class(parent); // value: bool(false) $class = get_class(self); // value: bool(false) $class = get_class(); // value: string(9) "BaseModel" $class = __CLASS__; // value: string(9) "BaseModel" // 2. Query the database with id $row = get_row_from_db_as_array(func_get_args()); // 3. Return the filled instance $obj = new $class(); $obj->data = $row; return $obj; } } class User extends BaseModel { protected $table = 'users'; protected $fields = array('id', 'name'); protected $primary_keys = array('id'); } class Section extends BaseModel { // [...] } $my_user = User::get(3); $my_user->name = 'Jean'; $other_user = User::get(24); $other_user->name = 'Paul'; $my_user->save(); $other_user->save(); $my_section = Section::get('apropos'); $my_section->delete(); Obviously, this is not the behavior I was expecting (although the actual behavior also makes sense).. So my question is if you guys know of a mean to get, in the parent class, the name of child class.

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• Encapsulate update method inside of object or have method which accepts an object to update

  - by Tom

  Hi, I actually have 2 questions related to each other: I have an object (class) called, say MyClass which holds data from my database. Currently I have a list of these objects ( List < MyClass ) that resides in a singleton in a "communal area". I feel it's easier to manage the data this way and I fail to see how passing a class around from object to object is beneficial over a singleton (I would be happy if someone can tell me why). Anyway, the data may change in the database from outside my program and so I have to update the data every so often. To update the list of the MyClass I have a method called say, Update, written in another class which accepts a list of MyClass. This updates all the instances of MyClass in the list. However would it be better instead to encapulate the Update() method inside the MyClass object, so instead I would say foreach(MyClass obj in MyClassList) { obj.update(); } What is a better implementation and why? The update method requires a XML reader. I have written an XML reader class which is basically a wrapper over the standard XML reader the language natively provides which provides application specific data collection. Should the XML reader class be in anyway in the "inheritance path" of the MyClass object - the MyClass objects inherits from the XML reader because it uses a few methods. I can't see why it should. I don't like the idea of declaring an instance of the XML Reader class inside of MyClass and an MyClass object is meant to be a simple "record" from the database and I feel giving it loads of methods, other object instances is a bit messy. Perhaps my XML reader class should be static but C#'s native XMLReader isn't static.? Any comments would be greatly appreciated Thanks Thomas

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• The new operator in C# isn't overriding base class member

  - by Dominic Zukiewicz

  I am confused as to why the new operator isn't working as I expected it to. Note: All classes below are defined in the same namespace, and in the same file. This class allows you to prefix any content written to the console with some provided text. public class ConsoleWriter { private string prefix; public ConsoleWriter(string prefix) { this.prefix = prefix; } public void Write(string text) { Console.WriteLine(String.Concat(prefix,text)); } } Here is a base class: public class BaseClass { protected static ConsoleWriter consoleWriter = new ConsoleWriter(""); public static void Write(string text) { consoleWriter.Write(text); } } Here is an implemented class: public class NewClass : BaseClass { protected new static ConsoleWriter consoleWriter = new ConsoleWriter("> "); } Now here's the code to execute this: class Program { static void Main(string[] args) { BaseClass.Write("Hello World!"); NewClass.Write("Hello World!"); Console.Read(); } } So I would expect the output to be Hello World! > Hello World! But the output is Hello World Hello World I do not understand why this is happening. Here is my thought process as to what is happening: The CLR calls the BaseClass.Write() method The CLR initialises the BaseClass.consoleWriter member. The method is called and executed with the BaseClass.consoleWriter variable Then The CLR calls the NewClass.Write() The CLR initialises the NewClass.consoleWriter object. The CLR sees that the implementation lies in BaseClass, but the method is inherited through The CLR executes the method locally (in NewClass) using the NewClass.consoleWriter variable I thought this is how the inheritance structure works? Please can someone help me understand why this is not working?

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• How to model has_many with polymorphism?

  - by Daniel Abrahamsson

  I've run into a situation that I am not quite sure how to model. Suppose I have a User class, and a user has many services. However, these services are quite different, for example a MailService and a BackupService, so single table inheritance won't do. Instead, I am thinking of using polymorphic associations together with an abstract base class: class User < ActiveRecord::Base has_many :services end class Service < ActiveRecord::Base validates_presence_of :user_id, :implementation_id, :implementation_type belongs_to :user belongs_to :implementation, :polymorphic = true delegate :common_service_method, :name, :to => :implementation end #Base class for service implementations class ServiceImplementation < ActiveRecord::Base validates_presence_of :user_id, :on => :create has_one :service, :as => :implementation has_one :user, :through => :service after_create :create_service_record #Tell Rails this class does not use a table. def self.abstract_class? true end #Default name implementation. def name self.class.name end protected #Sets up a service object def create_service_record service = Service.new(:user_id => user_id) service.implementation = self service.save! end end class MailService < ServiceImplementation #validations, etc... def common_service_method puts "MailService implementation of common service method" end end #Example usage MailService.create(..., :user_id => user.id) BackupService.create(...., :user_id => user.id) user.services.each do |s| puts "#{user.name} is using #{s.name}" end #Daniel is using MailService, Daniel is using BackupService So, is this the best solution? Or even a good one? How have you solved this kind of problem?

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• What is happening in Crockford's object creation technique?

  - by Chris Noe

  There are only 3 lines of code, and yet I'm having trouble fully grasping this: Object.create = function (o) { function F() {} F.prototype = o; return new F(); }; newObject = Object.create(oldObject); (from Prototypal Inheritance) 1) Object.create() starts out by creating an empty function called F. I'm thinking that a function is a kind of object. Where is this F object being stored? Globally I guess. 2) Next our oldObject, passed in as o, becomes the prototype of function F. Function (i.e., object) F now "inherits" from our oldObject, in the sense that name resolution will route through it. Good, but I'm curious what the default prototype is for an object, Object? Is that also true for a function-object? 3) Finally, F is instantiated and returned, becoming our newObject. Is the "new" operation strictly necessary here? Doesn't F already provide what we need, or is there a critical difference between function-objects and non-function-objects? Clearly it won't be possible to have a constructor function using this technique. What happens the next time Object.create() is called? Is global function F overwritten? Surely it is not reused, because that would alter previously configured objects. And what happens if multiple threads call Object.create(), is there any sort of synchronization to prevent race conditions on F?

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• using STI and ActiveRecordBase<> with full FindAll

  - by oillio

  Is it possible to use generic support with single table inheritance, and still be able to FindAll of the base class? As a bonus question, will I be able to use ActiveRecordLinqBase< as well? I do love those queries. More detail: Say I have the following classes defined: public interface ICompany { int ID { get; set; } string Name { get; set; } } [ActiveRecord("companies", DiscriminatorColumn="type", DiscriminatorType="String", DiscriminatorValue="NA")] public abstract class Company<T> : ActiveRecordBase<T>, ICompany { [PrimaryKey] private int Id { get; set; } [Property] public String Name { get; set; } } [ActiveRecord(DiscriminatorValue="firm")] public class Firm : Company<Firm> { [Property] public string Description { get; set; } } [ActiveRecord(DiscriminatorValue="client")] public class Client : Company<Client> { [Property] public int ChargeRate { get; set; } } This works fine for most cases. I can do things like: var x = Client.FindAll(); But sometimes I want all of the companies. If I was not using generics I could do: var x = (Company[]) FindAll(Company); Client a = (Client)x[0]; Firm b = (Firm)x[1]; Is there a way to write a FindAll that returns an array of ICompany's that can then be typecast into their respective types? Something like: var x = (ICompany[]) FindAll(Company<ICompany>); Client a = (Client)x[0]; Or maybe I am going about implementing the generic support all wrong?

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• base destructor called twice after derived object?

  - by sil3nt

  hey there, why is the base destructor called twice at the end of this program? #include <iostream> using namespace std; class B{ public: B(){ cout << "BC" << endl; x = 0; } virtual ~B(){ cout << "BD" << endl; } void f(){ cout << "BF" << endl; } virtual void g(){ cout << "BG" << endl; } private: int x; }; class D: public B{ public: D(){ cout << "dc" << endl; y = 0; } virtual ~D(){ cout << "dd" << endl; } void f(){ cout << "df" << endl; } virtual void g(){ cout << "dg" << endl; } private: int y; }; int main(){ B b, * bp = &b; D d, * dp = &d; bp->f(); bp->g(); bp = dp; bp->f(); bp->g(); }

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• Specifying different initial values for fields in inherited models (django)

  - by Shawn Chin

  Question : What is the recommended way to specify an initial value for fields if one uses model inheritance and each child model needs to have different default values when rendering a ModelForm? Take for example the following models where CompileCommand and TestCommand both need different initial values when rendered as ModelForm. # ------ models.py class ShellCommand(models.Model): command = models.Charfield(_("command"), max_length=100) arguments = models.Charfield(_("arguments"), max_length=100) class CompileCommand(ShellCommand): # ... default command should be "make" class TestCommand(ShellCommand): # ... default: command = "make", arguments = "test" I am aware that one can used the initial={...} argument when instantiating the form, however I would rather store the initial values within the context of the model (or at least within the associated ModelForm). My current approach What I'm doing at the moment is storing an initial value dict within Meta, and checking for it in my views. # ----- forms.py class CompileCommandForm(forms.ModelForm): class Meta: model = CompileCommand initial_values = {"command":"make"} class TestCommandForm(forms.ModelForm): class Meta: model = TestCommand initial_values = {"command":"make", "arguments":"test"} # ------ in views FORM_LOOKUP = { "compile": CompileCommandFomr, "test": TestCommandForm } CmdForm = FORM_LOOKUP.get(command_type, None) # ... initial = getattr(CmdForm, "initial_values", {}) form = CmdForm(initial=initial) This feels too much like a hack. I am eager for a more generic / better way to achieve this. Suggestions appreciated. Other attempts I have toyed around with overriding the constructor for the submodels: class CompileCommand(ShellCommand): def __init__(self, *args, **kwargs): kwargs.setdefault('command', "make") super(CompileCommand, self).__init__(*args, **kwargs) and this works when I try to create an object from the shell: >>> c = CompileCommand(name="xyz") >>> c.save() <CompileCommand: 123> >>> c.command 'make' However, this does not set the default value when the associated ModelForm is rendered, which unfortunately is what I'm trying to achieve. Update 2 (looks promising) I now have the following in forms.py which allow me to set Meta.default_initial_values without needing extra code in views. class ModelFormWithDefaults(forms.ModelForm): def __init__(self, *args, **kwargs): if hasattr(self.Meta, "default_initial_values"): kwargs.setdefault("initial", self.Meta.default_initial_values) super(ModelFormWithDefaults, self).__init__(*args, **kwargs) class TestCommandForm(ModelFormWithDefaults): class Meta: model = TestCommand default_initial_values = {"command":"make", "arguments":"test"}

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• Javascript Getting Objects to Fallback to One Another

  - by Ian

  Here's a ugly bit of Javascript it would be nice to find a workaround. Javascript has no classes, and that is a good thing. But it implements fallback between objects in a rather ugly way. The foundational construct should be to have one object that, when a property fails to be found, it falls back to another object. So if we want a to fall back to b we would want to do something like: a = {sun:1}; b = {dock:2}; a.__fallback__ = b; then a.dock == 2; But, Javascript instead provides a new operator and prototypes. So we do the far less elegant: function A(sun) { this.sun = sun; }; A.prototype.dock = 2; a = new A(1); a.dock == 2; But aside from elegance, this is also strictly less powerful, because it means that anything created with A gets the same fallback object. What I would like to do is liberate Javascript from this artificial limitation and have the ability to give any individual object any other individual object as its fallback. That way I could keep the current behavior when it makes sense, but use object-level inheritance when that makes sense. My initial approach is to create a dummy constructor function: function setFallback(from_obj, to_obj) { from_obj.constructor = function () {}; from_obj.constructor.prototype = to_obj; } a = {sun:1}; b = {dock:2}; setFallback(a, b); But unfortunately: a.dock == undefined; Any ideas why this doesn't work, or any solutions for an implementation of setFallback? (I'm running on V8, via node.js, in case this is platform dependent)

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• OOP design issue: Polymorphism

  - by Graham Phillips

  I'm trying to solve a design issue using inheritance based polymorphism and dynamic binding. I have an abstract superclass and two subclasses. The superclass contains common behaviour. SubClassA and SubClassB define some different methods: SubClassA defines a method performTransform(), but SubClassB does not. So the following example 1 var v:SuperClass; 2 var b:SubClassB = new SubClassB(); 3 v = b; 4 v.performTransform(); would cause a compile error on line 4 as performTransform() is not defined in the superclass. We can get it to compile by casting... (v as SubClassA).performTransform(); however, this will cause a runtime exception to be thrown as v is actually an instance of SubClassB, which also does not define performTransform() So we can get around that by testing the type of an object before casting it: if( typeof v == SubClassA) { (cast v to SubClassA).performTransform(); } That will ensure that we only call performTransform() on v's that are instances of SubClassA. That's a pretty inelegant solution to my eyes, but at least its safe. I have used interface based polymorphism (interface meaning a type that can't be instantiated and defines the API of classes that implement it) in the past, but that also feels clunky. For the above case, if SubClassA and SubClassB implemented ISuperClass that defined performTransform, then they would both have to implement performTransform(). If SubClassB had no real need for a performTransform() you would have to implement an empty function. There must be a design pattern out there that addresses the issue.

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