In Entity Framework 5 model first, there seem to be some breaking changes due to the way the class files are generated (No more code generation, but T4 templates)
2 examples:
The generated context file doesn't implement IDisposable anymore
There isn't a constructor which takes a connectionstring anymore
Are there more breaking changes? And what is the solution to them?
The default code generated from a model in Entity Framework 5 now inherits DbContext instead of ObjectContext.
This still implements IDisposable, but if you're getting an error from a line of code similar to this:
using (var mymodel = new MyModelContext()) { ... }
...complaining about not implementing IDisposable, then your problem is most likely that your model is defined in a separate assembly that references EF5 and you have not added an EF5 reference to your project.
As Ladislav Mrnka has already mentioned in his answer, if you want to pass a connection string to the constructor you have to create your own constructor manually to do this.
If you want to switch Entity Framework back to the older style of generated code, which will automatically generate the constructor you're looking for, then follow these steps:
Click on the designer surface of your EDMX file, and look at the properties window. Find a property called "Code Generation Strategy" and set this to "Default" instead of "None". This will tell Visual Studio to start creating the code for your object model in MyModel.Designer.cs in one big file, this time using ObjectContext instead of DbContext.
Delete the following sub files from below your EDMX file: MyModel.Context.tt, MyModel.tt. These are the auto generated files that you don't want anymore. If you don't delete them you'll get class naming conflicts because your objects will be created twice.
The generated context file doesn't implement IDisposable anymore
IDisposable is still implemented by the parent context type. The generated type is still disposable.
There isn't a constructor which takes a connectionstring anymore
It now uses convention to get connection string but you can add your own constructor either to template or to your partial class part of the context.
Are there more breaking changes? And what is the solution to them?
It is whole breaking change because it uses different API - DbContext API instead of ObjectContext API which means different types, different methods, POCO entities etc. If you want to get back to original code generation you have to delete those T4 templates and enable code generation as described in .Designer.cs file but the current recommended way is to use POCOs and DbContext API.
I was having the same issue with the using statement needing a type that extended IDisposable... Turns out that I forgot to reference System.Data.Entity in my project... Added the reference and it fixed the problem.
Just clean and build the project, don't forget to add the reference to your entity.
Related
I am trying to use Entity Framework data migrations, as described in this post.
However, when I try to execute the Enable-Migrations step, I receive the following error in Package Manager Console:
The target context 'MyDataContext' is not constructible. Add a default constructor or provide an implementation of IDbContextFactory
So, I created a factory class that implements IDbContextFactory in the project that contains my DbContext class, but data migrations doesn't appear to recognize it.
Is there something that I should explicitly do to instruct data migrations to use this factory class?
I also hit this problem as i wrote my context to take a connection string name (and then used ninject to provide it).
The process you've gone through seems correct, here is a snippet of my class implementation if it's of any help:
public class MigrationsContextFactory : IDbContextFactory<MyContext>
{
public MyContext Create()
{
return new MyDBContext("connectionStringName");
}
}
That should be all you need.
Like #Soren pointed out, instead of using IDbContextFactory, not supported on some earlier EF Core releases (i.e. EF Core 2.1), we can implement IDesignTimeDbContextFactory<TContext>, which supports the missing ConnectionString parameter.
For a settings.json based aproach, which you can use with either of the referred interfaces, check #Arayn's sample which allows us to define "ConnectionStrings:DefaultConnection" value path
Update 1
According to #PaulWaldman's comment, on EF Core 5 support for IDbContextFactory was reintroduced. For further details, check his comment below.
few days ago i read tutorial about GenericRepository and Unit Of Work patterns http://www.asp.net/mvc/tutorials/getting-started-with-ef-using-mvc/implementing-the-repository-and-unit-of-work-patterns-in-an-asp-net-mvc-application. I use web forms and i have EntityFramework CTP4 package installed. (I can't using EF 5).
I want to code generic repository for my project but i was stuck at this line:
this.dbSet = context.Set<TEntity>();
I know that this line doesn't work because a use ObjectContext in my project and database first approach. How can i deal with it? Can I code generic repository without migration to code first (which is not an option in my case) ?
This is the equivalent for ObjectContext:
this.dbSet = context.CreateObjectSet<TEntity>();
Now this creates an ObjectSet<TEntity> rather than a DbSet<TEntity>, but for your pattern you can use it in the same way.
UPDATE
The ObjectSet class does not have a utility method like that matches the Find() method of the DbSet. In order to "Get by key" you would need to construct an EntityKey and use the ObjectContext.GetEntityByKey(), unfortunately that's not a really simple thing to do.
There really isn't a simple way to tackle this, that I've found. In my case what I've done is to base all of my entities from a common class (using custom T4 templates to generate the classes from the model). And then I can add a generic constraint to my repositories, like:
public class MyRepository<TEntity> where TEntity : MyEntityBaseClass
And then my common base class has an Id field which is inherited by all the entities so I can can simply do:
return myObjectSet.SingleOrDefault(x => x.Id == myId);
I'm sure there are other approaches, that might be a good topic for another question.
1. You want to add the DbContextGenerator template to your visual studio templates:
2. After this make sure you clear out the default generation tool on your .edmx file.
3. Now you can implement the GenericRepository pattern as you wish.
For various reasons I would like to not store the connection string for my Entity Framework DB model in one of the various .config files. (I am using the latest and greatest DBContext API with the new Entity Framework version 4.1, .NET 4 and Visual Studio 2010 C#.) However, the code generation template for DBContext only creates a single parameterless constructor. (If I don't use the DBContext API, then my entity framework model has 7 different constructors to chose from, including the one I want.)
The only way I could figure out how to do this was to directly modify the code-generation template (context.tt file) to give me the constructor I want (example code below). This works, but it feels like I'm doing it "the hard way". What is the correct way to get a DBContext constructor that accepts a connection string?
public <#=Code.Escape(container)#>(string connectionString)
: base(connectionString)
{
<#
WriteLazyLoadingEnabled(container);
#>
}
One final note in case it might help somebody else. Although this method works, it took me a while to realize that the "connection string" is not strictly the DB connection string, but rather the special entity framework connection string which contains the DB connection string (just like what would be stored in the app.config file).
Your approach looks like the most correct way to do this. It's what the t4 templates were created for, and you're basically doing the same thing that the model-first templates do by default.
Another possibility would be to make the db context class be partial (if it isn't by default) and create another partial class file alongside it to add the constructor you want. But it seems likely that you'll want all of your t4-generated contexts to follow this pattern, so I think it's best to leverage the code generation to do this automatically the way you do in the question.
I have a subclass in a different assembly to its base class. The parent is a POCO class used for EF Code First.
When I try to add an instance of inherited class to the database I get InvalidOperationException: "Object mapping could not be found for Type with identity 'Foo.Bar.MyInheritedClass'".
It works fine if subclass is in same assembly as base class.
In regular EF the solution seems to be a call to ObjectContext.MetadataWorkspace.LoadFromAssembly(assembly). But I can't figure out how this relates to Code First.
Any advice?
I'm using Entity Framework 4.1 RC.
I solved this by inheriting from the first assembliy's DbContext, adding a DbSet<> for the derived class, and then adding new instances of derived type to to that.
Posted code on MSDN forum here.
I know this post is a bit old, but I was able to accomplish this using #Dave's recomendation inside the constructor:
public Context() {
((IObjectContextAdapter)this).ObjectContext.MetadataWorkspace.LoadFromAssembly(
System.Reflection.Assembly.GetAssembly(
typeof(--[Inherited DbContext]--)));
}
I'm quite new to EF (Entity Framework 4) and I got the same exception when I made changes in the model.
My problem turned out to be that I did not know EF need all the names on all the navigation properties to agree, not only their type. For example if there is a navigation property named foo, then there needs to be a declared variable in the corresponding class with the very same name.
One strange thing i've got to see in Entity Framework 4.0 V2 Auto Generated Classes(tt) is that the classes are not marked as Serializable. Although they are having DataContract attribute for WCF.
Now the problem is, when I store the POCO object into viewstate it throws me an exception saying that the class is not serializable.
If I generate the classes without the t4 templates or using the defualt class generating scheme, what i see is that the generated classes are having the Serializable attribute on them.
But unfortunately i can not make use of the default Entity Generation Scheme. Since I want to accomodate some custom logic to the autogenerated class which is possible through t4 templates only.
Now what i want to know is:
1) Why the Serializable Attribute is not there in the autogenerated class or am I making any mistake or i am towards a wrong approach.
2) Is it a good idea to customize the EF 4.0 T4 template to accomodate Serializable attribute.
Looking for your valuable suggestion.
Thanks,
Burhan Ghee
Yes you can modify the template. Look at Adding [DataMember] [DataContract] attributes in Entity Framework POCO Template for exactly what you need to do to fix your template. Look specifically at the WriteHeader function in the template.
The purpose of T4 template is allow you to customize. Customize it fearlessly! You are not only encouraged to customize template, but also the edmx file that your template is based on.
see this http://blogs.msdn.com/adonet/archive/2010/03/05/updated-data-model-designer-extension-starter-kit.aspx