How to extend DbContext with partial class and partial OnModelCreating method in EntityFramework Core

Question

I'm using EF Core and DatabaseFirst approach. My dbContext is created automatically by Scaffold-DbContext command.

I need to add some new DbSets into a dbContext and add into OnModelCreating method some additional code but after each scaffolding that added code are erased and I have to add it each time again.

What I want to do is to create another partial dbContext class and mark protected override void OnModelCreating(ModelBuilder modelBuilder) method as partial

but get errors:

A partial method cannot have access modifiers or the virtual, abstract, override, new, sealed, or extern modifiers.

A partial method may not have multiple implementing declarations

Here is a pseudo code:

MyDbContext1.cs - generated by Scaffold-DbContext

public partial class MyDbContext : DbContext
{
    public MyDbContext()
    {
    }

    public MyDbContext(DbContextOptions<MyDbContext> options)
        : base(options)
    {
    }

    public virtual DbSet<Client> Clients { get; set; }

    protected override partial void OnModelCreating(ModelBuilder modelBuilder)
    {
        modelBuilder.Entity<Client>(entity =>
        {
            // some code ...
        }
    }
}

MyDbContext2.cs - this code I added each time into dbContext after scaffolding:

public partial class MyDbContext
{
    public virtual DbSet<JustAnotherEntity> AnotherEntity { get; set; }

    protected override partial void OnModelCreating(ModelBuilder modelBuilder)
    {
        modelBuilder.Entity<JustAnotherEntity>(entity =>
        {
            entity.HasKey(e => new {e.Id, e.IdAction, e.IdState})
                .ForSqlServerIsClustered(false);
        });
    }
}

Danelaw

kind of duplicated by this post. It is not possible to split a methods logic via partial. Wich method should be executed first, or how should the compiler know, how to merge partial methods? From microsoft docs: A partial method declaration consists of two parts: the definition, and the implementation. 1/2 – Reprography

You will have to remove partial from your method declaration and put your logic in one of your classes. 2/2 – Reprography

EF Core Power Tools does this for you – Volnay

Hey Dmitry, please consider making Simon Weaver's answer the accepted one. Four years later, things have changed a bit. This will help others find the best answer. – Body

hi @BrianMacKay, if I asked my question today I definetelly would have marked his answer as right one. But I asked four years ago and at that time there was not OnModelCreatingPartial solution, so the accepted answer helped me a lot. I believe it deserves to be the accepted answer inspite of Simon's answer is best workaround for nowadays. – Danelaw

@DmitryStepanov Thanks for the thoughtful response! StackOverflow is intended to help people find the current correct answer to problems, but your answer caused me to do some research and I discovered there is some debate about this topic, or at least there was: meta.stackexchange.com/questions/62252/… ... So do with that as you will. The community will still be able to find the current best via voting. – Body

Answer 1

EFCore 3 - They FINALLY fixed this!

You can now implement OnModelCreatingPartial in a partial class like this. Note the partial keyword on both the class and method:

public partial class RRStoreContext : DbContext
{
    partial void OnModelCreatingPartial(ModelBuilder builder)
    {
        builder.Entity<RepeatOrderSummaryView>().HasNoKey();
    }
}

If you look at the generated context file - right at the very end of OnModelCreating(...) you'll see...

 OnModelCreatingPartial(modelBuilder);

Note: I use scaffolding, but I needed to manually add HasNoKey for a stored procedure (with a custom return type that wasn't otherwise scaffolded).

Frippery

Any idea where I can find any official documentation on this OnModelCreatingPartial method or how we should be using it? I mean I can hazard a guess, but I can't see to find it documented anywhere. – Kendall

@SimonGymer This question is the top Google match and I can't find it either! Here's a blog post though irina.codes/extending-ef-core-dbcontext – Frippery

@Frippery how is this official. this is a custom implementation looks like – Foresaid

@Foresaid I'm not quite sure what you mean. The 'official' part is that the generated code (by the official scaffolder) now calls OnModelCreatingPartial(...) if you've defined it. This is much easier than it was before. The code shown above is all you need to do to add to the generated code. You've always been able to modify the scaffolded code, but this allows you to regenerate the scaffolded code without interfering with changes you may have made. – Frippery

This should be the accepted answer IMO. – Body

@Brian I usually reserve bold text and !!! for when I think my answer deserves exactly that ;-) – Frippery

Answer 2

An alternative would be creating another context class that inherit from MyDbContext that actually include all the custom code. and then use this new class as your context. This way, there is no need to update the generated code.

public class MyDbContext2 : MyDbContext 
{
    public MyDbContext2()
    {
    }

    public MyDbContext2(DbContextOptions<MyDbContext> options)
        : base(options)
    {
    }

    public virtual DbSet<JustAnotherEntity> AnotherEntity { get; set; }

    protected override void OnModelCreating(ModelBuilder modelBuilder)
    {
        base.OnModelCreating(modelBuilder);
        modelBuilder.Entity<JustAnotherEntity>(entity =>
        {
            entity.HasKey(e => new {e.Id, e.IdAction, e.IdState})
                .ForSqlServerIsClustered(false);
        });
    }
}

Landrum

Considerably better than the accepted answer. The base class scaffolds without any manual changes. – Bloodcurdling

I've marked this solution as a right answer because it's really no need to change anything in MyDbContext after scaffolding. – Danelaw

how this can be done in MVC4 and Entity Framework 6. As below code this only work with Entity core.. public MyDbContext2(DbContextOptions<MyDbContext> options) : base(options) { } – Homiletics

Just did this in an EF Core 3 project to extend auto generated classes. Thank you. Best answer. Perfect answer. ;) – Prone

I had to remove the empty constructor when i registered it in my DI container – Adman

Answer 3

You can't override methods in a partial class because all of the "parts" become a single class. But you can accomplish this by having the main OnModelCreating call a partial method. Like this:

public partial class Db : DbContext
{
    partial void OnModelCreating2(ModelBuilder modelBuilder)
    {
       //additional config
    }
}

public partial class Db : DbContext
{

    public DbSet<Person> Persons { get; set; }

    partial void OnModelCreating2(ModelBuilder modelBuilder);
    protected override void OnModelCreating(ModelBuilder modelBuilder)
    {
        base.OnModelCreating(modelBuilder);

        OnModelCreating2(modelBuilder);
    }
    protected override void OnConfiguring(DbContextOptionsBuilder optionsBuilder)
    {
        optionsBuilder.UseSqlServer("Server=localhost;database=efcore2test;integrated security=true");
        base.OnConfiguring(optionsBuilder);
    }
}

Brezin

That only kicks the can down the road: the contexts isn't generated with the OnModelCreating2(modelBuilder); call, so they still have to keep modifying the generated code. It would have been an option if the scaffolding code was extensible, for example by being based on an editable tt template. – Debenture

good workaround but still need to add these lines OnModelCreating2(ModelBuilder modelBuilder); OnModelCreating2(modelBuilder); – Morentz

@GertArnold something like this maybe https://mcmap.net/q/233773/-azure-functions-database-connection-string – Foresaid

Answer 4

This is the complete solution :

You have a base DbContext Like this :

public partial class OrderContext : DbContext
{

public OrderContext(DbContextOptions<OrderContext> options)  : base(options)
{
}

public virtual DbSet<Order> Orders { get; set; }

protected override void OnModelCreating(ModelBuilder modelBuilder)
{
    //some code

    OnModelCreatingPartial(modelBuilder);
}

partial void OnModelCreatingPartial(ModelBuilder modelBuilder);
}

Then you have a second partial class like this :

public partial class OrderContext : DbContext
{
      partial void OnModelCreatingPartial(ModelBuilder modelBuilder)
      {
         // some other code
      }
  }

Terrenceterrene