Entity Framework Core 使用 ToTable 创建映射表

Question

I'm trying to create a mapping table using EF core and I had thought it was possible to create the mapping table without creating a specific entity.

Currently I have EntityA and EntityB; EntityA needs to be changed to have a collection of EntityB as below:

EntityA
{
    int Id
    string Name
    ICollection<EntityB>
}

EntityB
{
    int Id
    string Name
}

Each entity is configured in its own class that implements IEntityTypeConfiguration

I want the resuling table to look like the following:

Table: EntityA_EntityB_Mapping
int Id
int EntityAId
int EntityBId

Is it possible to use EntityTypeBuilder.ToTable on the fly without creating an separate entity for EntityAEntityB ?

What's the proper way to achieve this?

TIA

Answer 1

I believe what you are referring to is you want:

public class EntityA
{
    public virtual ICollection<EntityB> Bs { get; set; }
}

public class EntityB
{
    public virtual ICollection<EntityA> As { get; set; }
}

rather than:

public class EntityA
{
    public virtual ICollection<EntityAB> ABs { get; set; }
}

public class EntityB
{
    public virtual ICollection<EntityAB> ABs { get; set; }
}

public class EntityAB
{
    public virtual EntityA A { get; set; }
    public virtual EntityB B { get; set; }
}

Which for a while was the only way to implement many-to-many relationships in EF Core. With EF Core 6 (and possibly 5?) you can implement the relationship directly either with a non-referenced joining entity (EntityAB but no dbSet and the collections still refer to EntityA or EntityB rather than the EntityAB) or with configuration just for the joining table.

The detail for the options can be found in the EF Core documentation: https://learn.microsoft.com/en-us/ef/core/modeling/relationships?tabs=fluent-api%2Cfluent-api-simple-key%2Csimple-key under the many-to-many relationship type config, but without creating a joining entity you can use:

modelBuilder
    .Entity<EntityA>()
    .HasMany(x => x.Bs)
    .WithMany(x => x.As)
    .UsingEntity(a => a.ToTable("ABs"));

This needs to be expanded out if you want to configure a specific table structure for things like the PK/FK names. (Done within the UsingEntity ) If you define a joining entity you would set up the configuration for that EntityAB entity minus relationships then use .UsingEntity<EntityAB>() when configuring the A & B's.