Entity Framework's DbModel: How to map a one to many relationship using a connection table?

Question

I'm trying to map via DbModel this relationship present on the database.

CREATE TABLE core.Institutes 
(
    ID INT NOT NULL PRIMARY KEY IDENTITY(1,1),
    Name NVARCHAR(128) NOT NULL,
    OldID INT NULL
)
GO

CREATE TABLE core.InstitutePlaces
(
    FKInstituteID INT NOT NULL PRIMARY KEY REFERENCES core.Institutes(ID),
    FKPlaceID INT NOT NULL REFERENCES core.Places(ID)
)
GO

CREATE TABLE core.Places
(
    ID INT NOT NULL PRIMARY KEY IDENTITY(1,1),
    Name NVARCHAR(128) NOT NULL,
    FKParentID INT NULL REFERENCES core.Places(ID),
    OldID INT NULL
)
GO

on this model

public class Place
{
    public int Id { get; set; }
    public string Name { get; set; }
    public int? ParentId { get; set; }
    public Place Parent { get; set; }
}

public class Institute
{
    public int Id { get; set; }
    public string Name { get; set; }
    public Place Place { get; set; }
}

we're using something like this to do the mapping

modelBuilder.Entity<Institutes.Institute>().HasOptional(i => i.Place);

but it doesn't work :(

This scenario is perfectly managed by the EDML file, so the problem is only about the mapping.

Answer 1

Something like this will give you (almost) the desired schema with one caveat: Code First does not create a 1:1 relationship in entity splitting scenarios which your desired schema (creating a 1:* association using a join table) is a special case of it.

public class Place
{
    public int Id { get; set; }
    public string Name { get; set; }
    public int? ParentId { get; set; }
    public Place Parent { get; set; }
}

public class Institute
{
    [DatabaseGenerated(DatabaseGenerationOption.None)]
    public int Id { get; set; }
    public string Name { get; set; }

    public int? PlaceId { get; set; }
    public Place Place { get; set; }
}

public class Context : DbContext
{
    public DbSet<Place> Places { get; set; }
    public DbSet<Institute> Institutes { get; set; }

    protected override void OnModelCreating(ModelBuilder modelBuilder)
    {
        modelBuilder.Entity<Institute>().Map(mc =>
        {
            mc.Properties(p => new { p.Id, p.Name });
            mc.ToTable("Institutes");
        })
        .Map(mc =>
        {
            mc.Properties(p => new { p.Id, p.PlaceId });
            mc.ToTable("InstitutePlaces");
        });

        modelBuilder.Entity<Place>()
                    .HasOptional(p => p.Parent)
                    .WithMany()
                    .HasForeignKey(p => p.ParentId);
    }
}

I had to switch off identity generation due to a bug that I explained here.