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

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.