I am using Entity Framework 6.0.2 with an existing database in which tags are stored in a single table that looks like this:
Id
: int, primary key
TagType
: string, determine the type of tag, either "usertag" or "movietag"
ItemId
: int, contains the Id of the item to which is referred (either a User Id or a Movie Id)
The following classes describe this situation:
public class User
{
public int Id { get; set; }
}
public class Movie
{
public int Id { get; set; }
}
public abstract class Tag
{
public int Id { get; set; }
public int ItemId { get; set; }
}
public class UserTag : Tag
{
public virtual User User { get; set; }
}
public class MovieTag : Tag
{
public virtual Movie Movie { get; set; }
}
As you can see my derived classes have navigation properties, which are backed by the value of the ItemId
property in the base class. My mapping is as follows:
public class Context : DbContext
{
public DbSet<Tag> Tags { get; set; }
protected override void OnModelCreating(DbModelBuilder modelBuilder)
{
modelBuilder.Entity<Tag>()
.Map<UserTag>(m => m.Requires("TagType").HasValue("usertag"))
.Map<MovieTag>(m => m.Requires("TagType").HasValue("movietag"));
modelBuilder.Entity<UserTag>()
.HasRequired(m => m.User).WithMany().HasForeignKey(m => m.ItemId);
modelBuilder.Entity<MovieTag>()
.HasRequired(m => m.Movie).WithMany().HasForeignKey(m => m.ItemId);
}
}
Now when I try to use this mapping using the following code, I get an exception:
using System.Data.Entity;
class Program
{
static void Main()
{
using (var db = new Context())
{
db.Database.Delete();
db.Database.Initialize(false);
}
}
}
The exception that is thrown is:
Unhandled Exception: System.InvalidOperationException: The foreign key component 'ItemId' is not a declared property on type 'UserTag'. Verify that it has not been explicitly excluded from the model and that it is a valid primitive property
Yes the ItemId
property is not declared on the type UserTag
, but it is inherited from the base Tag
class. To me it seems that this mapping should be possible. Is this a bug or a restriction in Entity Framework 6?