Entity Framework Code First Many to Many Setup For Existing Tables - entity-framework

Question

I have the following tables Essence, EssenseSet, and Essense2EssenceSet
Essense2EssenceSet is the linking table that creates the M:M relationship.
I've been unable to get the M:M relationship working though in EF code first though.
Here's my code:
[Table("Essence", Schema = "Com")]
public class Essence
{
public int EssenceID { get; set; }
public string Name { get; set; }
public int EssenceTypeID { get; set; }
public string DescLong { get; set; }
public string DescShort { get; set; }
public virtual ICollection<EssenceSet> EssenceSets { get; set; }
public virtual EssenceType EssenceType { get; set; }
}
[Table("EssenceSet", Schema = "Com")]
public class EssenceSet
{
public int EssenceSetID { get; set; }
public int EssenceMakerID { get; set; }
public string Name { get; set; }
public string DescLong { get; set; }
public string DescShort { get; set; }
public virtual ICollection<Essence> Essences { get; set; }
}
[Table("Essence2EssenceSet", Schema = "Com")]
public class Essence2EssenceSet
{
//(PK / FK)
[Key] [Column(Order = 0)] [ForeignKey("Essence")] public int EssenceID { get; set; }
[Key] [Column(Order = 1)] [ForeignKey("EssenceSet")] public int EssenceSetID { get; set; }
//Navigation
public virtual Essence Essence { get; set; }
public virtual EssenceSet EssenceSet { get; set; }
}
public class EssenceContext : DbContext
{
public DbSet<Essence> Essences { get; set; }
public DbSet<EssenceSet> EssenceSets { get; set; }
public DbSet<Essence2EssenceSet> Essence2EssenceSets { get; set; }
protected override void OnModelCreating(DbModelBuilder mb)
{
mb.Entity<Essence>()
.HasMany(e => e.EssenceSets)
.WithMany(set => set.Essences)
.Map(mc =>
{
mc.ToTable("Essence2EssenceSet");
mc.MapLeftKey("EssenceID");
mc.MapRightKey("EssenceSetID");
});
}
}
This is the code I'm trying to run:
Essence e = new Essence();
e.EssenceTypeID = (int)(double)dr[1];
e.Name = dr[2].ToString();
e.DescLong = dr[3].ToString();
//Get Essence Set
int setID = (int)(double)dr[0];
var set = ctx.EssenceSets.Find(setID);
e.EssenceSets = new HashSet<EssenceSet>();
e.EssenceSets.Add(set);
ctx.Essences.Add(e);
ctx.SaveChanges();
And here's the error:
An error occurred while saving entities that do not expose foreign key properties for their relationships. The EntityEntries property will return null because a single entity cannot be identified as the source of the exception.
I'm not able to find the problem. I'd greatly appreciate help setting this up right.
Thanks!

Answer 1

Remove your Essence2EssenceSet model class. If junction table contains only keys of related entities participating in many-to-many relations it is not needed to map it as entity. Also make sure that your fluent mapping of many-to-many relations specifies schema for table:
mb.Entity<Essence>()
.HasMany(e => e.EssenceSets)
.WithMany(set => set.Essences)
.Map(mc =>
{
mc.ToTable("Essence2EssenceSet", "Com");
mc.MapLeftKey("EssenceID");
mc.MapRightKey("EssenceSetID");
});