mapping - Entity Framework Code First Many to Many Setup For Existing Tables

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");
      });