The navigation on entity type has not been added to the model, or ignored, or entityType ignored

Question

The navigation 'Tags' on entity type 'Notepad.Models.Note' has not been added to the model, or ignored, or entityType ignored.

public class Note
    {
        public Note()
        {
            CreationDate = DateTime.Now;
            Tags = new HashSet<Tag>();
            Parts = new HashSet<Part>();
        }

        public int ID { get; set; }
        public virtual ICollection<Tag> Tags { get; set; }
        public virtual ICollection<Part> Parts { get; set; }
        public DateTime? CreationDate { get; set; }
    }


public class Tag
    {
        public Tag()
        {
            Notes = new HashSet<Note>();
        }

        public int ID { get; set; }
        public string Name { get; set; }

        public virtual ICollection<Note> Notes { get; set; }
    }

It happens while adding a migration:

dnx ef migrations add DbData -c DataDbContext

Why do you think it happens?

EDIT: DataDbContext:

public class DataDbContext : DbContext
    {
        public DbSet<Note> Notes { get; set; }
        public DbSet<Tag> Tags { get; set; }
        public DbSet<Part> Parts { get; set; }
    }

Answer 1

You have Many-to-many relationship there. As the documentation says: http://docs.efproject.net/en/latest/modeling/relationships.html#id21

Many-to-many relationships without an entity class to represent the join table are not yet supported. However, you can represent a many-to-many relationship by including an entity class for the join table and mapping two separate one-to-many relationships.

So you must create additional "join" class like this:

public class NoteTag
    {
        public int NoteId { get; set; }
        public Note Note { get; set; }

        public int TagId { get; set; }
        public Tag Tag { get; set; }
    }

then, replace

 ICollection<Tag> Tags {set;get}

in your Note class to

 ICollection<NoteTag> NoteTags {set;get}

and also in Tag class:

ICollection<Note> Notes {set;get;}

to

ICollection<NoteTags> NoteTags {set;get}

and then override OnModelCreating method in DbContext:

protected override void OnModelCreating(ModelBuilder modelBuilder)
        {
            modelBuilder.Entity<NoteTag>()
                .HasKey(t => new { t.NoteId, t.TagId });

            modelBuilder.Entity<NoteTag>()
                .HasOne(pt => pt.Note)
                .WithMany(p => p.NoteTags)
                .HasForeignKey(pt => pt.NoteId);

            modelBuilder.Entity<NoteTag>()
                .HasOne(pt => pt.Tag)
                .WithMany(t => t.NoteTags)
                .HasForeignKey(pt => pt.TagId);
        }

Answer 2

I am using EF 7, this problem took around 2 hours of my week end. :) So, here is the simple solution - I am having a profile class like this -

[Table("Profile")]
public class Profile
{
    public Profile()
    {

    }

    [Column(Order = 1)]
    [Key]        
    public Guid ProfileID { get; set; }
    [JsonIgnore]
    public virtual ICollection<StudentLivingWith> StudentProfileMap { get; set; }

    [JsonIgnore]
    public virtual ICollection<StudentLivingWith> ParentProfileMap { get; set; }
}

I am using the ProfileID as a F-Key reference in my another table named "StudentLivingWith". (ya, I know the name is bit strange. :)) As you can see in below class, both the columns "StudentProfileID" and "ParentProfileID" refering to the same column "profileID" of my "Profile" table.

[Table("StudentLivingWith")]
public class StudentLivingWith
{
    public StudentLivingWith()
    {

    }

    [Column(Order = 1)]
    [Key]
    [DatabaseGenerated(DatabaseGeneratedOption.Identity)]
    public int StudentLivingWithID { get; set; }

    [Column(Order = 2)]
    [ForeignKey("StudentProfileID")]
    public Guid StudentProfileID { get; set; }

    [Column(Order = 3)]
    [ForeignKey("ParentProfileID")]
    public Guid ParentProfileID { get; set; }

    [JsonIgnore]
    [InverseProperty("StudentProfileMap")]
    public virtual ICollection<Profile> StudentProfile { get; set; }

    [JsonIgnore]
    [InverseProperty("ParentProfileMap")]
    public virtual ICollection<Profile> ParentProfile { get; set; }
}    

So the conclusion is - you just need to add [InverseProperty] tag on the reference, and this simple solution did the trick for me.

I hope this will help. Thanks.