Why is Entity Framework creating an extra column that doesn't map to any field and is always null?

Question

I have my POCO set up like this:

public class Notification : BaseClass
{

    public User Receiver { get; set; }

    [Required, Index]
    public long ReceiverID { get; set; }

    public virtual User ContextualUser { get; set; }

    public long? ContextualUserID { get; set; }

    public virtual User Actor { get; set; }

    public long? ActorID { get; set; }

    public string Content { get; set; }

    public string Uri { get; set; }

    [Required]
    public string Type { get; set; }

}

My base class has only three properties: ID, CreateDate, and IsDeleted, none are related to User class:

public abstract class BaseClass
{
    [Key]
    public long ID { get; set; }

    [Required]
    public DateTime CreateDate { get; set; }

    public bool IsDeleted { get; set; }

    public BaseClass()
    {
        CreateDate = DateTime.UtcNow;
    }
}

However, Entity Framework creates an extra column, User_ID, which is always null, as seen below:

It doesn't apparently map to anything. None of my other entities (either deriving from the same base class or not) have this phantom column. Why is this column created? I am on EF 6.1.3 code-first with SQL Azure.

UPDATE: I've also ran update-database in PM console but nothing has changed, I still have this column.

UPDATE 2: There is even an index and a foreign key defined on column. I've tried dropping it (it's not a live app yet) but it's being referenced by an index:

alter table notifications drop column User_ID

Msg 5074, Level 16, State 1, Line 1
The index 'IX_User_ID' is dependent on column 'User_ID'.
Msg 5074, Level 16, State 1, Line 1
The object 'FK_dbo.Notifications_dbo.Users_User_ID' is dependent on column 'User_ID'.
Msg 4922, Level 16, State 9, Line 1
ALTER TABLE DROP COLUMN User_ID failed because one or more objects access this column.

Answer 1

you can use data annonations for this :

public abstract class BaseClass
{
    [Key]
    public long ID { get; set; }

    [Required]
    public DateTime CreateDate { get; set; }

    public bool IsDeleted { get; set; }

    public BaseClass()
    {
        CreateDate = DateTime.UtcNow;
    }
}

public class Notification : BaseClass
{
    [ForeignKey("ReceiverID")]
    [InverseProperty("Receivers")]
    public User Receiver { get; set; }

    [Required, Index]
    public long ReceiverID { get; set; }


    [ForeignKey("ContextualUserID")]
    [InverseProperty("ContextualUsers")]
    public virtual User ContextualUser { get; set; }

    public long? ContextualUserID { get; set; }


    [ForeignKey("ActorID")]
    [InverseProperty("Actors")]
    public virtual User Actor { get; set; }

    public long? ActorID { get; set; }

    public string Content { get; set; }

    public string Uri { get; set; }

    [Required]
    public string Type { get; set; }

}

public class User
{
    [InverseProperty("Receiver")]
    public ICollection<Notification> Receivers { get; set; }

    [InverseProperty("ContextualUser")]
    public ICollection<Notification> ContextualUsers { get; set; }

    [InverseProperty("Actor")]
    public ICollection<Notification> Actors { get; set; }
}

for more information :

Many to many relation

Navigation property

Answer 2

It was navigation property on my User class configured incorrectly. Because Notification class didn't have a property called User but rather Receiver, and because I've forgot to add the correct name (Receiver) as an inverse property on my User class which had a Notifications collection, this extra field was generated for navigating from User to Notifications. I've added the annotation [InverseProperty("Receiver")] to my collection on User class and it now works correctly.