Is it possible to directly reference a many-to-many table using entity framework, code first

问题

I am using the entity framework and modelling a many-to-many relationship.

I have created the relationship between the two entities using the fluent API (let's say users and groups):

this.HasMany(t => t.Users)
.WithMany(t => t.Groups)
.Map(
m =>
{
  m.ToTable("GroupMembers");
  m.MapLeftKey("Group_Id");
  m.MapRightKey("User_Id");
});

This works great, but I'd like to also be able to reference the GroupMembers table directly. To do that, I have something like:

[Table("GroupMembers")]
public class GroupMember
{
    #region Properties

    /// <summary>
    /// Gets or sets the group.
    /// </summary>
    public virtual Group Group { get; set; }

    /// <summary>
    /// Gets or sets the Id of rht group.
    /// </summary>
    [Key]
    [Column("Group_Id", Order = 1)]
    public int GroupId { get; set; }

    /// <summary>
    /// Gets or sets the user.
    /// </summary>
    public virtual User User { get; set; }

    /// <summary>
    /// Gets or sets the Id of the user.
    /// </summary>
    [Key]
    [Column("User_Id", Order = 2)]
    public int UserId { get; set; }

    #endregion
}    

However, I will get the following error during initializing of the DbContext:

Schema specified is not valid. Errors: (381,6) : error 0019: The EntitySet 'GroupUser' with schema 'dbo' and table 'GroupMembers' was already defined. Each EntitySet must refer to a unique schema and table.

I believe this is because the entity framework does not realize that the GroupMembers table specified in the fluent API and the GroupMembers entities table are actually one in the same. In other words, if I remove the fluent API code which is describing the many-to-many relationship, then I am successfully able to initialize the DbContext.

Can I have a many-to-many table that I can also reference directly?

回答1:

No, you can't. If you want to have access to the join table via a separate entity you must replace your many-to-many relationship by two one-to-many relationships and change the navigation properties in User and Group to refer to GroupMember:

public class Group
{
    public int GroupId { get; set; }
    public virtual ICollection<GroupMember> Members { get; set; }
}

public class User
{
    public int UserId { get; set; }
    public virtual ICollection<GroupMember> Members { get; set; }
}

modelBuilder.Entity<Group>()
    .HasMany(g => g.Members)
    .WithRequired(gm => gm.Group);

modelBuilder.Entity<User>()
    .HasMany(u => u.Members)
    .WithRequired(gm => gm.User);

Why do you want this GroupMember entity? It doesn't contain any business meaning and has only references and keys. Usually you can get and modify any content of the join table by writing LINQ queries and by using the Group and User DbSets/entities and their navigation properties.

回答2:

We got around this by creating a view that is only used to define the relationship. We had other fields in the lookup table that we needed to access.

The view just selects the join fields from the lookup table.

回答3:

Yes you can... If you use another Context.

If you try to map more than one entity to the same table in the same Context then you will get the error. But if you put the two mappings in separate Contexts then there will be no issue. This way you can have one mapping that describes a many-many relationship with the joining table transparent as if there are only foreign keys in it... And you can have another mapping, with its own context, to expose the actual join table if you want to get at any properties of the relationship.

回答4:

Yes, you can! Check this out. Also, remove GroupMembers entity and IDbSet collection.

来源：https://stackoverflow.com/questions/9810285/is-it-possible-to-directly-reference-a-many-to-many-table-using-entity-framework