title: Why EF migrations adds field?
body: <p>I use Identity 2.0. I have <code>CustomUserRole</code></p>
  
  <pre><code>public class CustomUserRole : IdentityUserRole&lt;long&gt;
  {
  }
  </code></pre>
  
  <p>When i try generate a migration all is good. But if i add a configuration to <code>CustomUserRole</code> </p>
  
  <pre><code> public class CustomRoleConfiguration : EntityTypeConfiguration&lt;CustomRole&gt;
      {
          public CustomRoleConfiguration()
          {
              Property(r =&gt; r.Id)
                  .HasDatabaseGeneratedOption(DatabaseGeneratedOption.None);
          }
      }
  </code></pre>
  
  <p>EF adds a field <code>CustomRole_Id</code></p>
  
  <pre><code>            CreateTable(
              "dbo.AspNetUserRoles",
              c =&gt; new
                  {
                      UserId = c.Long(nullable: false),
                      RoleId = c.Long(nullable: false),
                      CustomRole_Id = c.Long(),
                  })
              .PrimaryKey(t =&gt; new { t.UserId, t.RoleId })
              .ForeignKey("dbo.AspNetUsers", t =&gt; t.UserId, cascadeDelete: true)
              .ForeignKey("dbo.CustomRoles", t =&gt; t.CustomRole_Id)
              .Index(t =&gt; t.UserId)
              .Index(t =&gt; t.CustomRole_Id);
  </code></pre>
  
  <p>Now i have two field for <code>Role</code>. I dont understand why.</p>