Intro

I'm trying to build a code first table which will have a 3rd table relating it to the ApplicationUser. I've been searching SO and Google, as I always do, to find a solution to my problem as I'm sure I'm not the first to run into this, but all the solutions I've tried are not working. Maybe it's because of the version of ASP.NET Identity (v 2.2.1) I'm using requires a different solution than what I'm finding or There are concepts I'm not familiar with yet.

Essentially, what I'm trying to build is exactly what the IdentityUserRole class is with different meaning. I want to build it using DataAnnotations and not the method they used. While I do appreciate other solutions and ways of doing the same thing, and I'm working on a real project, I do like to learn and at the very least want to learn how to do it with DataAnnotation.

With that said, here's the good stuff:

Classes

Sections class, which would be relative to the IdentityRole class

public class Sections {
    [Key]
    [StringLength(128)]
    public virtual String Id { get; set; }

    [Required]
    [Index("SectionsNameIndex", IsUnique = true)]
    [MaxLength(256)]
    [Display(Name = "Section Name")]
    public virtual String Name { get; set; }
}

Here is the UserSections class, which would be relative to the IdentityUserRole class

Version 1

public class UserSections {
    [Key, Column(Order = 1)]
    [Index]
    [StringLength(128)]
    [ForeignKey("User")]
    public virtual String UserId { get; set; }
    
    public virtual ApplicationUser User { get; set; }
    
    [Key, Column(Order = 2)]
    [Index]
    [StringLength(128)]
    [ForeignKey("Section")]
    public virtual String SectionId { get; set; }
    
    public virtual Sections Section { get; set; }
}

Version 2

public class UserSections {
    [Key, Column(Order = 1)]
    [Index]
    [StringLength(128)]
    public virtual String UserId { get; set; }
    
    [ForeignKey("UserId")]
    public virtual ApplicationUser User { get; set; }
    
    [Key, Column(Order = 2)]
    [Index]
    [StringLength(128)]
    public virtual String SectionId { get; set; }
    
    [ForeignKey("SectionId")]
    public virtual Sections Section { get; set; }
}

Problem

Problem is for either version I get the following error:

One or more validation errors were detected during model generation:

{MyProjectName}.DataContexts.IdentityUserLogin: : EntityType 'IdentityUserLogin' has no key defined. Define the key for this EntityType.

{MyProjectName}.DataContexts.IdentityUserRole: : EntityType 'IdentityUserRole' has no key defined. Define the key for this EntityType.

IdentityUserLogins: EntityType: EntitySet 'IdentityUserLogins' is based on type 'IdentityUserLogin' that has no keys defined.

IdentityUserRoles: EntityType: EntitySet 'IdentityUserRoles' is based on type 'IdentityUserRole' that has no keys defined.

Question(s)

How can I use DataAnnotations to get this to work like the IdentityUserRoles without, if possible, creating a third class to extend the ApplicationUser class?

Update #1

Based on answers given, here is some more info. I did create a secondary context to keep it away from the identity context. When I tried using the same context used with identity portion it worked. However, is there a way to do this using a different context?