Extending ASP.NET Identity Roles: IdentityRole is not part of the model for the current context

You have to specify during the creation of User Store that AspNetRole is used instead of IdentityRole. You can achieve this by using the UserStore class with 6 type parameters:

new UserStore<AspNetUser, AspNetRole, string, IdentityUserLogin, IdentityUserRole, IdentityUserClaim>(new PayrollDBEntities());

This indicates changes at User Manager creation as well. Here is a simplified example about the creation of needed instances:

public class AspNetUser : IdentityUser { /*customization*/ }

public class AspNetRole : IdentityRole { /*customization*/ }

public class PayrollDBEntities : IdentityDbContext //or : IdentityDbContext <AspNetUser, AspNetRole, string, IdentityUserLogin, IdentityUserRole, IdentityUserClaim> 
{
}

public class Factory 
{
    public IdentityDbContext DbContext 
    { 
        get 
        {
            return new PayrollDBEntities();
        } 
    }

    public UserStore<AspNetUser, AspNetRole, string, IdentityUserLogin, IdentityUserRole, IdentityUserClaim> UserStore
    {
        get 
        {                
            return new UserStore<AspNetUser, AspNetRole, string, IdentityUserLogin, IdentityUserRole, IdentityUserClaim>(DbContext);
        }
    }

    public UserManager<AspNetUser, string> UserManager
    { 
        get 
        {
            return new UserManager<AspNetUser, string>(UserStore);
        } 
    }

    public RoleStore<AspNetRole> RoleStore 
    {
        get 
        {
            return new RoleStore<AspNetRole>(DbContext);
        }
    }

    public RoleManager<AspNetRole> RoleManager 
    {
        get 
        {
            return new RoleManager<AspNetRole>(RoleStore);
        }
    }
}

After a few days of trying to get this to work in a clean manner, I've come to the conclusion that if you're using Database first and want to integrate ASP.NET Identity into your app, by far the easiest and cleanest solution is to create your own membership provider by overriding ASP.NET Identity. It's actually pretty easy, so far I've implemented UserStore and RoleStore to my liking. I've added columns/relations specific to my domain in my database, and whenever I create a user or a role, I take care of my database commits by adding the required relations. My UserStore implementation is quite similar to this. My RoleStore implementation is something like this:

public class ApplicationRoleStore : IRoleStore<ApplicationRoleDTO>
{
    private PayrollDBEntities _context;
    public ApplicationRoleStore() { }

    public ApplicationRoleStore(PayrollDBEntities database)
    {
        _context = database;
    }

    public Task CreateAsync(ApplicationRoleDTO role)
    {
        if (role == null)
        {
            throw new ArgumentNullException("RoleIsRequired");
        }
        var roleEntity = ConvertApplicationRoleDTOToAspNetRole(role);
        _context.AspNetRoles.Add(roleEntity);
        return _context.SaveChangesAsync();

    }

    public Task DeleteAsync(ApplicationRoleDTO role)
    {
        var roleEntity = _context.AspNetRoles.FirstOrDefault(x => x.Id == role.Id);
        if (roleEntity == null) throw new InvalidOperationException("No such role exists!");
        _context.AspNetRoles.Remove(roleEntity);
        return _context.SaveChangesAsync();
    }

    public Task<ApplicationRoleDTO> FindByIdAsync(string roleId)
    {
        var role = _context.AspNetRoles.FirstOrDefault(x => x.Id == roleId);

        var result = role == null
            ? null
            : ConvertAspNetRoleToApplicationRoleDTO(role);

        return Task.FromResult(result);
    }

    public Task<ApplicationRoleDTO> FindByNameAsync(string roleName)
    {

        var role = _context.AspNetRoles.FirstOrDefault(x => x.Name == roleName);

        var result = role == null
            ? null
            : ConvertAspNetRoleToApplicationRoleDTO(role);

        return Task.FromResult(result);
    }

    public Task UpdateAsync(ApplicationRoleDTO role)
    {

        return _context.SaveChangesAsync();
    }

    public void Dispose()
    {
        _context.Dispose();
    }
    private ApplicationRoleDTO ConvertAspNetRoleToApplicationRoleDTO(AspNetRole aspRole)
    {
        return new ApplicationRoleDTO{
            Id = aspRole.Id,
            EnterpriseId = aspRole.EnterpriseId,
            Name = aspRole.Name
        };
    }

    private AspNetRole ConvertApplicationRoleDTOToAspNetRole(ApplicationRoleDTO appRole)
    {
        return new AspNetRole{
            Id = appRole.Id,
            EnterpriseId = appRole.EnterpriseId,
            Name = appRole.Name,
        };
    }
}

And my ApplicationRoleDTO:

public class ApplicationRoleDTO : IRole
{
    public ApplicationRoleDTO()
    {
        Id = Guid.NewGuid().ToString();
    }

    public ApplicationRoleDTO(string roleName)
        : this()
    {
        Name = roleName;
    }
    public string Id { get; set; }
    public string Name { get; set; }
    public Guid EnterpriseId { get; set; }
}

I also found these 2 articles pretty helpful:

Overview of Custom Storage Providers for ASP.NET Identity

Implementing a Custom MySQL ASP.NET Identity Storage Provider

I'll explain here with the code exampels :).

The trick is, they are already in the IdentityDbContext (AspNetRoles, AspNetUserClaims, AspNetUsers, ....)

In the IdentityModel you will see ApplicationUser is empty at the top. If you want to customize these users or roles, just add properties here and then update your database via the console

Example of my context

public class ApplicationDbContext : IdentityDbContext<ApplicationUser>
{
    public ApplicationDbContext()
        : base("DefaultConnection")
    {
    }

    public DbSet<Request> Requests { get; set; }
    public DbSet<Reservation> Reservations { get; set; }
    public DbSet<PriceType> PriceTypes { get; set; }
    public DbSet<Product> Products { get; set; }
    public DbSet<Price> Prices { get; set; }
    public DbSet<GuestbookPost> Posts { get; set; }
    public DbSet<Count> Counts { get; set; }
    public DbSet<Invoice> Invoices { get; set; }
    public DbSet<InvoiceLine> InvoiceLines { get; set; }

    ...

}

So no application user is defined here, but I did add more properties to it, example:

public class ApplicationUser : IdentityUser
{
    public string FirstName { get; set; }
    public string LastName { get; set; }
    public string GroupName { get; set; }
    public string Email { get; set; }
    [StringLength(15)]
    public string Phone { get; set; }
    public string Remark { get; set; }
    public DateTime? BirthDate { get; set; }
    public DateTime ValidFrom { get; set; }
    public DateTime ValidUntil { get; set; }

    public string Street { get; set; }
    public string ZipCode { get; set; }
    public string City { get; set; }

    public virtual ICollection<Request> Requests { get; set; } 
}