title: Add new entities to DbContext
body: <p>I use ASP.NET Core with Identity and want to extend default Db context. If I want to add not linked table I just add a new class:</p>
  
  <pre><code>public partial class Table1
  {
      public int Id { get; set; }
      public string Txt { get; set; }
  }
  </code></pre>
  
  <p>and extend my ApplicationDbContext:</p>
  
  <pre><code>public class ApplicationDbContext : IdentityDbContext&lt;ApplicationUser&gt;
  {
      public ApplicationDbContext(DbContextOptions&lt;ApplicationDbContext&gt; options)
          : base(options)
      {
      }
  
      public virtual DbSet&lt;Table1&gt; Table1 { get; set; }
  
      protected override void OnModelCreating(ModelBuilder builder)
      {
          base.OnModelCreating(builder);
          // Customize the ASP.NET Identity model and override the defaults if needed.
          // For example, you can rename the ASP.NET Identity table names and more.
          // Add your customizations after calling base.OnModelCreating(builder);
  
          builder.Entity&lt;Table1&gt;(entity =&gt;
          {
              entity.ToTable("Table_1");
  
              entity.Property(e =&gt; e.Id).HasColumnName("ID");
  
              entity.Property(e =&gt; e.Txt)
                  .IsRequired()
                  .HasMaxLength(50);
          });
      }
  }
  </code></pre>
  
  <p>then create a migration and update db. It works. But if I want to add a new table, which linked to table from IdentityDbContext:</p>
  
  <pre><code>public partial class Users
  {
      public int Id { get; set; }
      public string UserId { get; set; }
      public string FirstName { get; set; }
      public string LastName { get; set; }
  
      public virtual AspNetUser User { get; set; }
  }
  </code></pre>
  
  <p>of course, AspNetUser class does not exist (it's created by IdentityDbContext, as I understand). How to do it correctly?</p>