title: Unable to add DbModelBuilder and .HasRequired() to modelBuilder
body: <p>I am trying to create a ModelBuilder within my API and can't seem to add <code>.HasRequired()</code> to my code.  I am assuming this is due to the fact that it lives within <code>DBModelBuilder</code>, however, I cannot add that also.</p>
  
  <p>It will only allow me to use <code>ModelBuilder</code>.
  Otherwise it throws an error: <code>OnModelCreating(DbModelBuilder): no suitable method found to override</code></p>
  
  <p>Am I missing something here?</p>
  
  <p>My DbContext looks like so:</p>
  
  <pre><code>public class TicketContext : DbContext
      {
          public DbSet&lt;Tickets&gt; Tickets { get; set; }
          public DbSet&lt;Users&gt; Users { get; set; }
  
          protected override void OnModelCreating(DbModelBuilder modelBuilder) // &lt;-- Not allowing me to add DbModelBuilder here
          {
              modelBuilder.Entity&lt;Tickets&gt;().HasRequired(t =&gt; t.Users);
  
          }
      }
  </code></pre>
  
  <h2>Update</h2>
  
  <p>I have (as a test) started a new project from scratch.  I created a new ASP.NET 5.0 project using a Web API template.  Straight away my project doesn't recognize <code>DbModelBuilder</code>.  I added reference to EntityFramework.dll and still no good.  I then added <code>using System.Data.Entity;</code> and it then accepted <code>DbModelBuilder</code> but still complains that the namespace 'DbModelBuilder' could not be found.</p>
  
  <p>I can't understand how I can have this error upfront on a brand new project?</p>
  
  <p>As soon as I try and add the package Entity Framework from NuGet, I get more errors that version 6.1.3 is not compatible with DNX Core 5.0</p>
  
  <p>I can't seem to find any examples/solutions to any of these errors.</p>
  
  <h2>Update 2</h2>
  
  <p>I have managed to get DbModelBuilder recognized now by adding the EntityFramework.dll reference to the DNX Core 5.0 Assembly as well as the DNX 4.5.1 assembly, however, now it has thrown even more errors wanting System.Core added and mscorlib.  I really can't believe how much trouble it is to create a (what I thought would be simple) Web API project.  </p>