title: Add field from another model Django
body: <p>I have the follow Models (an example, not really one):</p>
  
  <pre><code>class ModelB(models.Model):
    name = models.CharField(max_length=50)
  
    def __str__(self):
      return self.name
  
  class ModelA(models.Model):
    code = models.CharField(max_length=50, unique=True, help_text="Code unique")
    foreignkey = models.ForeignKey(ModelB, unique=True)
  </code></pre>
  
  <p>And in my admin.py I have:</p>
  
  <pre><code>class ModelBAdmin(admin.ModelAdmin):
    list_display = ('name',)
  
  class ModelAAdmin(admin.ModelAdmin):
    list_display = ('code', 'foreignkey')
  
  admin.site.register(ModelA, ModelAAdmin)
  admin.site.register(ModelB, ModelBAdmin)
  </code></pre>
  
  <p>I would make something similar to:</p>
  
  <pre><code>class ModelBAdmin(admin.ModelAdmin):
     list_display = ('name', 'code')
  </code></pre>
  
  <p>The code must be the code-relation from ModelA-code. How I can make this?</p>
  
  <p>P.D.: Sorry for my english...</p>
  
  <p>Thx a lot,
  Antonio.</p>