title: Django filter iexact on a list
body: <p>I have a user model set up like this.</p>
  
  <pre><code>class ExternalUserModel(models.Model):
      email = models.EmailField()
      # other fields
  
  class MyUserModel(models.Model):
      external_user = models.ForeignKey(ExternalUserModel)
      # other fields
  </code></pre>
  
  <p>And I am trying to get a list of MyUserModel from a list of emails.</p>
  
  <p>This is the query I'm tying to perform:</p>
  
  <pre><code>MyUserModel.objects.filter(external_user__email__iexact__in=user_emails)
  </code></pre>
  
  <p>But I'm getting this error:
  <code>Unsupported lookup 'iexact' for EmailField or join on the field not permitted.</code></p>
  
  <p>I need iexact since the list of emails are based on user input and may not match the casing stored in the data base.</p>
  
  <p>How should I go about making this query?</p>