title: How to create a field with a list of instances in factory_boy
body: <p>I am using factory boy to test some models (not django) and I was wondering how can I display a field containing a list with multiple instances of another factory. For example having the clases <strong>User</strong> and <strong>Group</strong>:</p>
  
  <pre class="lang-py prettyprint-override"><code>class User:
    name = StringType(required=True)
  
  class Group:
    name = StringType(required=True)
    user = ModelType(User)
  </code></pre>
  
  <p>I would like to display a field called <strong>groups</strong> within the <strong>User</strong> factory which contains all the groups to which such user belongs to. Showing two groups by default ['<strong>group1</strong>', '<strong>group2</strong>'] when running the factory.</p>
  
  <pre class="lang-py prettyprint-override"><code>class UserFactory:
      name = factory.Faker('first_name')
      groups = factory.RelatedFactory(GroupFactory, 'user')
  
      class Meta:
          model = User
  
  
  class GroupFactory:
      name = factory.Faker('word')
      user = factory.SubFactory(UserFactory)
  
      class Meta:
          model = Group
  </code></pre>
  
  <p>I have tried using a related factory as displayed above, but I don't know how to define default values for the related fields. Is there any Factory-boy guru around who can bring some light to this issue?</p>