title: Django + Factory Boy: How to use a named parameter to control behavior of factory.Maybe + factory.RelatedFactory
body: <p>Is there a way to define a named parameter (that is not a model attribute) to control the behavior of <strong>factory.Maybe</strong>?</p>
  
  <p>For example: I want to create a named parameter that controls the creation of a <strong>RelatedFactory</strong> through a <strong>Maybe</strong> condition, I tried to do this:</p>
  
  <pre><code># factories.py
  class Purchase(factory.DjangoModelFactory):
      user = factory.SubFactory('project.factories.UserFactory')
      total = uniform(10, 100)
  
      class Meta:
          model = Purchase
  
  class UserFactory(factory.DjangoModelFactory):
      first_name = factory.Faker('first_name')
      last_name = factory.Faker('last_name')
  
      has_purchase = False
      purchases = factory.Maybe(
          'has_purchase',
          yes_declaration=factory.RelatedFactory(Purchase, 'purchase'),
          no_declaration=None
      )
  
      class Meta:
          exclude = ['has_purchase', 'purchases']
          model = User
  
  # tests.py
  user_without_purchase = UserFactory.create()
  user_with_purchase = UserFactory.create(has_purchase=True)
  </code></pre>
  
  <p>It seems that I can't change the value of <strong>has_purchase</strong> through the named parameter, it's always set as False. I don't know if it's because both fields are declared in <strong>exclude</strong> Meta attribute. And I have to put them in <strong>exclude</strong> or else <em>DjangoModelFactory</em> tries to use these fields when saving it to the database, which causes an Integrity error.</p>
  
  <p>Is there a way to do that?</p>