title: Django + Graphene + DjangoModelFormMutation : dissociating create and update
body: <p>I am using a <code>DjangoModelFormMutation</code> from Graphene-python to expose a generic mutation for a model, like so :</p>
  <pre><code>from graphene import Field
  from graphene_django.forms.mutation import DjangoModelFormMutation
  from django.db import models
  
  class Pet(models.Model):
      name = models.CharField()
      legs = models.PositiveIntegerField()
  
  class PetForm(forms.ModelForm):
      class Meta:
          model = Pet
          fields = ('name', 'legs')
  
  # This will get returned when the mutation completes successfully
  class PetType(DjangoObjectType):
      class Meta:
          model = Pet
  
  class PetMutation(DjangoModelFormMutation):
      pet = Field(PetType)
  
      class Meta:
          form_class = PetForm
  
  </code></pre>
  <p>Then in my schema file, I register the mutation like this :</p>
  <pre><code>create_pet = mutations.PetMutation.Field()
  update_pet = mutations.PetMutation.Field()
  </code></pre>
  <p>This works, but there are two desired behaviors I can't achieve this way :</p>
  <ol>
  <li><strong>Have two different mutations</strong> : one for create that can <em>only</em> create and not update, and one for update that can <em>only</em> update, and not create.</li>
  <li><strong>Have the update mutation work partially as well</strong> , i.e only updating the client-provided fields.</li>
  </ol>
  <p>How can I achieve the desired behavior ?</p>