mutate_object does not handle OneToOneFields

[lukavdplas]

The mutate_object method of the LettercraftMutation (for handling mutation requests to the backend) has separate handlers for relational fields. The method decides on the handler by checking field.many_to_many and field.many_to_one:

lettercraft/backend/graphql_app/LettercraftMutation.py

Lines 110 to 115 in dd3fb51

	if field.many_to_many is True:
	many_to_many_fields.append(key)
	elif field.many_to_one is True:
	one_to_many_fields.append(key)
	else:
	simple_fields.append(key)

This does not account for the OneToOneField - this is also a relational field, but it is not many_to_many or many_to_one, so it won't be handled properly and the mutation will return an error.

A OneToOneField is essentially a ForeignKey with a uniqueness requirement. I think you could just use the handler for ForeignKey fields here. In that case, you could just adjust this tree as follows:

 if field.many_to_many is True: 
     many_to_many_fields.append(key) 
 elif field.is_relation is True: 
     one_to_many_fields.append(key) 
 else: 
     simple_fields.append(key) 

I didn't test this yet, however.