Schema Validation on Patch

So I was trying t implement a simple Patch use case but its proving to be a difficult affair. As Patch allows for partial submission of schema fields I tried the obvious solution and added the partial=True argument to my schema declaration.

@blp.arguments(UserInputSchema(partial=True))

This however leads to a problem related to the marshmallow model schema class, since it immediately tries to create a model instance from the passed object.

Am I missing something or does patch just not play well with the intended behavior of marshmallow’s ModelSchema?

Thanks for the help in advance.

[lafrech]

Patching is indeed complicated stuff.

What you’re trying to do is not a real patch but a put-like patch. It does not allow deleting a field, for instance.

A real patch would require something like json-patch. Search for json-patch on the Internet to get a better idea of what I’m talking about. I don’t do that and I have no recommendation.

This said, I think a lot of people live happily with put-like patches, which are much easier to achieve. I don’t do that either, I only use PUT.

This however leads to a problem related to the marshmallow model schema class, since it immediately tries to create a model instance from the passed object.

I think this is due to your schema. You must have some post_load decorator instantiating a model class, right?

I generally don’t to that. The schema deserializes the data and passes it to the view function as a dict. Then the view function uses it to update the data.

I think passing partial=True should do what you want.

@blp.route(.../<item_id>)
class UserInputById(MethodView):
    @blp.arguments(UserInputSchema(partial=True))
    @blp.response(UserInputSchema)
    def patch(self, new_data, user_input_id):
        # 1/ Get item object (using ORM/ODM)
        user_input = UserInput.get(user_input_id)
        # At this stage, new_data is a dict, user_input is an object
        # 2/ Update item with new_data
        # I don't have a generic method for that.
        # I suppose it depends on the application, the ORM/ODM, etc.
        # Doing it field by field can be verbose.
        [...]
        # 3/ Save item
        user_input.save()
        # 3/ Return item
        return user_input

Hope this helps.

[lafrech]

You shouldn’t have to override init. There won’t be any 1 appended if you only ever call the Edit schema with partial=True.