Marshmallow doesn't merge nested data

from marshmallow.schema import Schema, fields
class MyPayload(Schema):
    class ActionAdd(Schema):
        # Parameters for actionAdd
        item_name = fields.Str()

    class ActionRemove(Schema):
        # Parameters for actionRemove
        ref_number = fields.Int()

    add_items = fields.Nested(ActionAdd, many=True, dump_to="items")
    remove_items = fields.Nested(ActionRemove, many=True, dump_to="items")


data = {
    "add_items": [
        {
            "item_name": "notebook"
        }
    ],
    "remove_items": [
        {
            "ref_number": 10
        }
    ]
}

request_data, errors = MyPayload().dump(data)

>>> request_data
Out[3]: {'items': [{'item_name': 'notebook'}]}

In an example shown above I am trying to dump a dictionary that will construct a payload to my API. the set of attributes of “ActionAdd” is differ from “ActionRemove”. They both need to be dumped to the same “items” dictionary. From the example you can see that the request_data is missing “ref_number”. Once “item_name” is removed from “data” then “ref_number” will be visible in request_data. Whats wrong with my implementation ?

[deckar01]

I’m not sure that we should implicitly merge when duplicate keys are used for dump_to. I think we should error instead of silently overwriting data.

Have you tried implementing this behavior with a post_dump method instead of dump_to?

from marshmallow import Schema, fields, post_dump

class MyPayload(Schema):
    class ActionAdd(Schema):
        item_name = fields.Str()
    
    class ActionRemove(Schema):
        ref_number = fields.Int()
    
    add_items = fields.Nested(ActionAdd, many=True)
    remove_items = fields.Nested(ActionRemove, many=True)
    
    @post_dump
    def merge_items(self, data):
        data['items'] = data['add_items'] + data['remove_items']
        del data['add_items']
        del data['remove_items']
        return data

If the use case ever needs special ordering or conditional logic, this pattern would have to be used anyway.

[lafrech]

In marshmallow v3, an error is raised in case of attribute or data_key collision (excluding dump_only/load_only fields): https://github.com/marshmallow-code/marshmallow/pull/992.

A custom field seems relevant to your need. Feel free to reopen if still stuck with this.