Serialize multiple attributes using Pluck

Can Pluck be used to deserialize multiple source attributes to a single field and then serialize back to multiple fields? I read in #1315 that the intent of Pluck is to go from flat -> nested -> flat, but I couldn’t figure out how to get from the input:

{'baz': 'blue', 'qux': 'orange'}

by deserializing to

{'bar': {'baz': 'blue', 'qux': 'orange'}}

and then serializing back to the above.

I could get eitherbaz or qux but not both.

I attempted something like

class Bar(Schema):
    baz = fields.String()
    qux = fields.String()


class Foo(Schema):
    bar_1 = fields.Pluck(Bar, 'baz', data_key='baz', attribute='bar')
    bar_2 = fields.Pluck(Bar, 'qux', data_key='qux', attribute='bar')

But this throws the error:

ValueError: The attribute argument for one or more fields collides with another field's name or attribute argument. Check the following field names and attribute arguments: ['bar']

Is there something I’m missing, or is this use case not supported by Pluck?

[erdnaxeli]

@plondino to solve your issue, you could do this:

class Foo(Schema):
    bar_1 = fields.String(attribute='bar.baz', data_key='baz')
    bar_2 = fields.String(attribute='bar.qux', data_key='qux')

Foo().load({'baz': 'blue', 'qux': 'orange'})  # => {'bar': {'baz': 'blue', 'qux': 'orange'}}
Foo().dumps({'bar': {'baz': 'blue', 'qux': 'orange'}})  # => {"baz": "blue", "qux": "orange"}

It makes me a bit wonder why the Pluck field type exists in the first place, but it just works 🤷

[plondino]

Thanks for the analysis, very helpful. I was thinking that I could do the flattening and nesting in a post_dump and pre_load, respectively, but was getting a little tripped up in whether the philosophy is to have the schema match the input/serialized data or the internalized representation (and not sure it’s explicit in the docs).

If the latter is the case, the above approach seems like good syntax for what is probably a common use case.

I also have nested schemas (i.e. two or more levels down) that I’d like to be able to flatten on serialization; fields.Nested supports the dot notation as an argument to only but it doesn’t flatten, it only excludes. I wonder if the above syntax could support dotted attributes as well.