How to pass parameters to a nested schema which is referred by name

I would like to refer to a schema by its name when I use it as a nested one, but still I’d like to pass custom arguments to its constructor.

For example instead of doing this:

class AuthorSchema(Schema):
    books = fields.Nested(BookSchema(enveloped=True, many=True))
    class Meta:
        fields = ('id', 'name', 'books')

I’d like to use something like this:

class AuthorSchema(Schema):
    books = fields.Nested('BookSchema', enveloped=True, many=True)
    class Meta:
        fields = ('id', 'name', 'books')

Please note that in this case the BookSchema’s __init__ method is looking for a custom parameter called enveloped.

Is there any official way that I can achieve this using only the native Nested field?

I’m using the following versions of marshmallow: marshmallow==2.16.3 flask-marshmallow==0.9.0

[sloria]

I wonder if we should instead allow for a callable, like in SQLAlchemy.

class AuthorSchema(Schema):
    books = fields.List(fields.Nested(lambda: BookSchema(enveloped=True)))

Thoughts?

[deckar01]

There is currently not a way to pass custom arguments when the nested schema is referenced as a string. What you can do instead is subclass the schema with the custom Meta properties and use that schema name instead.

class NestedBookSchema(BookSchema):
    class Meta:
        enveloped = True

...
    books = fields.Nested('NestedBookSchema', many=True)

I’m not exactly sure where enveloped comes from, so if this is not part of the Meta then you might have to get a little creative with passing the arg through the constructor.

It seems like this could be made a lot more convenient by adding an argument that passes schema constructor args through the nested field constructor.