jsonapi_marshmallow_meta.py

from marshmallow_jsonapi import fields, Schema
import json


class ProductSchema(Schema):
    class Meta:
        type_ = "products"

    id = fields.String()
    processes = fields.Relationship(
        type_="processes",
        id_field="id",
        include_resource_linkage=True,
        schema="ProcessSchema",
        many=True
    )


class ProcessSchema(Schema):
    class Meta:
        type_ = "processes"

    id = fields.String()
    meta = fields.ResourceMeta()
    some_field = fields.String()


serialized = ProductSchema().dump({
    "id": "1",
    "processes": [
        {
            "id": 15,
            "some_field": 123,
            "meta": {
                "order": "1"
            }
        }
    ]
})

print(json.dumps(serialized, indent=4))


assert 'meta' in serialized['data']['relationships']['processes']['data'][0]