Flask-restful: marshal complex object to json

Question

I have a question regarding flask restful extension. I'm just started to use it and faced one problem. I have flask-sqlalchemy entities that are connected many-to-one relation and I want that restful endpoint return parent entity with all its children in json using marshaller. In my case Set contains many parameters. I looked at flask-restful docs but there wasn't any explanation how to solve this case.

Seems like I'm missing something obvious but cannot figure out any solution. Here is my code:

# entities
class Set(db.Model):
    id = db.Column("id", db.Integer, db.Sequence("set_id_seq"), primary_key=True)
    title = db.Column("title", db.String(256))

    parameters = db.relationship("Parameters", backref="set", cascade="all")


class Parameters(db.Model):
    id = db.Column("id", db.Integer, db.Sequence("parameter_id_seq"), primary_key=True)
    flag = db.Column("flag", db.String(256))
    value = db.Column("value", db.String(256))
    set_id = db.Column("set_id", db.Integer, db.ForeignKey("set.id"))


# marshallers

from flask.ext.restful import fields

parameter_marshaller = {
    "flag": fields.String,
    "value": fields.String
}

set_marshaller = {
    'id': fields.String,
    'title': fields.String,
    'parameters': fields.List(fields.Nested(parameter_marshaller))
}

# endpoint    

class SetApi(Resource):

    @marshal_with(marshallers.set_marshaller)
    def get(self, set_id):
        entity = Set.query.get(set_id)
        return entity


restful_api = Api(app)
restful_api.add_resource(SetApi, "/api/set/<int:set_id>")

Now when i call /api/set/1 I get server error:

TypeError: 'Set' object is unsubscriptable

So I need a way to correctly define set_marshaller that endpoint return this json:

{
  "id": : "1",
  "title": "any-title",
  "parameters": [
       {"flag": "any-flag", "value": "any-value" },
       {"flag": "any-flag", "value": "any-value" },
       .....
   ]
}

I appreciate any help.

Answer 1

I found solution to that problem myself.

After playing around with flask-restful i find out that i made few mistakes:

Firstly set_marshaller should look like this:

set_marshaller = {
    'id': fields.String,
    'title': fields.String,
    'parameters': fields.Nested(parameter_marshaller)
}

Restless marshaller can handle case if parameter is list and marshals to json list.

Another problem was that in API Set parameters has lazy loading, so when i try to marshall Set i got KeyError: 'parameters', so I need explicitly load parameters like this:

class SetApi(Resource):

     @marshal_with(marshallers.set_marshaller)
     def get(self, set_id):
        entity = Set.query.get(set_id)
        entity.parameters # loads parameters from db
        return entity

Or another option is to change model relationship:

parameters = db.relationship("Parameters", backref="set", cascade="all" lazy="joined")