understanding marshmallow nested schema with list data

Question

Am new to python and am using marshmallow serialization. Unable to use the nested schema. My code:

from sqlalchemy import Column, Float, Integer, String, Text, text,ForeignKey
from sqlalchemy.ext.declarative import declarative_base
from sqlalchemy.orm import relationship

Base = declarative_base()
metadata = Base.metadata




class CompanyDemo(Base):
    __tablename__ = 'company_demo'

    company_id = Column(Integer, primary_key=True,                                                     
    server_default=text("nextval('company_demo_company_id_seq'::regclass)"))
    name = Column(Text, nullable=False)
    address = Column(String(50))
    location = Column(String(50))


 class UsersDemo(Base):
    __tablename__ = 'users_demo'

    id = Column(Integer, primary_key=True,                                                                                                                    
    server_default=text("nextval('users_demo_id_seq'::regclass)"))
    company_id = Column(Integer,ForeignKey('company_demo.company_id'), nullable=False)
    email = Column(String)

company = relationship('CompanyDemo')

Schema:

    from marshmallow import Schema, fields, pprint


    class CompanySchema(Schema):
        company_id = fields.Int(dump_only=True)
        name = fields.Str()
        address = fields.Str()
        location = fields.Str()


    class UserSchema(Schema):
        email = fields.Str()   
        company = fields.Nested(CompanySchema)


    user = UserSchema()
    user = UserSchema(many=True)
    company = CompanySchema()
    company = CompanySchema(many=True)

and my flask app:

    from flask import Flask, jsonify, url_for, render_template
    from sqlalchemy import create_engine
    from sqlalchemy.orm import sessionmaker
    from flask_sqlalchemy import SQLAlchemy
    from model import CompanyDemo, UsersDemo
    from schemas.userschema import user, company
    
    app = Flask(__name__)
    app.secret_key = "shiva"
    
    
    def db_connect():
        engine = create_engine('postgresql://ss@127.0.0.1:5432/test')
        Session = sessionmaker(autocommit=False, autoflush=False, bind=engine)
        # create a Session
        session = Session()
        session._model_changes = {}
        return session
    
    
    @app.route('/company', methods=["GET", "POST"])
    def get_all_company():
        db = db_connect()
        allcompany = db.query(CompanyDemo).join(UsersDemo).all()
        return jsonify(company.dump(allcompany, many=True).data)  # company is marshmallow schema
    
    
    if __name__ == '__main__':
        app.run(host='0.0.0.0', port=15418, debug=True)

Is there anything wrong in my code? I am facing problems with nested schema and unable to get the nested data in output.

The output is below:

[ { "address": "qqq ", "company_id": 1, "location": "www ", "name": "eee" }, { "address": "www ", "company_id": 2, "location": "qqq ", "name": "aaa" } ]

Answer 1

Self contained example using in-memory SQLite:

from flask import Flask, jsonify
from flask_sqlalchemy import SQLAlchemy
from marshmallow import Schema, fields, pprint

app = Flask(__name__)
app.config['DEBUG'] = True
app.config['SECRET_KEY'] = 'super-secret'
app.config['SQLALCHEMY_TRACK_MODIFICATIONS'] = False
app.config['SQLALCHEMY_DATABASE_URI'] = 'sqlite:///:memory:'
app.config['SQLALCHEMY_ECHO'] = True
db = SQLAlchemy(app)


class CompanyDemo(db.Model):
    __tablename__ = 'company_demo'

    company_id = db.Column(db.Integer, primary_key=True)
    name = db.Column(db.Text, nullable=False)
    address = db.Column(db.String(50))
    location = db.Column(db.String(50))

    def __unicode__(self):
        return u"{name} ({address})".format(name=self.name, address=self.address)


class UsersDemo(db.Model):
    __tablename__ = 'users_demo'

    id = db.Column(db.Integer, primary_key=True,)
    company_id = db.Column(db.Integer, db.ForeignKey('company_demo.company_id'), nullable=False)
    company = db.relationship('CompanyDemo')
    email = db.Column(db.String)

    def __unicode__(self):
        return u"{email}".format(email=self.email)


class CompanySchema(Schema):
    company_id = fields.Int(dump_only=True)
    name = fields.Str()
    address = fields.Str()
    location = fields.Str()


class UserSchema(Schema):
    email = fields.Str()
    company = fields.Nested(CompanySchema)

user_schema = UserSchema()
company_schema = CompanySchema()


@app.route('/')
def index():
    return "<a href='/dump_company'>Dump Company</a><br><a href='/dump_user'>Dump User</a>"

@app.route('/dump_user')
def dump_user():
    user = UsersDemo.query.first()
    return jsonify(user_schema.dump(user).data)

@app.route('/dump_company')
def dump_company():
    company = CompanyDemo.query.first()
    return jsonify(company_schema.dump(company).data)


def build_db():
    db.drop_all()
    db.create_all()
    company = CompanyDemo(name='Test 1', address='10 Downing Street', location='wherever')
    db.session.add(company)
    user = UsersDemo(email='fred@example.com', company=company)
    db.session.add(user)
    db.session.commit()


@app.before_first_request
def first_request():
    build_db()

if __name__ == '__main__':
    app.run(debug=True, port=7777)