Filtering after a join in Flask-SQLAlchemy

Question

I have two tables( location and country ) that am trying to query; which are represented by the models below in my flask application

from sqlalchemy import Column, DateTime, ForeignKey, Integer, \
                       Numeric, SmallInteger, String, Table
from sqlalchemy.orm import relationship
from sqlalchemy.schema import FetchedValue
from flask_sqlalchemy import SQLAlchemy


db = SQLAlchemy()


class Country(db.Model):
    __tablename__ = 'country'

    country_id = db.Column(db.Integer, primary_key=True)
    country_name = db.Column(db.String(30), nullable=False)
    full_country_name = db.Column(db.String(90), nullable=False)
    country_code = db.Column(db.String(4), nullable=False)

    def __str__(self):
        return '%s' % self.country_name

    def __repr__(self):
        return '<Country %r>' % self.country_name


class Location(db.Model):
    __tablename__ = 'location'

    location_id = db.Column(db.Integer, primary_key=True)
    location_name = db.Column(db.String(75), nullable=False)
    country_id = db.Column(db.ForeignKey('mashamba.country.country_id'), nullable=False, index=True)

    country = db.relationship('Country', primaryjoin='Location.country_id == Country.country_id', backref='locations')

    def __str__(self):
        return '%s' % self.location_name

    def __repr__(self):
        return '<Location %r>' % self.location_name

What am trying to do is get all columns from both tables by performing a join using the following code

Location.query.join(Country).\
                filter_by(location_name='Cairo',
                          country_id=67).first()

Problem is when i run the code i get the following error

sqlalchemy.exc.InvalidRequestError: Entity '<class 'app.models.Country'>' has no property 'location_name'

Everything works fine when you run for example this code

Location.query.join(Country).all()

What is wrong here and how can this be solved?

Answer 1

filter_by() applies to the primary entity of the query, or the last entity that was the target of a join(). In your case that is Country, which does not have the required attribute. Either use filter() or move the call to filter_by(location_name=...) before the join:

Location.query.\
    filter_by(location_name='Cairo').\
    join(Country).\
    filter_by(country_id=67).\
    first()