FranGoitia FranGoitia - 7 months ago 25
Python Question

SQL Alchemy Strange behaviour Join Multiple Columns from same table

I'm trying to perform a query that involves joining two columns from the same column.

These are my models:

class Match(Base):
__tablename__ = 'matches'

id = Column(Integer, primary_key=True)
date = Column(Date, nullable=False)
time = Column(Time, nullable=True)
league_id = Column(ForeignKey('leagues.id'), nullable=False, index=True)
league = relationship('League', backref='matches')
type = Column(enums.matches_types)
home_id = Column(ForeignKey('teams.id'), nullable=False, index=True)
home = relationship('Team', foreign_keys=[home_id], backref='home_matches')
away_id = Column(ForeignKey('teams.id'), nullable=False, index=True)
away = relationship('Team', foreign_keys=[away_id], backref='away_matches')


class Team(Base):
__tablename__ = 'teams'

id = Column(Integer, primary_key=True)
name = Column(String, nullable=False)
country_id = Column(ForeignKey('countries.id'), nullable=False, index=True)
country = relationship('Country', backref='teams')


This is what I wrote in SQLAlchemy:

self.db_match = Session.query(Match).join(home, Match.home).options(contains_eager(Match.home)
).join(away, Match.away).options(contains_eager(Match.away)
).join(Match.b_reference).options(contains_eager(Match.b_reference)
).filter(home.name == self.match['home']['name']
).filter(away.name == self.match['away']['name']
).filter(Match.date == self.match['date'])


The translation that SQLAlchemy does to pure SQL is the following:

SELECT teams_1.name, teams_2.name, matches.id, matches_b_reference_codes.code
FROM teams, matches JOIN teams AS teams_1 ON teams_1.id = matches.home_id
JOIN teams AS teams_2 ON teams_2.id = matches.away_id
JOIN matches_b_reference_codes ON matches.id = matches_b_reference_codes.match_id
WHERE teams_1.name = 'Portland Trail Blazers' AND teams_2.name = 'Miami Heat' AND matches.date = '2011-01-09';


But this is what I need:

SELECT teams_1.name, teams_2.name, matches.id, matches_b_reference_codes.code
**FROM matches JOIN teams AS teams_1 ON teams_1.id = matches.home_id**
JOIN teams AS teams_2 ON teams_2.id = matches.away_id
JOIN matches_b_reference_codes ON matches.id = matches_b_reference_codes.match_id
WHERE teams_1.name = 'Portland Trail Blazers' AND teams_2.name = 'Miami Heat' AND matches.date = '2011-01-09';

Answer

You are using contains_eager incorrectly. You need to specify the alias parameter when you're joining to an alias:

home = aliased(Team)
away = aliased(Away)
Session.query(Match).join(home, Match.home) \
                    .options(contains_eager(Match.home, alias=home)) \
                    .join(away, Match.away) \
                    .options(contains_eager(Match.away, alias=away)) \
                    ... \
                    .filter(...)