JOIN same table twice with aliases on SQLAlchemy

Question

I am trying to port the following query to SQLAlchemy:

SELECT u.username, GROUP_CONCAT(DISTINCT userS.name)
FROM Skills AS filterS 
INNER JOIN UserSkills AS ufs ON filterS.id = ufs.skill_id
INNER JOIN Users AS u ON ufs.user_id = u.id
INNER JOIN UserSkills AS us ON u.id = us.user_id
INNER JOIN Skills AS userS ON us.skill_id = userS.id
WHERE filterS.name IN ('C#', 'SQL')
GROUP BY u.id;

I don't understand how to achieve AS statement in SQLAlchemy. Here is what I currently have:

# User class has attribute skills, that points to class UserSkill
# UserSkill class has attribute skill, that points to class Skill
db.session.query(User.id, User.username, func.group_concat(Skill.name).label('skills')).\
   join(User.skills).\
   join(UserSkill.skill).filter(Skill.id.in_(skillIds)).\
   order_by(desc(func.count(Skill.id))).\
   group_by(User.id).all()

Please help.

Solution 1

I figured this out. Here are the classes that are used in my Flask app:

class User(Model):
    id = db.Column(db.Integer, primary_key=True, autoincrement=True)
    username = Column(db.String(80), unique=True, nullable=False)
    skills = db.relationship('UserSkill')

class Skill(Model):
    id = db.Column(db.Integer, primary_key=True, autoincrement=True)
    name = Column(db.String(80))

class UserSkill(Model):
    status = db.Column(db.Enum(SkillStatus))
    user_id = db.Column(db.Integer, db.ForeignKey('users.id'), primary_key=True)
    skill_id = db.Column(db.Integer, db.ForeignKey('skills.id'), primary_key=True)
    skill = db.relationship("Skill")

So, the actual code would look like this:

from sqlalchemy.orm import aliased

userSkillF = aliased(UserSkill)
userSkillI = aliased(UserSkill)
skillF = aliased(Skill)
skillI = aliased(Skill)
            
db.session.query(User.id, User.username,\
         func.group_concat(func.distinct(skillF.name)).label('skills'),\
         func.group_concat(func.distinct(skillI.name)).label('other_skills')).\
    join(userSkillF, User.skills).\
    join(userSkillI, User.skills).\
    join(skillF, userSkillF.skill).filter(skillF.id.in_(skillIds)).\
    join(skillI, userSkillI.skill).\
    group_by(User.id).all()

Many thanks Ilja Everilä, fresh look on SqlAlchemy docs made me understand aliased now.

Solution 2

We can do the join without relationships as well. Explicitly mention the condition on join.

Example

    from sqlalchemy.orm import aliased

    user1 = aliased(UserSkill)
    user2 = aliased(UserSkill)

    query_result = db.session.query(
        func.distinct(User.id).label('user_id'),
        User.username,
    ).join(
        user1,
        User.id == user1.user_id,
    ).join(
        user2,
        user2.id == User.id,
    ).filter(
        user1.user_id == id,
    ).all()

Comments

[Ilja Everilä]

"Using Aliases" is a good start, then "Querying with Joins".