SQLAlchemy how to update children when old = [1, 2] new = [2, 3]

Question

I have two models:

class Person(Model):
    id
    name
    skills = relationship(Skill)

class Skill(Model):
    id
    skill
    person_id

At the beginning, for example:

jack = Person(name='jack')
jack.skills = [Skill(s) for s in ['python', 'ruby']]
jack.save()

Then, one day, jack lost his skill 'ruby' but earned 'swift' so his skill is ['python', 'swift'].

My current way of doing this update is:

1. look for existing skills, i get old = ['python', 'ruby']
2. get the new list new = ['python', 'swift']
3. make old, new to set(old), set(new)
4. unchanged = old.intersection(new) , so i get the skill that does not change
5. I add every skill in set(new - unchanged)
6. I delete every skill in set(old-unchanged)

Is there a easier way to do this?

Answer 1

Use collection_class=set on the relationship to treat it as a set instead of a list.

Here's a working example of how to relate people with skills. This is a many-to-many relationship, instead of each skill being related to one person_id, each skill can be related to many people through the person_skill table. The relationship collection is a set , and Skill has a __hash__ function to make skills with the same name hash the same.

from sqlalchemy import create_engine, Column, Integer, String, ForeignKey, Table
from sqlalchemy.ext.declarative import declarative_base
from sqlalchemy.orm import sessionmaker, relationship

engine = create_engine('sqlite:///:memory:', echo=True)
Base = declarative_base(bind=engine)
Session = sessionmaker(bind=engine)
session = Session()


class Person(Base):
    __tablename__ = 'person'

    id = Column(Integer, primary_key=True)
    name = Column(String, nullable=False, unique=True)

    # many-to-many relation, as a set
    skills = relationship('Skill', 'person_skill', collection_class=set)


class Skill(Base):
    __tablename__ = 'skill'

    id = Column(Integer, primary_key=True)
    name = Column(String, nullable=False, unique=True)

    def __hash__(self):
        # so that the set collection will handle duplicate entries
        return hash((self.__class__, self.name))


# many-to-many table, relate a person to a skill
person_skill = Table(
    'person_skill', Base.metadata,
    Column('person_id', Integer, ForeignKey(Person.id), primary_key=True),
    Column('skill_id', Integer, ForeignKey(Skill.id), primary_key=True)
)


# create the tables
Base.metadata.create_all()

# populate some skills and people
s1 = Skill(name='python')
s2 = Skill(name='sqlalchemy')
s3 = Skill(name='questions')
s4 = Skill(name='ruby')

p1 = Person(name='davidism', skills={s1, s2, s4})
p2 = Person(name='user2653947', skills={s3})

session.add_all([p1, p2])
session.commit()

# change some skills on people
p1.skills.discard(s4)
p2.skills.add(s2)
session.commit()

This is not a complete solution. You could for instance plug in the unique object pattern demonstrated in this answer to make sure the skills you create are never duplicated.