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

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

i have 2 models:

class person(model): id name skills = relationship(skill) class skill(model): id skill person_id

at beginning, example:

jack = person(name='jack') jack.skills = [skill(s) s in ['python', 'ruby']] jack.save()

then, 1 day, jack lost skill 'ruby' earned 'swift' skill ['python', 'swift'].

my current way of doing update is:

look existing skills, old = ['python', 'ruby'] get new list new = ['python', 'swift'] make old, new set(old), set(new) unchanged = old.intersection(new), skill not change i add together every skill in set(new - unchanged) i delete every skill in set(old-unchanged)

is there easier way this?

use collection_class=set on relationship treat set instead of list.

here's working illustration of how relate people skills. many-to-many relationship, instead of each skill beingness related 1 person_id, each skill can related many people through person_skill table. relationship collection set, , skill has __hash__ function create skills same name hash same.

from sqlalchemy import create_engine, column, integer, string, foreignkey, table sqlalchemy.ext.declarative import declarative_base sqlalchemy.orm import sessionmaker, relationship engine = create_engine('sqlite:///:memory:', echo=true) base of operations = 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, 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): # set collection handle duplicate entries homecoming hash((self.__class__, self.name)) # many-to-many table, relate person 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 tables base.metadata.create_all() # populate skills , 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() # alter skills on people p1.skills.discard(s4) p2.skills.add(s2) session.commit()

this not finish solution. instance plug in the unique object pattern demonstrated in answer create sure skills create never duplicated.

python sqlalchemy