How to remove SQLAlchemy Many-To-Many Orphans from database? - sql

Question

Context
I have a simple MySQL database written with SQLAlchemy. The following are my two models, Subreddit and Keyword, that have a many-to-many relationship, along with their association table:
subreddits_keywords = db.Table('subreddits_keywords', db.Model.metadata,
db.Column('subreddit_id', db.Integer, db.ForeignKey('subreddits.id', ondelete='CASCADE')),
db.Column('keyword_id', db.Integer, db.ForeignKey('keywords.id', ondelete='CASCADE')),
)
class Subreddit(db.Model, JsonSerializer):
__tablename__ = 'subreddits'
id = db.Column(db.Integer, primary_key=True)
subreddit_name = db.Column(db.String(128), index=True)
# Establish a parent-children relationship (subreddit -> keywords).
keywords = db.relationship('Keyword', secondary=subreddits_keywords, backref='subreddits', cascade='all, delete', passive_deletes=True, lazy='dynamic')
// ...
class Keyword(db.Model, JsonSerializer):
__tablename__ = 'keywords'
id = db.Column(db.Integer, primary_key=True)
keyword = db.Column(db.String(128), index=True)
// ...
As test data, I've created the following data set:
Subreddit:
test_subreddit
Keywords:
test_keyword1
test_keyword2
test_keyword3
In other words, test_subreddit.keywords should return [test_keyword1, test_keyword2, test_keyword3].
Problem
When I remove test_subreddit, test_keyword1, test_keyword2, test_keyword3 still persist in the database.
I understand that with many-to-many relationships, there is technically no parent so cascade's technically will not work according to this post:
https://stackoverflow.com/a/803584/10426919.
What I've Tried
I followed this link: https://github.com/sqlalchemy/sqlalchemy/wiki/ManyToManyOrphan.
This link provides a library function that should fix my exact problem.
However, the function does not work when integrated into my Model file in the following ways:
Method #1:
from app.extensions import db
from werkzeug.security import generate_password_hash, check_password_hash
from sqlalchemy.inspection import inspect
from sqlalchemy_utils import auto_delete_orphans <------ # library
subreddits_keywords = db.Table('subreddits_keywords', db.Model.metadata,
db.Column('subreddit_id', db.Integer, db.ForeignKey('subreddits.id', ondelete='CASCADE')),
db.Column('keyword_id', db.Integer, db.ForeignKey('keywords.id', ondelete='CASCADE')),
)
class Subreddit(db.Model, JsonSerializer):
__tablename__ = 'subreddits'
id = db.Column(db.Integer, primary_key=True)
subreddit_name = db.Column(db.String(128), index=True)
# Establish a parent-children relationship (subreddit -> keywords).
keywords = db.relationship('Keyword', secondary=subreddits_keywords, backref='subreddits', cascade='all, delete', passive_deletes=True, lazy='dynamic')
// ...
class Keyword(db.Model, JsonSerializer):
__tablename__ = 'keywords'
id = db.Column(db.Integer, primary_key=True)
keyword = db.Column(db.String(128), index=True)
// ...
auto_delete_orphans(Subreddit.keywords) <------ # Library function
However, this function does not seem to do anything. There is no error that is output to help guide me towards the right direction. When I check my database in MySQL workbench, the Subreddit, test_subreddit, is deleted, but the keywords [test_keyword1, test_keyword2, test_keyword3] are still in the database under the Keywords table.
Method #2:
I tried integrating the actual function, that the library function is based on, into my code as well:
from app.extensions import db
from werkzeug.security import generate_password_hash, check_password_hash
from sqlalchemy.inspection import inspect
from sqlalchemy_utils import auto_delete_orphans
# for deleting many-to-many "orphans".
from sqlalchemy import event, create_engine
from sqlalchemy.orm import attributes, sessionmaker
subreddits_keywords = db.Table('subreddits_keywords', db.Model.metadata,
db.Column('subreddit_id', db.Integer, db.ForeignKey('subreddits.id', ondelete='CASCADE')),
db.Column('keyword_id', db.Integer, db.ForeignKey('keywords.id', ondelete='CASCADE')),
)
class Subreddit(db.Model, JsonSerializer):
__tablename__ = 'subreddits'
id = db.Column(db.Integer, primary_key=True)
subreddit_name = db.Column(db.String(128), index=True)
# Establish a parent-children relationship (subreddit -> keywords).
keywords = db.relationship('Keyword', secondary=subreddits_keywords, backref='subreddits', cascade='all, delete', passive_deletes=True, lazy='dynamic')
// ...
class Keyword(db.Model, JsonSerializer):
__tablename__ = 'keywords'
id = db.Column(db.Integer, primary_key=True)
keyword = db.Column(db.String(128), index=True)
// ...
engine = create_engine("mysql://", echo=True)
Session = sessionmaker(bind=engine)
#event.listens_for(Session, 'after_flush')
def delete_tag_orphans(session, ctx):
# optional: look through Session state to see if we want
# to emit a DELETE for orphan Tags
flag = False
for instance in session.dirty:
if isinstance(instance, Subreddit) and \
attributes.get_history(instance, 'keywords').deleted:
flag = True
break
for instance in session.deleted:
if isinstance(instance, Subreddit):
flag = True
break
# emit a DELETE for all orphan Tags. This is safe to emit
# regardless of "flag", if a less verbose approach is
# desired.
if flag:
session.query(Keyword).\
filter(~Keyword.subreddits.any()).\
delete(synchronize_session=False)
Again, the keywords persisted despite being attached to no parent.
What I'm trying to accomplish
When children in the database no longer have a parent, I would like them to be removed from the database. What am I doing wrong?

Answer 1

Rather than using auto_delete_orphans, I created a method that I can call when I want to delete children. This method checks the child in question, and sees if it has any parents. If it does have a parent, we leave it be, but if it does not have a parent, we then delete the children.
Here is how I implemented this method, given that a Subreddit is a parent and a Keyword is a child of Subreddit.
def check_for_keyword_orphans(keyword):
# check if each keyword has an associated subreddit
if len(keyword.subreddits) == 0:
db.session.delete(keyword)
return True # keyword deleted
else:
return False # keyword has an associated subreddit
And here is how I used the method in my API route:
keywords = subreddit.keywords
for keyword in keywords:
check_for_keyword_orphans(keyword)
db.session.commit()