How do I check if many-to-many row already exists in sqlalchemy to prevent duplicate? - flask-sqlalchemy

Question

Here is code for my models:
class IllustratorAsset(db.Model):
id = db.Column(db.Integer, primary_key=True)
timestamp = db.Column(db.DateTime, index=True, default=datetime.utcnow)
uploader = db.Column(db.Integer, db.ForeignKey('user.id'))
project = db.Column(db.Integer, db.ForeignKey('project.id'))
fragments = db.relationship('AssetFragment', backref='ai', lazy='dynamic')
file_hash = db.Column(db.String(128), unique=True)
file_name = db.Column(db.String(128))
def __repr__(self):
return '<Picture {}>'.format(self.id)
tags_association = db.Table(
'tags_association',
db.Column('fragment_id', db.Integer(), db.ForeignKey('asset_fragment.id'),index=True),
db.Column('tag_id', db.Integer(), db.ForeignKey('tag.id'),index=True),
PrimaryKeyConstraint('fragment_id', 'tag_id')
)
class AssetFragment(db.Model):
id = db.Column(db.Integer, primary_key=True)
asset = db.Column(db.Integer, db.ForeignKey('illustrator_asset.id'))
timestamp = db.Column(db.DateTime, index=True, default=datetime.utcnow)
uploader = db.Column(db.Integer, db.ForeignKey('user.id'))
tags = db.relationship('Tag', secondary=tags_association, backref='fragments', lazy='dynamic')
file_name = db.Column(db.String(128))
file_hash = db.Column(db.String(128))
class Tag(db.Model):
id = db.Column(db.Integer, primary_key=True)
tag = db.Column(db.String(64), index=True, unique=True)
def __repr__(self):
return '<Tag {}>'.format(self.tag)
The idea is illustrations storage, there is Asset (Adobe Illustrator file), it has Fragments (renders of its different fragments), the fragment has some Tags for quick search which should be working through many-to-many relationship tags_association.
Currently, I'm stuck at trying to check if an association already exists, I'm sure there is some simple way to do it, but I'm missing some point. I came up with the code below, and I still get the exception of constraints.
for tag in new_tags: #list of tags from form
t = Tag.query.filter_by(tag=tag).first()
if (t is not None) and not(t in fragment.tags):
fragment.tags.append(t)
else:
new_tag = Tag(tag=tag)
if not(new_tag in fragment.tags):
fragment.tags.append(new_tag)
DB is Postgres

Answer 1

Well answer came shortly after I've tried to do it in flask shell. Shell output
I've realized, that it seems to be something wrong with the comparator, simple override makes it work:
def __eq__(self, __o: object) -> bool:
return self.tag == __o.tag
Also needed to change checking by a bit:
for tag in new_tags:
t = Tag.query.filter_by(tag=tag).first()
if t is None:
new_tag = Tag(tag=tag)
print(new_tag)
if not(new_tag in fragment.tags):
fragment.tags.append(new_tag)
else:
print(t)
if not(t in fragment.tags):
fragment.tags.append(t)