self referential many to many flask-sqlalchemy

Question

I for the life of me cannot figure out why this self-referential many-to-many will not be happy:

minor_contains = db.Table(
    'minor_contains',
    db.Column('parent_id', db.Integer, db.ForeignKey('minors.id'),
            primary_key=True),
    db.Column('contains_id', db.Integer, db.ForeignKey('minors.id'),
            primary_key=True))

class Minor(db.Model):
    __tablename__ = 'minors'
    id = db.Column(db.Integer, primary_key=True)
    name = db.Column(db.String())
    ...
    contains = db.relationship(
        "Minor",
        secondary=minor_contains,
        primaryjoin="id == minor_contains.c.parent_id",
        secondaryjoin="id == minor_contains.c.contains_id",
        backref="contained_by",
        lazy='dynamic')

I've tried reworking it a few different ways based on examples I've seen for SQLAlchemy and for Flask-SQLAlchemy, but I consistently end up where either I get the following error message or I end up in an infinite loop somewhere.

E ArgumentError: Could not locate any simple equality expressions involving locally mapped foreign key columns for primary join condition 'minor_contains.parent_id = :parent_id_1' on relationship Minor.contains. Ensure that referencing columns are associated with a ForeignKey or ForeignKeyConstraint, or are annotated in the join condition with the foreign() annotation. To allow comparison operators other than '==', the relationship can be marked as viewonly=True.

UPDATE

I'm really failing to understand the error message because it shows the column from the join table being compared to itself, where the condition on the join should be the PK of the minors table compared to the FK in the join table.

I'll also add a version of this that just hangs forever. You'll see I've been reworking variable names and such just rewriting it over and over hoping if I take a fresh stab at it, I'll somehow be smarter the second or fifth time around.

minor_contains = db.Table(
    'minor_contains',
    db.Column('parent_minor_id', db.Integer, db.ForeignKey('minors.id'),
            primary_key=True),
    db.Column('contains_minor_id', db.Integer, db.ForeignKey('minors.id'),
            primary_key=True))

class Minor(db.Model):
    __tablename__ = 'minors'
    id = db.Column(db.Integer, primary_key=True)
    name = db.Column(db.String())
    ...
    contains = db.relationship(
        "Minor",
        secondary=minor_contains,
        primaryjoin=id==minor_contains.c.parent_minor_id,
        secondaryjoin=id==minor_contains.c.contains_minor_id,
        backref=db.backref("minor_contains", lazy='dynamic'))

Answer 1

I think it's necessary to specify the model name in the join conditions.

contains = db.relationship(
    "Minor",
    secondary=minor_contains,
    primaryjoin="Minor.id == minor_contains.c.parent_id",
    secondaryjoin="Minor.id == minor_contains.c.contains_id",
    backref=db.backref('minor_contains', lazy='dynamic'),
    lazy='dynamic')

I had the same issue and it fixed the problem. I found a useful answer here : link