Flask-SqlAlchemy Adjacency List Relationship backfref unexpected error

Question

I am working on a small project where I am using Flask-SqlAlchemy to implement an adjacency list relationship. I have a model (table) which has an attribute for db.Relationship() which references the parent_id column of the same table.

Here is the following code (partial):

class Node(db.Model):
    id = db.Column(db.Integer, primary_key = True)
    title = db.Column(db.String(80))
    folder_id = db.Column(db.Integer, db.ForeignKey('node.id'))

    children = db.relationship('Node', backref = 'parent', remote_side=[id])

When I try to add a child folder to a parent folder using the backref attribute on the child in the Python Shell such as the following code(partial) :

parentNode = Node('title1')
db.session.add(parent)
db.session.commit()

childNode = Node('title2')
child.parent = parentNode

However, I get the following erros after I try that in the Python shell:

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>

TypeError: Incompatible collection type: Node is not list-like

I have read the SqlAlchemy docs to figure out a solution and they have similar syntax for adjacency list relationships. It has a similar example but does not show how one can add a parent node to a child node like I was trying in the shell. The reason for adding the parent node into the child's backref property is that it works with one-to-many relationships. Any help in finding the problem with the code would be greatly appreciated... Please feel free to suggest alternate solutions.

Thanks!

Answer 1

You configure the relationship in a wrong way. Please do this one:

from sqlalchemy.orm import backref
class Node(db.Model):
    # ...
    children = db.relationship('Node', backref=backref('parent', remote_side=[id]))

or

class Node(db.Model):
    # ...
    parent = db.relationship("Node", backref='children', remote_side=[id])