How to remove a child from a parent in SQLAlchemy relationship

Question

Here are my models (ignoring imports):

class Parent(Base): __tablename__ = 'parents' id = Column(Integer, primary_key=True) name = Column(String) children = relationship('Child', backref='parent', lazy='dynamic') class Child(Base): __tablename__ = 'children' id = Column(Integer, primary_key=True) name = Column(String) parent_id = Column(Integer, ForeignKey('parents.id'))

Next I create a parent and a child, and relate them:

dbsession = session() child = Child(name='bar') dbsession.add(child) parent = Parent(name='foo') parent.children.append(child) dbsession.add(parent) dbsession.commit()

And all that works fine (so ignore any errors I may have made copying it to here). Now I'm trying to break the relationship, while keeping both the parent and the child in the database, and I'm coming up empty.

I appreciate any help.

Community · Accepted Answer · 2017-05-23 11:45:28Z

3

I'm not sure exactly what you mean by break a relationship or why but I think this might work:

child = dbsession.query(Child).filter(Child.name=='bar').one() child.parent_id = None dbsession.add(child) dbsession.commit()

This post gives more info about blanking a foreign key: Can a foreign key be NULL and/or duplicate?

edited May 23, 2017 at 11:45

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answered Feb 10, 2017 at 19:36

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2 Comments

drewd423 Over a year ago

Looks like that worked. Basically what I'm trying to do is say that the child does not belong to that parent any more. Thank you very much!

snakecharmerb May 25 at 8:12

The add shouldn’t be necessary - the child is already in the session.

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