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Removing child entity without setting parent_id to NULL in SQLAlchemy

If we have two sqlalchemy models, where a Child model has deletion flag:
class Parent(Base):
__tablename__ = 'parent'
id = Column(Integer, primary_key=True)
children = relationship('Child', primaryjoin='and_(Parent.id==Child.parent_id, not_(Child.is)deleted))')
class Child(Base):
__tablename__ = 'child'
id = Column(Integer, primary_key=True)
parent_id = Column(Integer, ForegnKey('parent.id'), nullable=False)
is_deleted = Column(Boolean, default=False)
parent = relationship(Parent)
When I remove child from parent with:
parent.children.remove(child)
SQLAlchemy sets child.parent_id to NULL. Is there any way to remove child from parent's children list, but keep parent_id? Right now I do:
child.is_deleted = True
session.flush()
session.refresh(parent)
which removes child from children list, but maybe there is a better way?

I don't think so, not as long as your relationship is dependent on Child.parent_id.

How to remove a child from a parent in SQLAlchemy relationship

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.

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?

One to Many Relationship sqlalchemy id's

Following this tutorial, for the one to many relationships, I have this simple code:
class Parent(Base):
__tablename__ = 'parent'
id = Column(Integer, primary_key=True)
children = relationship("Child", back_populates="parent")
class Child(Base):
__tablename__ = 'child'
id = Column(Integer, primary_key=True)
parent_id = Column(Integer, ForeignKey('parent.id'))
parent = relationship("Parent", back_populates="children")
Now the problem is populating parent_id, whenever I set the children as a list, and commit the results to a database as follows:
# assume I have a session
children = Child(), Child(), Child()
p = Parent(children=children)
session.add(p)
session.commit()
If I check the database at this point, parent_id is not populated. I guess this makes sense, because I haven't explicitly defined parent_id anywhere, but is there a way to get the children to get the parents id?

Parent accept children as a list so:
children = [Child(), Child(), Child()]
...
p = Parent.query.first()
for child in p.children:
print(child.parent_id)

Get inserted key before commit session

class Parent(db.Model):
id = db.Column(db.Integer, primary_key=True)
class Child(db.Model):
id = db.Column(db.Integer, primary_key=True)
parent_id = db.Column(db.Integer, db.ForeignKey('parent.id'))
parent = Parent()
db.session.add(parent)
child = Child()
child.parent_id = parent.id
db.session.add(child)
db.session.commit()
I want to INSERT into both parent and child tables inside a session considering that the parent_id must be included in the child table. In the moment I create the child object, parent.id is None.
How can I achieve that?

You could use flush() to flush changes to the database and thus have your primary-key field updated:
parent = Parent()
db.session.add(parent)
db.session.flush()
print parent.id # after flush(), parent object would be automatically
# assigned with a unique primary key to its id field
child = Child()
child.parent_id = parent.id
db.session.add(child)

Delete parent if the child is deleted

I want to delete the parent row if the associated rows in child tables have been removed.
class Child(Base):
__tablename__ = "children"
id = Column(Integer, primary_key=True)
parent_id = Column(Integer, ForeignKey("parents.id", ondelete='CASCADE'))
class Parent(Base):
__tablename__ = "parents"
id = Column(Integer, primary_key=True)
child = relationship(Child, backref="parent", passive_deletes=True)
If I remove the child
child_obj = session.query(Child).first()
session.delete(child_obj)
session.commit()
It does delete the child obj but parent remains as it is. I want to remove the parent as well using cascading.

You can read this thread:
Linking Relationships with Backref
class User(Base):
__tablename__ = 'user'
id = Column(Integer, primary_key=True)
name = Column(String)
addresses = relationship("Address", backref="user")
class Address(Base):
__tablename__ = 'address'
id = Column(Integer, primary_key=True)
email = Column(String)
user_id = Column(Integer, ForeignKey('user.id'))
And you can define it in your child class:
parent = relationship(Parent, backref=backref("children", cascade="all,delete"))

You could do this for simple* cases by creating a listener that intercepts deletions of Child instances and deletes the parent if there are no other children.
import sqlalchemy as sa
#sa.event.listens_for(sessionmaker, 'persistent_to_deleted')
def intercept_persistent_to_deleted(session, object_):
# We are only interested in instances of Child.
if not isinstance(object_, Child):
return
p = object_.parent
# Handle null parents.
if p is None:
return
cs = session.query(Child).filter(Child.parent == p).count()
if cs == 0:
session.delete(p)
* I would recommend thorough testing if your code is doing things like deleting children and then creating new children with the deleted children's parents in the same session. The listener works in this case:
c = session.query(Child).first()
p = c.parent
session.delete(c)
c1 = Child(parent=p)
session.add(c1)
session.commit()
but hasn't been tested on anything more complicated.

The behavior you're referring to is kinda simple. But once the application gets larger, you'd need more sophisticated methods. I, personally, implement a static method remove for every class. It takes the id and any other parameter necessary to know the delete scheme.
The answer is already given... But that's - not efficient - approach just to give you an example.
class Child(Base):
__tablename__ = "children"
id = Column(Integer, primary_key=True)
parent_id = Column(Integer, ForeignKey("parents.id", ondelete='CASCADE'))
#staticmethod
def remove(_id_):
child = Child.query().get(_id_)
Parent.remove(child.parent_id)
session.delete(child)
class Parent(Base):
__tablename__ = "parents"
id = Column(Integer, primary_key=True)
child = relationship(Child, backref="parent", passive_deletes=True)
#staticmethod
def remove(_id_):
parent = parent.query().get(_id_)
session.delete(parent)
If you, for example, added a Column named is_old - in the Child class - that's a binary integer 0 or 1 and wanted to delete the parent of the Child that has is_old == 1, It'll be very easy task.