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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.

Flask Sqlalchemy ORM how to add a property to return the count of child

This has been driving me crazy and I'm sure it is more straight forward than I'm making it. But so far, no end of searching and trying various combinations, has failed to get me anywhere.
I'm using Flask and SQLAlchemy and I'm new to both..
So, if i have two classes/tables defined like this
class Child(db.Modal):
__tablename__ = 'children'
id = db.Column(db.Integer, primary_key=True)
kidsname = db.Column(db.String(20))
parent_id = db.Column(db.Integer, db.ForeignKey('parent.id'), nullable=False)
class Parent(db.Model):
__tablename__ = 'parent'
id = db.Column(db.Integer, primary_key=True)
name = db.Column(db.String(20))
children = db.relationship('Child', backref='parent', lazy=True)
What I want is to be able to add a property to the parent class to return the number of children e.g.
def child_count(self):
return ????
I would like to be able to do something like this when an instance of parent is passed though to my html template..
<p>Parent: {{ parent.name }} # of children: {{parent.child_count}}</p>
Any help or pointer in the right direction would be greatly appreciated..

You can use column_property https://docs.sqlalchemy.org/en/latest/orm/mapped_sql_expr.html
from sqlalchemy.orm import column_property
from sqlalchemy import select, func
class Parent(db.Model):
__tablename__ = 'parent'
id = db.Column(db.Integer, primary_key=True)
name = db.Column(db.String(20))
children = db.relationship('Child', backref='parent', lazy=True)
child_count = column_property(select([func.count(children.id)]).\
where(children.parent_id==id))
Also, you can find a solution to your problem with the hybrid_property in this question SQLAlchemy - Writing a hybrid method for child count

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)

In SQLAlchemey, Can I add new object to the relationship object?

Can I add new object to the relationship object?
Class Parent():
id = Column(Integer, primary_key=True)
name = Column(String(10))
child = relationship('Child', backref='parent', cascade="all, delete-orphan")
Class Child():
id = Column(Integer, primary_key=True)
parent = Column(Integer, ForeignKey('parent.id'))
name = Column(String(10))
parent = Parent('parent1', [Child(name='child1'), Child(name='child2')])
session.add(parent)
session.commit()
The above code is insert one parent and its 2 child.
My Question is,
Can I add new child from exist parent like this?
parent = session.query(Parent).filter(Parent.id=1).first()
parent.children.append(Child(name='child3'))
session.merge(parent)
I tried above it shows the IntegrityError: (IntegrityError) datatype mismatch u'UPDATE child.
Had I made any mistake here?

Try this...
parent = session.query(Parent).filter(Parent.id=1).first()
child = Child(parent=parent, name='child3')
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.