I've seen slight variations on this topic, but not exactly what I'm looking
for.
I want to use a column_property (or something like it) to map the existence
of some Child relationship.
Say for example, I have two summary pages in a web app. One for displaying
rows of Parent objects, and one for displaying rows of Child objects.
I have mapped Parent to Child as a relationship.
Now, on the Parent summary there will be a link to the Child summary.
However, we only want the link if there are indeed Child relationships for
that Parent. Since a count is not needed, I would prefer to use an EXISTS
clause. Some Parents could have thousands of Child objects, so count would
not be the most performant solution.
Here is a contrived example of the Flask-SQLAlchemy models:
class Child(db.Model):
__tablename__ = "child"
child_key = db.Column(db.Integer, primary_key=True)
parent_key = db.Column(db.Integer, db.ForeignKey("Parent.parent_key"))
class Parent(db.Model):
__tablename__ = "parent"
parent_key = db.Column(db.Integer, primary_key=True)
children = db.relationship("Child",
primaryjoin="Parent.parent_key==Child.parent_key",
foreign_keys="[Child.parent_key]")
So, how could I implement some kind of property or method that will tell me
if Parent.has_children, using an EXISTS query?
I've seen the examples of using db.select() in a column_property, but I
can't get my head around the syntax for doing an exists. I am looking for
some approach that would leverage the defined relationship for children,
because in practice it could be a quite complex relationship, which I do
not wish to define more than once.
Any help is appreciated.
--
You received this message because you are subscribed to the Google Groups
"sqlalchemy" group.
To unsubscribe from this group and stop receiving emails from it, send an email
to sqlalchemy+unsubscr...@googlegroups.com.
To post to this group, send email to sqlalchemy@googlegroups.com.
Visit this group at http://groups.google.com/group/sqlalchemy.
For more options, visit https://groups.google.com/d/optout.
