Optimize DB accesses by using an SQL JOIN when retrieving a user. (opr #679)

[jwag956]

Datastore.find_user() is used on every page load to verify the user in
the current session; until now, find_user() would trigger 2+ DB
accesses, since the roles relationship uses lazy loading by default.
This is useful to reduce the rows accessed when retrieving multiple
users from the DB.

However, in find_user() we always retrieve only one, so
it makes sense to retrieve all roles in one SQL operation instead of
always triggering 2 or more.

This would be unnecessary if the roles relationship was defined with
lazy="joined". However, that would greatly increase the number of rows
accessed on all queries (even when querying all users), so I think it's
better to be selective and optimize only these accesses which happen on
every page load.

[madstacks]

After this change I am seeing this new error. Do you have any recommendations?

Got exception during processing: <class 'sqlalchemy.exc.InvalidRequestError'> - 
'User.roles' does not support object population - eager loading cannot be applied.

My roles relationship on the User model has lazy='dynamic' set.

[madstacks]

Removing lazy='dynamic' fixed the issue, but perhaps this should be in the release notes as something to be aware of when upgrading from flask-security to flask-security-too.

[jwag956]

Thanks for working this out. I really don't want to break backwards compat if I can help it - this was a PR I got from the original flask-security.

Can you add to this thread the snippet of model code you had to change?

[madstacks]

This is what I had to change on the User model. However, it also requires updating any code that relies on roles returning a Query object. e.g. this will no longer work user.roles.count() .

Thanks for your work on keeping this project alive!

     roles = db.relationship('Role',
                             secondary=roles_users,
-                            backref=db.backref('users', lazy='dynamic'),
-                            lazy='dynamic')
+                            backref=db.backref('users', lazy='dynamic'))