python - How to find out if a lazy relation isn't loaded yet, with SQLAlchemy?

Question

With SQLAlchemy, is there a way to know beforehand whether a relation would be lazy-loaded?
For example, given a lazy parent->children relation and an instance X of "parent", I'd like to know if "X.children" is already loaded, without triggering the query.

Answer 1

You can get a list of all unloaded properties (both relations and columns) from sqlalchemy.orm.attributes.instance_state(obj).unloaded.

See: Completing object with its relations and avoiding unnecessary queries in sqlalchemy

An easier way is to use inspect(), which gives the same results:

from sqlalchemy import inspect
from sqlalchemy.orm import lazyload

user = session.query(User).options(lazyload(User.articles)).first()
ins = inspect(user)

ins.unloaded  # <- set or properties that are not yet loaded

Answer 2

I think you could look at the child's __dict__ attribute dictionary to check if the data is already there or not.

Answer 3

Slightly neater than Haes answer (though it effectively does the same thing) is to use hasattr(), as in:

>>> hasattr(X, 'children')
False