A predicate class constructed like Django Q objects, used to test whether a new or modified model would match a query
django-predicate provides a Q like object to facilitate the question: “would this model instance be part of a query” but without running the query or even saving the object.
pip install django-predicate
Then use the P object just as you would Q objects:
from predicate import P p = P(some_field__startswith="hello", age__gt=20)
You can then call the eval method with a model instance to check whether it passes the conditions:
model_instance = MyModel(some_field="hello there", age=21) other_model_instance = MyModel(some_field="hello there", age=10) p.eval(model_instance) >>> True p.eval(other_model_instance) >>> False
or you can use Python’s in operator.
model_instance in p >>> True
Even though a predicate is not a true container class - it can be used as (and was designed as being) a virtual “set” of objects that meets some condiiton.
Like Q objects, P objects can be &’ed and |’ed together to form more complex logic groupings.
In fact, P objects are actually a subclass of Q objects, so you can use them in queryset filter statements:
qs = MyModel.objects.filter(p)
P objects also support QuerySet-like filtering operations that can be applied to an arbitrary iterable: P.get(iterable), P.filter(iterable), and P.exclude(iterable):
model_instance = MyModel(some_field="hello there", age=21) other_model_instance = MyModel(some_field="hello there", age=10) p.filter([model_instance, other_model_instance]) == [model_instance] >>> True p.get([model_instance, other_model_instance]) == model_instance >>> True p.exclude([model_instance, other_model_instance]) == [other_model_instance] >>> True
If you have a situation where you want to use querysets and predicates based on the same conditions, it is far better to start with the predicate. Because of the way querysets assume a SQL context, it is non-trivial to reverse engineer them back into a predicate. However as seen above, it is very straightforward to create a queryset based on a predicate.