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A predicate class constructed like Django Q objects, used to test whether a new or modified model would match a query

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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.


Install django-predicate:

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)
>>> True
>>> 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.

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