django-conceptq 0.1.0

Tiny query wrapper for composable, cross-relation complex queries.

conceptq is a small, simple utility for pullng portable, composable “concepts” out of Django ORM queries, using Q objects.

the problem

The problem: say you have a bit of logic that applies to a model – for example, a pizza topping is “savory” if it’s salty or spicy, but not sweet. The models:

class ToppingManager(models.Manager)
    def savory(self):
        return self.filter(Q(savory=True) | Q(salty=True), sweet=False)

class Topping(models.Model):
    savory = models.BooleanField()
    salty = models.BooleanField()
    sweet = models.BooleanField()

    objects = ToppingManager()

class Pizza(models.Model):
    toppings = models.ManyToManyField(Topping)

This works great, until you want to carry that concept over to “Pizza” classes. How do you say that a pizza is savory? It’s savory if its toppings are. But when our logic for the toppings is bound up in the Topping manager, we end up having to duplicate it in the Pizza manager too:

class PizzaManager(models.Manager):
    def savory(self):
        # Oh noes!  Duplication city!
        return self.filter(Q(topping__savory=True) | Q(topping__salty=True),
            topping__sweet=False)

With each step down the related manager chain, we’re duplicating the query logic yet again. Not very DRY, bug magnet, etc.

conceptq

conceptq is primarily a single decorator, @concept, that you can apply to a manager method. It expects the method to return a Q object, rather than a queryset; but it wraps the Q object in a filter call so the manager methods still chain like normal. In addition, it provides a via method to prefix all the calls for you:

class ToppingManager(models.Manager):
    @concept
    def savory(self):
        return Q(Q(savory=True) | Q(salty=True), sweet=False)

This can then be used this way:

Topping.objects.savory()                # --> returns queryset
Topping.objects.savory().q              # --> returns the Q object
Topping.objects.savory().via("prefix")  # --> returns Q object with prefixed fields

Now we can have fun across relations:

Pizza.objects.filter(Topping.objects.savory().via("toppings"))
# filters with Q(Q(toppings__savory=True) | Q(toppings__salty=True), toppings__sweet=False)
Customer.objects.filter(Topping.objects.savory().via("favorite_pizza__toppings"))
# filters with Q(Q(favorite_pizza__toppings__savory=True) | Q(favorite_pizza__toppings__salty=True),
#                Q(favorite_pizza__toppings__sweet=False))

And we can use the manager class as a place to build a library of higher level concepts that can be composed together easily:

class ToppingManager(models.Manager):
    @concept
    def savory(self):
        return Q(Q(savory=True) | Q(salty=True), sweet=False)

    @concept
    def cajun(self):
        return Q(savory=True, burnt=True)

    @concept
    def high_calorie(self):
        return Q(calories__gte=300)

    @concept
    def diet_cajun(self):
        return ((self.savory().q | self.cajun().q) & ~self.high_calorie().q)


>>> diet_cajun_pizzas = Pizza.objects.filter(Topping.objects.diet_cajun().via("toppings"))
# filters with:
# Q(
#   Q(Q(toppings__savory=True) | Q(toppings__salty=True), Q(toppings__sweet=False)) |
#   Q(toppings__savory=True, toppings__burnt=True)
# ) & ~Q(toppings__calories__gte=300)
#

Oh, and it works with F expressions too.

For more, see the included testproject and the only 40 lines of source code.

license

Copyright (c) 2013, Charlie DeTar All rights reserved.

Redistribution and use in source and binary forms, with or without modification, are permitted provided that the following conditions are met:

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