Package: django-conceptq
conceptqis a small, simple utility for pullng portable, composable
“concepts” out of Django ORM queries, using Q objects.the problemThe 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.conceptqconceptqis primarily a single decorator,@concept, that you can apply to
a manager method.  It expects the method to return aQ object,
rather than a queryset; but it wraps the Q object in afiltercall so the
manager methods still chain like normal.  In addition, it provides aviamethod
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 fieldsNow 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 includedtestprojectand the only 40 lines of source code.licenseCopyright (c) 2013, Charlie DeTar
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