Added filters that filter multiple values at once using multiwidgets and multivaluefields

[jonathan-golorry]

This is a pretty basic implementation, but it can do a few fancy things like efficiently filtering to customers with at least one completed, unreturned purchase:

class CustomerFilter(filters.FilterSet):
    purchases = filters.RelatedMultiFilter(
        related_names=["completed", "returned"], distinct=True
    )
    class Meta:
        model = Customer
        fields = {}

Customer.objects.filter(purchases__complete=True, purchases__returned=False).distinct()

SELECT DISTINCT "customers"."id"
FROM "customers"
INNER JOIN "purchases" ON ("customers"."id" = "purchases"."customer_id")
WHERE ("purchases"."completed" = True and "purchases"."returned" = False);

Currently django-filter with two separate separate filters would return all customers with a completed purchase and an unreturned purchase (with no guarantee that they are the same purchase). See #745 and #1077 :

class CustomerFilter(filters.FilterSet):
    class Meta:
        model = Customer
        fields = {
            "purchases__completed": ["exact"],
            "purchases__returned": ["exact"],
        }

Customer.objects.filter(purchases__complete=True.filter(purchases__returned=False).distinct()

SELECT DISTINCT "customers"."id"
FROM "customers"
INNER JOIN "purchases" ON ("customers"."id" = "purchases"."customer_id")
INNER JOIN "purchases" T3 ON ("customers"."id" = T3."customer_id")
WHERE ("purchases"."completed" = True and T3."returned" = False);

This is similar to RelatedFilter in philipn/django-rest-framework-filters/pull/199, but without the use of subqueries which can be extremely slow for this sort of lookup.

class CustomerFilter(filters.FilterSet):
    purchases = filters.RelatedFilter(
        "PurchaseFilter", queryset=Purchase.objects.all(), lookups=["completed", "returned"]
    )
    class Meta:
        model = Product

subquery = Purchase.objects.filter(completed=True).filter(returned=False).values('pk')
Customer.objects.filter(purchases__in=subquery).distinct()

SELECT DISTINCT "customers"."id"
FROM "customers"
INNER JOIN "purchases" ON ("customers"."id" = "purchases"."customer_id")
WHERE "purchases"."id" IN (
    SELECT U0."id"
    FROM "purchases" U0
    WHERE (U0."completed" = True AND U0."returned" = False)
);

Having done all this, I'm not sure a MultiWidget is a good way to group filters. The main issue is that validation errors are always described at the MultiWidget level instead of the subwidget level (see https://stackoverflow.com/questions/12245887/how-to-render-validation-errors-in-a-multivaluefield-multiwidget-per-field). This makes validation error messages largely useless. Unfortunately this is a fairly core issue in django and not easily fixable.

The django-rest-framework-filters approach of having a separate form for each RelatedFilter might be better.

If people really like this MultiWidget approach, the remaining work is:

• Handle related_names that result in a ModelChoiceField (currently errors due to no queryset)
• Change the SuffixedMultiWidget template to show suffixes to the user
• Rewrite MultiValueField.clean to raise more useful validation errors. The best approach is probably to to make errors a dictionary, but I'm not sure how to set the keys or how Form will interpret a dictionary.
• Write tests for fields.

I use django-rest-framework, so I don't have much experience with widgets, fields, and forms.

[carltongibson]

Closing in favour of #1167.