Venues List page filtering is broken about cshc-web

[grahammcculloch]

So... django-filter will add separate filter() calls to the queryset for each set filter. This will create an independent INNER JOIN for each of these filters, resulting in a potentially huge set of results with lots of duplicate entries. To get Django to filter for only results that satisfy ALL the filters, we need to combine all filters into a single call to filter().

Django provides a way to force this behaviour, by using the QuerySet._next_is_sticky() method between each call to filter(). See this blog post for an explanation.

I've added a new AddFilter class that derives from django_filters.filters.BaseFilterSet and overrides the qs method to call _next_is_sticky() between each call to filter().

Custom filter classes (e.g. VenueFilter) can now derive from this new AddFilter class to ensure that filters are combined and only results that satisfy ALL the filters are returned.