Conceptual question: Does it make any sense to add a filter without a backing form field?

[is-pdoherty]

I'd mentioned this in #1418 but I figured this question is more broadly applicable and I didn't want to hijack that thread.

What I'm wondering is:

1. Does it make sense to add a filter for a model field which doesn't have a corresponding Django form?
2. If not, does it make sense to create a form for just this purpose (i.e. creating a custom Django filter subclass) or am I naively proceeding down the wrong path here? Should this be done via another means (e.g. queryset) instead?

I'm looking to create a custom filter for Django PG Vector columns. My use case is somewhat complicated because it involves PG Vector and Graphene but, distilled, I think it would be being able to translate:

Item.objects.order_by(L2Distance('embedding', [3, 1, 2]))[:5]

into something like:

ItemFilter(lookup_expr="order_by_l2_distance: [3, 1, 2]")

in order to use it in graphql like:

query {
   allItems(order_by_l2_distance: [3, 1, 2]) {
    edges {
      node {
        id
      }
    }
  }
}

[carltongibson]

The form field (plus its widget)) maps from the query string to the values passed to the filter.

So in your case you want multiple values passed: [3, 1, 2]

The query string for that would either be?item=3&item=1&item=2 using a multiple choice filter (say, for discussion) or else ?item=3,1,2 using a CSV filter.

The form field's job, really, is to exclude bad data, such as passing totally the wrong type:

?item=cat&item=hat&item=mat

So even in a GraphQL case, you're still going to want to use one. (I'd never pass raw input from the request straight to my backend. The ORM &co are good at avoiding injection but basic sanitation is still required.)