BaseInlineFormSet Adds N Entries to form._meta.fields For FK

[David Sanders]

Currently BaseInlineFormSet appends the FK name for the formset to the fields for the form Meta every time add_fields runs, so with you end up with N fields where N - 1 are duplicates. As each form is instantiated it'll see a different version of the list.

Example based on tests/inline_formsets/tests.py:

>>> poet = Poet.objects.create(name='test')
>>> poet.poem_set.create(name='first test poem')
>>> poet.poem_set.create(name='second test poem')
>>> poet.poem_set.create(name='third test poem')
>>> PoemFormSet = inlineformset_factory(Poet, Poem, fields=('name',), extra=0)
>>> formset = PoemFormSet(None, instance=poet)
>>> PoemFormSet.form._meta.fields
['name', 'poet', 'poet', 'poet']

[David Sanders]

Submitted ​PR #6503.

[Tim Graham]

Duplicate of #21332