Django - ModelChoiceField queryset caching

23 Apr 2020

It would be nice to know a better way to cache the ModelChoiceField’s queryset when it’s used in a form that runs “is_valid()” in a loop (like Formsets do). The best way I know how at the moment is by not using a ModelChoiceField at all. The solution requires using a ChoiceField, running the query for choices outside of the loop, then passing the choices into the form to override the ChoiceField choices.

Here’s an example:

	# See full example: https://github.com/pawl/django_modelchoicefield_caching/blob/master/myapp/views.py#L31-L51
	for song in playlist:
	form_data = {'title': song["title"], 'artist': song["artist"]}
	song_form = forms.SongFormWithModelChoiceField(data=form_data)
	song_form.is_valid() # runs a query to get the ModelChoiceField queryset each time
	print('ModelChoiceField - query count AFTER validating all songs:',
	len(connection.queries)) # 5 queries
	# query for choices outside of the loop to prevent unnecessary queries
	artist_choices = [(artist.pk, artist.name)
	for artist in models.Artist.objects.all()]
	for song in playlist:
	form_data = {'title': song["title"], 'artist': song["artist"]}
	# pass choices into the Form
	song_form = forms.SongFormWithChoiceField(
	artist_choices=artist_choices,
	data=form_data)
	song_form.is_valid()
	print('ChoiceField w/ choices passed in - query count AFTER validating all songs:',
	len(connection.queries)) # 6 queries (only 1 more query!)

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