#32244: ORM inefficiency: ModelFormSet executes a single-object SELECT query per
formset instance when saving/validating
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Reporter: Lushen Wu | Owner: nobody
Type: | Status: new
Cleanup/optimization |
Component: Database | Version: 3.1
layer (models, ORM) |
Severity: Normal | Keywords: formsets
Triage Stage: | Has patch: 0
Unreviewed |
Needs documentation: 0 | Needs tests: 0
Patch needs improvement: 0 | Easy pickings: 0
UI/UX: 0 |
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Conceptual summary of the issue:
Let's say we have a Django app with {{{Author}}} and {{{Book}}} models,
and use a {{{BookFormSet}}} to add / modify / delete books that are
created by a given {{{Author}}}. The problem is when the {{{BookFormSet}}}
is validated, {{{ModelChoiceField.to_python()}}} ends up calling
{{{self.queryset.get(id=123)}}} which results in a single-object SELECT
query for **each** book in the formset. That means if I want to update 15
books, Django performs 15 separate SELECT queries, which seems incredibly
inefficient. (Our actual app is an editor that can update any number of
objects in a single formset, e.g. 50+).
My failed attempts to solve this:
1. First I tried passing a queryset to the {{{BookFormSet}}}, i.e.
{{{formset = BookFormSet(data=request.POST,
queryset=Book.objects.filter(author=1))}}}, but the `ModelChoiceField`
still does its single-object SELECT queries.
2. Then I tried to see where the {{{ModelChoiceField}}} defines its
queryset, which seems to be in {{{BaseModelFormSet.add_fields()}}}. I
tried initiating the {{{ModelChoiceField}}} with the same queryset that I
passed to the formset, e.g. {{{Book.objects.filter(author=1)}}} instead of
the original code which would be
{{{Book._default_manager.get_queryset()}}}. But this doesn't help because
I guess the new queryset I defined isn't actually linked to what was
passed to the formset (and we don't have a cache running). So the multiple
SELECT queries still happen. (Note: I realize
{{{_default_manager.get_queryset}}} is necessary for use cases where you
would want to switch one Model instance for another one which might not be
in the original queryset passed to the {{{BaseModelFormset}}}, but this is
not our use case)
3. I noticed that {{{BaseFormSet._existing_object()}}} provides a way to
check whether an object exists in the queryset that was giving to the
FormSet constructor, which means that queryset is evaluated at most once
and the results stored in {{{BaseFormSet._object_dict}}}. I thought there
might be some way to have {{{ModelChoiceField.to_python()}}} do something
similar before calling {{{self.queryset.get(id=123)}}}, but I don't think
{{{ModelChoiceField}}} is aware of {{{BaseFormSet}}}, and it would seem an
anti-pattern to reach up the hierarchy like this.
The easiest solution seems to me to pass {{{BaseFormSet._object_dict}}} in
some way to each {{{ModelForm}}} that's created, and then allow the
{{{ModelChoiceField}}} to check {{{_object_dict}}} before making another
SELECT query.
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Ticket URL: <https://code.djangoproject.com/ticket/32244>
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