Optimizer results are not used in case of one-to-many relations where just one object is used
Describe the Bug
When you have a one-to-many relation in a model, so either a reverse lookup of a foreign key or a many to many field, the optimizer will prefetch it (good 👍). If the field in GraphQL however is declared as not returning a list or connection but just a single type, then the default resolver will use QuerySet#first / QuerySet#get to get the result, which will bypass the prefetched objects and issue new queries.
Example:
class Project(models.Model):
name = models.CharField(max_length=255)
class Issue(models.Model)
project = models.ForeignKey(Project, related_name='issues')
@strawberry_django.type(Issue, name='Issue')
class IssueType:
id: int
@strawberry_django.type(Project, name='Project')
class ProjectType:
id: int
name: str
latest_issue: IssueType | None = strawberry_django.field(field_name='issues')
@strawberry.type
class Query:
projects: ListConnectionWithTotalCount[ProjectType] = strawberry_django.connection()This example seems a bit contrived, however it should show the bug. When querying as follows:
query {
projects {
name latestIssue { id }
}There will be a query for projects (👍), a prefetch-query for issues (👍) and then another query for each project fetching the first issue (🙁).
A fix could look like this, implemented as a custom StrawberryDjangoField subclass here:
class StrawberryDjangoFieldWithFixedPrefetchInSomeCases(StrawberryDjangoField):
def get_queryset_hook(self, info: Info, **kwargs):
if self.is_connection or self.is_paginated or self.is_list:
return super().get_queryset_hook(info, **kwargs)
elif self.is_optional:
def qs_hook(qs: QuerySet): # type: ignore
qs = self.get_queryset(qs, info, **kwargs)
if is_optimized_by_prefetching(qs):
return next(iter(qs), None)
else:
return qs.first()
return qs_hook
else:
def qs_hook(qs: QuerySet): # type: ignore
qs = self.get_queryset(qs, info, **kwargs)
if is_optimized_by_prefetching(qs):
try:
return next(iter(qs))
except StopIteration:
raise qs.model.DoesNotExist(
"%s matching query does not exist." % qs.model._meta.object_name
)
else:
return qs.get()
return qs_hookI will submit a proper pull request if there are no objections.
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