Why does Django admin list_select_related not work in this case?

问题

I've got a ModelAdmin class that includes a foreign key field in its list_display. But the admin list page for that model is doing hundreds of queries, one query per row to get the data from the other table instead of a join (select_related()).

The Django docs indicate you can add list_select_related = True as an attribute to your ModelAdmin to make this go away, but it doesn't seem to work at all for me. This SO question seems to give a similar problem, but his resolution is unclear, and it doesn't work in my case.

Here's a cut-down version of my model and model admin:

class Device(models.Model):
    serial_number = models.CharField(max_length=80, blank=True, unique=True)
    label = models.CharField(max_length=80, blank=True)

    def __str__(self):
        s = str(self.serial_number)
        if self.label:
            s += ': {0}'.format(self.label)
        return s

class Event(models.Model):
    device = models.ForeignKey(Device, null=True)
    type = models.CharField(max_length=40, null=False, blank=True, default='')

class EventAdmin(admin.ModelAdmin):
    list_display = ('__str__', 'device')
    list_select_related = True

However, adding that list_selected_related = True didn't change anything. I still get lots of queries like this instead of an SQL join:

Lots of queries, not a join

Any ideas why the Django admin seems to be ignoring my list_select_related and doing N queries? I'm using Python 2.7 and Django 1.3.3.

回答1:

The issue here is that setting list_select_related = True just adds a basic select_related() onto the query, but that call does not by default follow ForeignKeys with null=True. So the answer is to define the queryset the changelist uses yourself, and specify the FK to follow:

class EventAdmin(admin.ModelAdmin):
    list_display = ('__str__', 'device')
    def queryset(self, request):
        return super(EventAdmin, self).queryset(request).select_related('device')

回答2:

Since Django 1.6, list_select_related accepts a boolean, list or tuple with the names of the fields to include in the select_related() call. Hence you can now use:

class EventAdmin(admin.ModelAdmin):
    list_display = ('__str__', 'device')
    list_select_related = ['device']

回答3:

Although select_related is generally the way to go, there are time when one requires more control, then overiding the get_queryset becomes more applicable, this is a more modern version of Daniel Roseman's answer:

Where foo and bar are foreign key fields:

    def get_queryset(self, request):
        qs = super().get_queryset(request)
        return qs.select_related('foo', 'foo__bar').only('foo__field1', 'foo__bar__field2')

来源：https://stackoverflow.com/questions/15211799/why-does-django-admin-list-select-related-not-work-in-this-case

标签

python

django

performance

django-admin