CODEWITHSUNDEEP
pythondjangodjango-querysetintersectiondjango-filter
SirDeimos
SirDeimos

Reputation: 93

Django filtering by list of model objects

I'm trying to filter a queryset by checking that the object is in a list of those objects.

employee_list = [<Employee: A>, <Employee: B>, <Employee: C>]
qs = Employee.objects.filter(id__in=employee_list, [other_filters])

After running above, qs is an empty list. I was thinking that I could make a new list such as

employee_ids = [emp.id for emp in employee_list]
qs = Employee.objects.filter(id__in=employee_ids, [other_filters])

I haven't done benchmarking on this method, but I imagine performance would probably take a hit. Alternatively I could intersect the lists afterwards like:

qs = Employee.objects.filter([other_filters])
filtered_qs = [emp for emp in employee_lids if emp in qs]

However, I think the performance hit would be even worse this way.

What's the best/fastest way to do this? Thanks.

Upvotes: 1

Views: 3097

Answers (2)

Jmills
Jmills

Reputation: 2422

As Martol1ni noted you want to filter at the SQL level whenever possible so I think your methods do get progressively slower, but another issue...

Based on the Django Docs: https://docs.djangoproject.com/en/dev/ref/models/querysets/ I think yourid__in should be a list of integer ids, not a list of models.

Edit: Oh I see he covered this in his answer, but it was not explicit that it was incorrect in your question.

Edit2: But yes, if you want to know for sure, what really matters is real world performance, which you can do with django-debug-toolbar. It seems to me though the real issue was the id__in misuse which lead you to find trickier ways to do what you wanted to do.

Upvotes: 1

Martol1ni
Martol1ni

Reputation: 4702

Rule of thumb is to filter as much as possible through SQL, so I would go for 

qs = Employee.objects.filter(id__in=[emp.id for emp in employee_list], [other_filters])

I do not have any performance testing to back this up with though.

Upvotes: 2

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