Django Filtering Specific QuerySet.values()

Question

Let's say I have a model Class Parent and a Class Child. And child has a field called status and a ForeignKey relationship to Parent.

Let's say I retrieve one parent by calling filter (so as to have a QuerySet) by calling p = Parent.objects.filter(pk=1)

Now if I call p.values('children__name') I will receive a list of dictionaries of the children names to that parent.

My question is, if I wanted to call p.values('children__name') but limit the values only if the status of the child was specific, how would I do that?

I also want to make sure the original QuerySet is unaltered, as I don't want to filter it down (for larger QuerySets). I just want to filter the values that are based on some parameter.

Is there any way to do this in Django?

Daniel Roseman · Accepted Answer · 2014-09-07 09:37:12Z

12

You would just filter:

p.filter(children__status='whatever').values('children__name')

answered Sep 7, 2014 at 9:37

Daniel Roseman

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3 Comments

user3286136 Over a year ago

I do not wish to alter the original QuerySet (which this does). So for example, if I want to list all the parents and children that have a status of 'SICK' then I cannot just do what you did because it would filter down the parents to that. I wish to still keep all the parents, just have the value of 'children__name' be filtered down to those with a specific status. Does that make sense?

Jura Brazdil Over a year ago

This gets Parents who have any children with a status 'whatever' and then gets all the children names of these parents. That gets you children of other statuses as well, if they had a "sibling" with the status 'whatever'

Dhanil Dinesan Over a year ago

works fine for me

djvg · Accepted Answer · 2021-07-07 07:48:41Z

You can filter child values on M2M relationships using Prefetch. Prefetch specifies how to get data from the through table between Parent and Child and prefetch_related triggers the actual query.

from django.db.models import Prefetch

pf = Prefetch('children', Child.objects.filter(status='SICK')
parents = Parent.objects.filter(pk=1).prefetch_related(pf)

sick_children_names = []
for parent in parents:
    sick_children_names.append([child.name for child in parent.children.all()])

Alternative approach would be to use the through table itself.

names = Parent.children.through.objects.filter(parent_id=1, child__status='SICK').values('children__name')

Or with an existing qs p:

names = Parent.children.through.objects.filter(parent_id__in=p, child_status='SICK').values('children__name')

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