Django queryset to match all related objects

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暗喜
暗喜 2021-01-13 18:14

Let\'s say I have a ForeignKey from Coconut to Swallow (ie, a swallow has carried many coconuts, but each coconut has been carried by only one swallow). Now let\'s say that

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  • If i understood your altered question correctly you should be able to compare the coconut_carried_set of the swallow with the list of coconuts that have been carried.

    see docs

    I'm not entirely sure that this is what you want - I guess it depends on if you know which swallow you want to check beforehand or if you want to check it against all swallows - in that case there may be a better solution.

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  • 2021-01-13 18:31

    See the docs on queries spanning multi-valued relationships -- you should chain filter calls.

    A simple way to go would be something like

    queryset = Swallow.objects.all()
    for coconut in coconuts:
        queryset = queryset.filter(coconuts_carried=coconut)
    

    A fancy way to do this in one line using reduce would be

    reduce(lambda q, c: q.filter(coconuts_carried=c), coconuts, Swallow.objects.all())
    
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  • 2021-01-13 18:47

    I can have swallow.objects.filter(coconuts_carried__husk_sements__in = husk_segment_list) to show that this swallow has gripped at least one husk segment in the list.

    No, this is wrong, this gives you a list of swallows which have carried at least one husk segment from *husk_segment_list*.

    If I've understood right, we are talking about checking for a specific swallow.

    So, from your description I guess your models look something like this:

    class Swallow(models.Model):
        name = models.CharField(max_length=100)
    
    
    class Coconut(models.Model):
        swallow = models.ForeignKey(Swallow, related_name='coconuts_carried')
    
    
    
    class HuskSegment(models.Model):
        coconut = models.ForeignKey(Coconut, related_name='husk_segments')
    

    If you already have the husk segment list you need to check againts the swallows segments, there's no reason you need to resolve it in a query. Get the swallows' segments and check if it's a superset of your husk segment list.

    So we have:

    #husk_segment_list = [<object: HuskSegment>, <object: HuskSegment>, <object: HuskSegment>...]
    husk_segments_set = set((husk.pk for husk in husk_segment_list))
    
    whitey = Swallow.object.get(name='Neochelidon tibialis')
    wh_segments_set = set((value[0] for value in HuskSegment.objects.filter(coconut__in=whitey.coconuts_carried.all()).values_list('id')))
    
    whitey_has_carried_all = wh_segments_set.issuperset(husk_segments_set)
    
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