Q() chaining with & returns empty set

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Bai Shun

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Mar 31, 2016, 10:22:22 AM3/31/16
to Django users
I've got a problem in using Q()


Let's say we have baskets of fruits. How to filter out those baskets that contains all fruits in a given basket?

In this document https://docs.djangoproject.com/en/dev/ref/models/querysets/#in 'in' method seems will return any basket which contains any of the given fruit. Is there any "set_contains" method to be used for filtering ? Such as Basket.objects.filter(fruit_set_containsAll=fruitSet) ?

Edit: I found that Q() is not working as expected. I'll post the test here:

class Basket(models.Model):
    weight = models.FloatField(default=1)
class Fruitname(models.Model):
    name = models.CharField(max_length=32)
class Fruit(models.Model):
    ofkind = models.ForeignKey(Fruitname, on_delete=models.CASCADE)  
    inbasket = models.ForeignKey(Basket, on_delete=models.CASCADE)  
    weight = models.FloatField(default=1)

the database is set as 'apple' only in 1 basket, 'pear' in 2 baskets, and 'banana' in 3 basket:

print Basket.objects.all()
print Fruitname.objects.all()
[<Basket: id 31 has 4 fruits 
    1. id - 53 apple(28), weight 1.00
    2. id - 54 apple(28), weight 2.00
    3. id - 55 apple(28), weight 3.00
    4. id - 62 banana(30), weight 10.00
>, <Basket: id 32 has 2 fruits 
    1. id - 56 pear(29), weight 4.00
    2. id - 57 banana(30), weight 5.00
>, <Basket: id 33 has 4 fruits 
    1. id - 58 pear(29), weight 6.00
    2. id - 59 banana(30), weight 7.00
    3. id - 60 pear(29), weight 8.00
    4. id - 61 pear(29), weight 9.00
>]
[<Fruitname: apple(28)>, <Fruitname: pear(29)>, <Fruitname: banana(30)>]

If I try to query with 'apple' and 'banana', it gives empty set !!

print Basket.objects.filter(Q(fruit__ofkind__name__in=['apple'])&Q(fruit__ofkind__name__in=['banana'])).distinct()
[]

similarly,

print Basket.objects.filter(Q(fruit__ofkind__name__in=['banana'])&Q(fruit__ofkind__name__in=['pear'])).distinct()
[]

Here's how I use 'in' to filter, it is not what I need which is supposed to be an empty set.

print Basket.objects.filter(Q(fruit__ofkind__name__in=['apple','pear'])).distinct()
[<Basket: id 31 has 4 fruits 
    1. id - 53 apple(28), weight 1.00
    2. id - 54 apple(28), weight 2.00
    3. id - 55 apple(28), weight 3.00
    4. id - 62 banana(30), weight 10.00
>, <Basket: id 32 has 2 fruits 
    1. id - 56 pear(29), weight 4.00
    2. id - 57 banana(30), weight 5.00
>, <Basket: id 33 has 4 fruits 
    1. id - 58 pear(29), weight 6.00
    2. id - 59 banana(30), weight 7.00
    3. id - 60 pear(29), weight 8.00
    4. id - 61 pear(29), weight 9.00
>]

The only way that is working properly is chaining with filter:

Basket.objects.filter(fruit__ofkind__name__in=['apple']).filter(fruit__ofkind__name__in=['banana']).distinct()
[<Basket: id 31 has 4 fruits 
    1. id - 53 apple(28), weight 1.00
    2. id - 54 apple(28), weight 2.00
    3. id - 55 apple(28), weight 3.00
    4. id - 62 banana(30), weight 10.00
>]

These code is tested with Django 1.9.4 Any explaining? I would undo the accepted answer for the moment.


Erik Cederstrand

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Apr 1, 2016, 2:32:02 AM4/1/16
to Django Users

> Den 31. mar. 2016 kl. 04.14 skrev Bai Shun <wany...@gmail.com>:
>
> I've got a problem in using Q()

To see what's going on, try to print the SQL that Django generates for working and non-working queryset:

> print Basket.objects.filter(Q(fruit__ofkind__name__in=['apple'])&Q(fruit__ofkind__name__in=['banana'])).distinct()

> Basket.objects.filter(fruit__ofkind__name__in=['apple']).filter(fruit__ofkind__name__in=['banana']).distinct()

Print SQL like this:

print(my_queryset.query)


Erik

Peter of the Norse

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Apr 22, 2016, 2:30:18 AM4/22/16
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They talk about this in the documentation.   https://docs.djangoproject.com/en/1.8/topics/db/queries/#spanning-multi-valued-relationships  Anything in a single filter() involves the same object.  For example, if you wanted to find all baskets containing a three pound banana, you would do Basket.objects.filter(fruit__weight=3.0, fruit__ofkind__name=‘banana’).  If you split them up into different filter()s, it would find baskets that have a three pound fruit of any type and a banana of any weight.

I think this is fairly new.  I’ve written code like Basket.objects.filter(fruit__in=Fruit.objects.filter(weight=3, ofkind__name=‘banana’)), but that uses a sub-select, and there are some databases *cough*MSSQL*cough* that choke on sub-selects.

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