test if model object fits into QuerySet without executing SQL?

[Thomas Klopf]

Hi all,

  Please I have a question, couldn't find any answer for it..

  Let's say I have 2 QuerySets:

  1) Select all records from table

  2) Select all records from table where color = "blue"

So QuerySet #2 is more restricted than QuerySet #1

So question is - if I get a model object from QuerySet #1, is it possible to check if the model object is 'filtered'  or fits into QuerySet #2, without actually executing the SQL for QuerySet #2 to find the record again?

I'm asking because running the SQL for QuerySet #2 is expensive, ideally would test if the record fits into the QuerySet in purely python/django.

Thanks in advance!

Tom

[jgi...@caktusgroup.com]

Tom,

Could you clarify "fits into"?

qs1 = Table.objects.all()
qs2 = Table.objects.filter(color='blue')


mo1 = qs1[0]

Are you trying to determine if mo1 is in qs2?

Best

[Phako Perez]

You can create query as

def get_query(value=None):

    if value:

          query = “select all records from table where color = ‘" + value + “‘

          query = “select all records from table”

    return query

Sent from my iPhone

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[Thomas Klopf]

Hello,

  Yes exactly that, if mo1 is in qs2. BUT without running the SQL on the database, if possible.

Thanks!

Tom

[Jani Tiainen]

Hi,

Note that Django querysets are lazy.

So running:

qs1 = MyModel.objects.all()

As is, doing that doesn't execute any queries. If you want filtered objects, just add filter:

qs2 = qs1.filter(color="blue")

qs2 is not even evaluated yet. You need actually cause something that evaluates queryset.

But if you want to fetch all objects from qs1 and then apply some "filtering" you can just evaluate whole queryset and it's results

will be cached. You can use list comprehensions or itertools to filter your objects in Python.

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Jani Tiainen

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https://blog.jani.tiainen.cc/

Always open for short term jobs or contracts to work with Django.

[jgi...@caktusgroup.com]

result = qs1.difference(qs2)

if Result is length 0 then all of 1 is in 2.