Django talk: RE: filter chaining v/s filter with multiple arguments.

Monday, April 3, 2017

RE: filter chaining v/s filter with multiple arguments.

Hi Mahendra,

Then that would suggest that there really is no difference for the purposes of generating the SQL statement in chaining filters as opposed to adding multiple arguments in one filter, wouldn't it?

So you might then wonder why do any chaining? Well, sometimes we assign a filter to a variable and then we want to add further filters based on some condition in the code. We can simply chain the filter in that situation rather than create a dict of arguments to the filter. I suppose it is a matter of style and preference. Since QuerySets are evaluated lazily, there is no performance degradation with either option. I personally prefer chaining the filters together.

q = Blog.objects.filter(entry__headline__contains='Lennon', entry__pub_date__year=2008)

if select_only_january:

     q = q.filter(entry__pub_date__month=0)

--or create a keyword dictionary--

kwargs = { 'entry__headline__contains': 'Lennon', 'entry__pub_date__year': 2008 }

if select_only_january:

     kwargs['entry__pub_date__month'] =0

q = Blog.objects.filter(**kwargs)

There are other ways of doing it to with Q objects.

I hope this helps.

From: django-users@googlegroups.com [mailto:django-users@googlegroups.com] On Behalf Of Mahendra Gaur
Sent: Monday, April 3, 2017 9:42 AM
To: django-users@googlegroups.com
Subject: RE: filter chaining v/s filter with multiple arguments.

Thanks for reply.

I have tried q.query on both statements but both are giving same query. I had tried this even before initiating this mail chain, and this was the point where I got confused.

Thanks and regards,

Mahendra Gaur

On 3 Apr 2017 6:55 p.m., "Matthew Pava" <Matthew.Pava@iss.com> wrote:

Hi Mahendra,

You can view the SQL that is generated by using the query attribute on the QuerySet.

q = Blog.objects.filter(entry__headline__contains='Lennon', entry__pub_date__year=2008)

print(str(q.query))

That should help in understanding what is going on.

Best wishes!

From: django-users@googlegroups.com [mailto:django-users@googlegroups.com] On Behalf Of Mahendra Gaur
Sent: Sunday, April 2, 2017 9:39 AM
To: django-users@googlegroups.com
Subject: filter chaining v/s filter with multiple arguments.

Hello everyone,

I am newbie to django. Now-a-days am reading django docs.

While reading models i got confusion that, What is the difference between filter chaining and filter with multiple arguments.

For example what is the diffrence between below two:

Blog.objects.filter(entry__headline__contains='Lennon', entry__pub_date__year=2008)

Blog.objects.filter(entry__headline__contains='Lennon').filter(entry__pub_date__year=2008)

can some one please help me to understand how these both works and most importantly what will be corresponding MYSQL queries for above two statements ?

Thanks and Regards,

Mahendra

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