Django query with variable number of filter arguments

Question

I have a Django query that fetches from MyModel based on certain conditions:

if beta:
    MyModel.object.filter(x=alpha, y=beta)
else:
    MyModel.object.filter(x=alpha)

Is it possible to eliminate the if beta: check and do it in a single line i.e. make the query filter on y only when beta is not None

Is this a good (Djangonic) way:

MyModel.object.filter(**{'x':alpha, 'b':beta} if beta else **{'x':alpha})

Or is it possible to do something like this (I know the following is wrong, but can it be fixed to give the correct meaning?):

MyModel.object.filter(Q('x'=alpha) & (beta && Q('y'=beta)))

Kamil Rykowski · Accepted Answer · 2014-05-20 07:35:39Z

8

In such situations I'm using solution like below:

filter_kwargs = {'x': alpha}
if beta:
    filter_kwargs['y'] = beta

 MyModel.objects.filter(**filter_kwargs)

It is easy to expand when new conditions come in into the project requirements, but unfortunately its not a one-liner solution.

answered May 20, 2014 at 7:35

Kamil Rykowski

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Comments

Community · Accepted Answer · 2017-05-23 12:14:40Z

1

One way to do this is to use Q objects, see this answer.

In your case :

query = Q(x=alpha)
if beta:
    query = query & Q(y=beta)
MyModel.object.filter(query)

Not shorter than other examples, but it could be cleaner if you were to add more variables to test.

HTH,

edited May 23, 2017 at 12:14

CommunityBot

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answered May 20, 2014 at 7:55

Ambroise

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Comments

Sławek Kabik · Accepted Answer · 2014-05-20 08:12:56Z

0

In my opinion it isn't good solution. I have problem to read and understand the code.

You can also do something like this:

objects_ = MyModel.objects.filter(x=alpha, y=beta) if beta else MyModel.objects.filter(x=alpha)

return objects_

For me it's more clean.. but it's only my opinion.

edited May 20, 2014 at 8:12

answered May 20, 2014 at 7:29

Sławek Kabik

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2 Comments

Sławek Kabik Over a year ago

Yeah.. it's your opinion.. but it's not more repetitive. Python programmer always has to look for simple and clean solution.

Tonio Over a year ago

A line of python code longer than 80 characters is always unreadable. It is my opinion after 20 years.

RemcoGerlich · Accepted Answer · 2014-05-20 14:20:15Z

0

I would use

objects = MyObject.objects.filter(x=alpha)
if beta:
    # Additional filter because of ...
    objects = objects.filter(y=beta)

Your way is simply harder to read. Python should be easy to read. Note that this only works right for simple filters (no multi-valued relations), as buffer mentions. Otherwise, your original query looks best to me.

edited May 20, 2014 at 14:20

answered May 20, 2014 at 13:15

RemcoGerlich

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2 Comments

user Over a year ago

Careful with this. For multi-valued relations

Everything inside a single filter() call is applied simultaneously to filter out items matching all those requirements. Successive filter() calls further restrict the set of objects, but for multi-valued relations, they apply to any object linked to the primary model, not necessarily those objects that were selected by an earlier filter() call.

RemcoGerlich Over a year ago

Thanks, I didn't actually know the details of that. Updated answer a bit.

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