In Django, how does one filter a QuerySet with dynamic field lookups?

Question

Given a class:

from django.db import models

class Person(models.Model):
    name = models.CharField(max_length=20)

Is it possible (and if so how) to have a QuerySet that filters based on dynamic arguments, likeso:

 # instead of 
 Person.objects.filter(name__startswith='B')
 # and 
 Person.objects.filter(name__endswith='B')

 # is there some way, given:
 filter_by = '%s__%s' % ('name', 'startswith')
 filter_value = 'B'
 # You can run the equiv. of
 Person.objects.filter(filter_by=filter_value)
 # which throws an exception

Help is much appreciated & thank you in advance.

Answer 1

A really complex search forms usually indicates that a simpler model is trying to dig it's way out.

How, exactly, do you expect to get the values for the column name and operation? Where do you get the values of 'name' an 'startswith'?

 filter_by = '%s__%s' % ('name', 'startswith')

1. A "search" form? You're going to -- what? -- pick the name from a list of names? Pick the operation from a list of operations? While open-ended, most people find this confusing and hard-to-use.

   How many columns have such filters? 6? 12? 18?

   • A few? A complex pick-list doesn't make sense. A few fields and a few if-statements make sense.
   • A large number? Your model doesn't sound right. It sounds like the "field" is actually a key to a row in another table, not a column.

2. Specific filter buttons. Wait... That's the way the Django admin works. Specific filters are turned into buttons. And the same analysis as above applies. A few filters make sense. A large number of filters usually means a kind of first normal form violation.

A lot of similar fields often means there should have been more rows and fewer fields.

Answer 2

If, as S.Lott asks, this is the best way to solve your problem - you can use Python's argument expansion to achieve what you're looking for:

kwargs = {'%s__%s' % ('name', 'startswith'): 'A',
          '%s__%s' % ('name', 'endswith'): 'Z' }

Person.objects.filter(**kwargs)

Answer 3

A simplified example:

In a Django survey app, I wanted an HTML select list showing registered users. But because we have 5000 registered users, I needed a way to filter that list based on query criteria (such as just people who completed a certain workshop). In order for the survey element to be re-usable, I needed for the person creating the survey question to be able to attach those criteria to that question (don't want to hard-code the query into the app).

The solution I came up with isn't 100% user friendly (requires help from a tech person to create the query) but it does solve the problem. When creating the question, the editor can enter a dictionary into a custom field, e.g.:

{'is_staff':True,'last_name__startswith':'A',}

That string is stored in the database. In the view code, it comes back in as self.question.custom_query . The value of that is a string that looks like a dictionary. We turn it back into a real dictionary with eval() and then stuff it into the queryset with **kwargs:

kwargs = eval(self.question.custom_query)
user_list = User.objects.filter(**kwargs).order_by("last_name")

Answer 4

I have a query tool that does this. It is quite a lot of work but it uses dynamic forms, optgroups, and can't handle many to many. I am sure that many to many is possible but i just create functions that i use instead.