I’ve finally settled on a nice syntax for `OR`-ing [Django Q objects][q].

For a simple site search feature I needed to search for a term across several
fields in a model. Suppose the model looks like this:

class BlogPost(models.Model):
title = models.CharField(max_length=100)
body = models.TextField()
summary = models.TextField()

And you have a view method that accepts a parameter `q` for searching across
the `title`, `body` and `summary` fields. I want to find objects that contain
the `q` phrase in any of those fields. I need to build a [`QuerySet`][queryset]
with a filter that is the equivalent of

queryset = BlogPost.objects.filter(
Q(title__icontains=q) | Q(body__icontains=q) | Q(summary__icontains=q)
)

That’s not too much of a hassle for this simple example, but in cases where
the fields you are searching are chosen dynamically, or where you just have
an awful lot of fields to search against, I think it is nicer to do it like so:

import operator

search_fields = (‘title’, ‘body’, ‘summary’)
q_objects = [Q(**{field + ‘__icontains’:q}) for field in search_fields]
queryset = BlogPost.objects.filter(reduce(operator.or_, q_objects))

Nice one! The list comprehension gives me a list of `Q` objects generated from
the names in `search_fields`, so it is easy to change the fields to be searched.
And using [`reduce`][reduce] and [`operator.or_`][operator] gives me the
required `OR` filter in one line.

I see for Python 3 `reduce` has been [moved to the `functools` module][functools].

This stuff never used to be that obvious to me. It kind of isn’t even now.

P.S. I promise I am not writing a blog engine at this time, it was just for
the example.

[q]: http://docs.djangoproject.com/en/dev/topics/db/queries/#complex-lookups-with-q-objects
[queryset]: http://docs.djangoproject.com/en/dev/ref/models/querysets/
[operator]: http://docs.python.org/library/operator.html
[reduce]: http://docs.python.org/library/functions.html#reduce
[functools]: http://docs.python.org/library/functools.html