too many filters?

[Daniel Wong]

I've been reading about and trying to understand how indexes are
necessary in order to run queries, so I open my /_ah/admin/interactive
i.e. the Interactive Console and run the following program:

<code>
from app.models import Foo

if False: # True on first run. False on later runs
a = Foo(
title = 't',
category = 'c',
description = 'd' )

a.put()
print 'put new entity'

q = Foo.all()
q.filter('title =', 't')
q.filter('category =', 'c')

# when I uncomment the following line, an empty list gets printed
#q.filter('description =', 'd')

print [e.description for e in q]
</code>

Why is it that when I uncomment the last filter, I get no results, but
when I do, I get the one Foo that I just inserted? I believe I have
added the right config to my index.yaml:

- kind: Foo
properties:
- name: title
- name: category
- name: description

Yet, I still get no love from gae. What am I doing wrong here?