Pycassa: how to query parts of a Composite Type

Question

Basically I'm asking the same thing as in this question but for the Python Cassandra library, PyCassa.

Lets say you have a composite type storing data like this:

[20120228:finalscore] = '31-17'
[20120228:halftimescore]= '17-17'
[20120221:finalscore] = '3-14'
[20120221:halftimescore]= '3-0'
[20120216:finalscore] = '54-0'
[20120216:halftimescore]= '42-0'

So, I know I can easily slice based off of the first part of the composite type by doing:

>>> cf.get('1234', column_start('20120216',), column_finish('20120221',))
OrderedDict([((u'20120216', u'finalscore'), u'54-0'),
((u'20120216', u'halftimescore'), u'42-0')])

But if I only want the finalscore, I would assume I could do:

>>> cf.get('1234', column_start('20120216', 'finalscore'),
column_finish('20120221', 'finalscore'))

To get:

OrderedDict([((u'20120216', u'finalscore'), u'54-0')])

But instead, I get:

OrderedDict([((u'20120216', u'finalscore'), u'54-0'),
((u'20120216', u'halftimescore'), u'42-0')])

Same as the 1st call.

Am I doing something wrong? Should this work? Or is there some syntax using the cf.get(... columns=[('20120216', 'finalscore')]) ? I tried that too and got an exception.

According to http://www.datastax.com/dev/blog/introduction-to-composite-columns-part-1, I should be able to do something like this...

Thanks

Answer 1

If know all the components of the composite column then you should the 'columns' option:

cf.get('1234', columns=[('20120216', 'finalscore')])

You said you got an error trying to do this, but I would suggest trying again. It works fine for me.

When you are slicing composite columns you need to think about how they are sorted. Composite columns sort starting first with the left most component, and then sorting each component toward the right. So In your example the columns would look like this:

+------------+---------------+------------+---------------+------------+----------------+
| 20120216   | 20120216      | 20120221   | 20120221      | 20120228   | 20120228       |
| finalscore | halftimescore | finalscore | halftimescore | finalscore | halftimescore  |
+------------+---------------+------------+---------------+------------+----------------+

Thus when you slice from ('20120216', 'finalscore') to ('20120221', 'finalscore') you get both values for '20120216'. To make your query work as you want it to you could change the column_finish to ('20120216', 'halftimescore').