Hello all,
I know people have asked about custom aggregates/annotations before, but
I haven't quite been able to figure out how to translate those
discussions into what I need (or even if a custom aggregate is the way
to go). Any advice would be much appreciated.
What I would like to do is sequentially number the rows in a query,
within subgroups, as follows. Suppose I have three columns, subject_id,
experiment_id, and date, and the query is ordered by those columns
respectively (a la queryset.order('subject__id', 'experiment__id',
'date')). I would like each record in the result to be annotated with a
session_number that describes that record's sequence within the set of
records with the same experiment_id and subject_id. Here is an example
of what I mean:
| subject_id | experiment_id | date | session_number |
---------------------------------------------------------
| 1 | 4 | 5/27/09 | 0 |
| 1 | 4 | 5/28/09 | 1 |
| 1 | 4 | 5/29/09 | 2 |
---------------------------------------------------------
| 1 | 5 | 3/2/09 | 0 |
| 1 | 5 | 3/5/09 | 1 |
---------------------------------------------------------
| 2 | 4 | 5/22/09 | 0 |
| 2 | 4 | 5/23/09 | 1 |
and so on.
I confess I'm a bit lost as to where to begin with this. It seems like
the kind of thing that a relational database should be exceedingly good
at, so I would hope that a custom annotation wouldn't be hard to come up
with, but I don't know much about the ins and outs of SQL functions or
the right way to calculate what I want.
Thanks for any help you can provide!
Best,
Richard
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