[postmodern-devel] Help with query replacement

[Vamsee Kanakala]

Marijn Haverbeke wrote:
> 
> Unfortunately I don't quite understand what you are trying to do, but
> it seems that in the simple case something like this (untested) might
> work...
> 

Sorry, I forgot that using the word paging without the context can be
confusing. By paging, I meant paging in the web pages - if I have 100
results to a query, I would like to show that to the user in chunks of
20 rows over 5 pages, to avoid huge requests and make the user's life
easier.

To avoid putting this 100-row result in the session and then paging over
it (can be quite inefficient if there are lots of results), I want to
retrieve the count of results, and then retrieve small piece of them
according to page numbers with :offset and :limit.

So something like this:

(query (:select 'id 'name :from 'cities))

Would turn into:

(query-dao 'city (:limit (:offset (:order-by 								   (:select 'id
'name :from 'cities)								   'name) offset) limit))

But to calculate how many pages I have to create links to, I need to
have the number of results before the query actually runs. So, number of
results divided by limit should give me the number of pages this result
set would run into.

But to avoid writing a similar query twice, I'm trying to change the
first query into this (the aim is to write a macro which does this
automatically), to get the number of rows:

(query (:select :raw("count(*)") :from 'cities) :single)

Also, the element :count was not added to postmodern - I have checked
the source too. So, I have to use :raw for now - please correct me if
I'm wrong.

> (defun query-subst (query)
>  (let ((from (position :from query)))
>    `(:select (:count '*) ,@(nthcdr from query)))
> 

Perhaps you can already see that this does not help - position cannot
look into trees (works only with sequences) and the select statement can
be arbitrarily long. I think my problem can be simply put this way: I
need to replace everything between :select and :from with a single
(:count *) or (:raw "count(*)").

Perhaps I'm being finicky - I'm just looking to cut some extra lines of
code and move the entire paging-related code into one macro.

Thanks much,
Vamsee.