hi,
i have a problem with a certain web-application, and wonder if someone
has an idea how to solve it the best way....
imagine the following situation..
class Role(Model):
title = CharField(maxlength=100)
class User(Model):
name = CharField(maxlength=100)
roles = ManyToManyField(Role)
now, i would like to list all the Users and their details on a page,
in a html-table, and in the column for the "roles" i would like all the
roles's titles as comma-separated text.
for example:
Joe | pawn,bishop |
Bill | knight,rook,pawn |
Jane | queen |
as i assume this is a fairly common situation, how would you do it?
of course there is the most straightforward version, where you simply
ask the user instance for it's all roles, and join them by ",".
but this has the problem that if you have 100 users, then you will make
101 queries (1 to list all the users, and one for each user to query the
roles).
is there a faster/better way?
there are 2 ways i have thought about:
1. execute a "raw" sql query which simply selects all the roles for all
the users. then manually build a dictionary from this data (the key
being User.id, and the value is the comma-separated list). and then
access this data in the html page.
2. slightly faster: define an aggregate function for concatenating text
in the database, and do a "raw" sql query which returns a much better
data structure (a list of (user_id,text) pairs), and then access this
data in the html page.
are there any better ways?
my biggest problem with these approaches is:
a. i cannot use the generic view's pagination anymore, because i need
all the user_ids before i call the generic view
b. it's complicated to access such data-structure in the html page. the
best way i came up is to have a custom page-template that gets the data
from the dictionary. at first it seems simple, but then you realize that
if you have a dictionary where user_id => text, then it's not possible
(afaik) to ask in the page template for certain value in the dictionary.
means i cannot do {{ user_roles.{{ user.id }} }} . or can i?
as i said, this seems to be a common situation. how do you approach this
problem?
gabor
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