SQL/Paginate help needed for has_many :through relationship

rubyonrails-talk
1

Hey all. I'm doing a paginated find with will_paginate, with a fairly complex set of associations and ordering clauses.

Here's the associations relevant to this example:

Question   belongs_to :subject   #standard acts_as_taggable setup   has_many :taggings, :conditions => ["taggable_type => ?", "Question"]   has_many :tags, :through_taggings

What i'm trying to do, is, for a given tag, order the results so that the ones with that tag are at the top, and then the other ones in alphabetical order.

Here's an example of one of the generated find calls:

Question.find(:all, {:per_page=>30,               :conditions=>["questions.subject_id = ?", "9"],               :page=>1,               :order=>"(tags.name = 'piano') desc, tags.name",               :include=>[:subject, {:taggings=>:tag}]})

For the resultant sql, rails splits it into two sql calls. The first uses just the associations referred to in the order list, and gets the ids of the questions. This is where the problem is occurring: here's the first sql which rails generates:

SELECT DISTINCT `questions`.id FROM `questions` LEFT OUTER JOIN `taggings` ON `taggings`.taggable_id = `questions`.id AND `taggings`.taggable_type = 'Question' LEFT OUTER JOIN `tags` ON `tags`.id = `taggings`.tag_id WHERE (questions.subject_id = '9') ORDER BY (tags.name = 'piano') desc, tags.name LIMIT 0, 30

I can copy and run this myself in mysql and see that it's not having the desired result: the 30 question ids that it brings back don't belong to questions which have the 'piano' keyword.

I think that this is a grouping issue, or something similar: i think the 'distinct questions.id' part is interacting with the joined table of questions and taggings in such a way as to cut out the right ids. I think that if i had a working version of the above sql query i could work backwards and set up my find options appropriately.

Any advice, anyone? thanks, max

2

Max,

How about something like…

:order=>“(CASE WHEN tags.name = ‘piano’ THEN 1 ELSE 0 END) desc, tags.name”,

Anthony Crumley

http://commonthread.com

3

Anthony Crumley wrote:

How about something like...

:order=>"(CASE WHEN tags.name = 'piano' THEN 1 ELSE 0 END) desc, tags.name",

Thanks Anthony but that's just a different way of getting the same result. I think my ordering needs to take the join into account somehow, maybe with a 'group by' or something but i can't get my head around it :confused:

4

How about creating a method in your model that will do two finds. The first will return all questions with that tag. The second will return those without that tag.

Have this model return the sum of the two finds.

Using two named_scopes called with_tag and without_tag might be a nice way of doing this, and then in your class method

def self.with_tag_on_top(tag)   self.with_tag(tag) + self.without_tag(tag) end

so in your controller you can have something like

@questions = Question.with_tag_on_top(params[:tag]).paginate(:page => params[:page])

Franz

5

Anthony Crumley wrote:

Max,

How about something like...

:order=>"(CASE WHEN tags.name = 'piano' THEN 1 ELSE 0 END) desc, tags.name",

Anthony Crumley http://commonthread.com

Hi Max!

I have the same problem of this post

How did you solve that?

Thanks in advance, Raphael Costa