Association count sorting

rubyonrails-talk
1

I have 3 models GrandPa, Pa, Kid

GrandPa => has_many :pas Pa => has_many kids

When I list GrandPa I would like to present the following

2

Kid has one pa and has one grandpa through pa

Pa has one to many kids and has one grandpa

Grandpa has one to many kids and has one pa

<%= :name[gp]

?pa = kid if from gp?

I think you should just have one model with a foreign key to parent and quantity of child including nil.

3

Thanks.

Grandpa has one to many kids and has one pa

No, Grandpa has_many pas

I didnt quite get how you proposed listing the pas (and kid count for each pa) that belong to a certain grandpa

4

Thanks.

Grandpa has one to many kids and has one pa

No, Grandpa has_many pas How is this possible? That’s like saying I have two left arms.

I didnt quite get how you proposed listing the pas (and kid count for

each pa) that belong to a certain grandpa Because the sort is to easy to explain. What you’re sorting and why is the problem. So, now that I have two left arms how do I sort them by thumb length on my right arm. Well, I guess get a mirror.

5

Here is the answer if someone else runs into this..

GrandPa.all do |gpa| p gpa.name gpa.pas.all(:joins => :kids, :select => "pas.*, count(pas.id) AS kid_count"        :group => :id, :order => "kid_count DESC").each do |pa|    p "#{pa.name} : #{pa.kid_count}" end end

6

A crude way (untested), I am sure there are better as I suspect this will call count zillions of times. It should get you going and you can refactor when you want to. grandpa.pas will give you an array of pas, then simply sort this array by the number of kids. sorted = grandpa.pas.sort_by { |p| p.kids.count }

Colin

7

Your solution may certainly work... but is a bit "imperative" in coding terms, and does introduce an n+1 problem.

Rails offers a feature called "counter_cache" on associations - which gives you a column in associate keeping track of the amount of "belonging" records.

Have a look for it in the manual here: http://rails.rubyonrails.org/classes/ActiveRecord/Associations/ClassMethods.html

If you use it, you can then just do a normal find without the block and order by the counter-cache column, which avoids all the extra SQL counts.