How to solve N + 1 issue in select box? - sql

Question

In my Rails app I have people which can have many projects and vice versa. The two tables are linked by a join table jobs.
class Project < ActiveRecord::Base
belongs_to :user
has_many :people, :through => :jobs
def self.names_as_options
order(:name).map{ |p| [ p.name, p.id, :'data-people_count' => p.people.count ] }
end
end
In one of my forms I have this select box:
<%= f.select :project_id, Project.names_as_options %>
The problem is that the count on people gives me an N + 1 query for each project.
What is the best way to overcome this?
Thanks for any help.

Answer 1

Try use scope whit lambda, is for that, here is one example how this works:
scope :top, lambda { order('views DESC').limit(20) }
in controller just call
Project.top
this is the best way to filter results in Ruby on Rails.

Answer 2

If you use counts, you might better use also counter caches, they are automatically used when needed. http://guides.rubyonrails.org/association_basics.html

Answer 3

You could add a "counter cache" of the people count for each project. Ordinarily you would add a field to the projects table via a migration
add_column :projects, :people_count, :integer, :default => 0
and then declare to use :counter_cache in the Person model
class Person < ActiveRecord::Base
belongs_to :projects, :counter_cache => true
end
This probably won't do what you want as it stands, as you are going through a Job join. So, the Person#projects declaration is just a convenient finder, and not used in any callback. But, you get the idea.
You could add a column as suggested above, and then make use of some callback methods in the Job class.
class Job
def update_project_counter
project.update_people_counter
end
after_create :update_project_counter
after_destroy :update_project_counter
end
class Project
def update_people_counter
self.update_attribute :people_count, people.count
end
end
Or something similar thats appropriate. You should then only need the one query.
class Project < ActiveRecord::Base
def self.names_as_options
order(:name).map do |p|
[p.name, p.id, :'data-people_count' => p.people_count]
end
end
end

Answer 4

Eager loading will solve this issue, use 'includes' as follows.
Example,
class LineItem < ActiveRecord::Base
belongs_to :order, -> { includes :customer }
end
class Order < ActiveRecord::Base
belongs_to :customer
has_many :line_items
end
class Customer < ActiveRecord::Base
has_many :orders
end
Ref: http://guides.rubyonrails.org/association_basics.html