CODEWITHSUNDEEP
ruby-on-rails-3activerecordjoin
Tom
Tom

Reputation: 4087

Rails ActiveRecord...how make a join

sorry for my beginner question but i'm trying to learn RoR. In Rails 3.2 i've declared: 

class Project < ActiveRecord::Base
   attr_accessible :name, :description
   has_many :subprojects
end


class SubProject < ActiveRecord::Base
   attr_accessible :id_name, :description, :num_alloc, :project_id
   belongs_to :projects
end

How can i show, in a view, a table with the attributes id_name, num_alloc (from SubProject) and name (from Project)

How can i make the join? In the controller, if i make:

@results=  SubProject.joins('LEFT OUTER JOIN.......)

this, return only the SubProject attribute right?

Thanks

Upvotes: 1

Views: 102

Answers (1)

MurifoX
MurifoX

Reputation: 15089

There is a thing called eager loading. When you make a query with a join, and create the objects, these associations are prepopulated. For example: 

@results = SubProject.joins(:project)

In your view, or any other place, if you call the project object inside subproject, you will have direct access to its content, without making another query. 

@results.each do |r|
  puts r.project.name # The project is prepopulated from the db
end

If you don't care about performance, you can just fecth the subprojects and for each one of them query the project inside.

@results = SubProject.all

@results.each do |r|
  puts r.project.name # This will make other query
end

Upvotes: 2

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