CODEWITHSUNDEEP
ruby-on-railsactiverecord
wolak
wolak

Reputation: 739

has_many :through association through two different associations

I have four model classes:

class Group < ActiveRecord::Base
  has_many :projects
  has_many :personal_blogs
end

class Project < ActiveRecord::Base
  has_many :events, :as => :event_producer
end

class PersonalBlog < ActiveRecord::Base
  has_many :events, :as => :event_producer
end

class Event < ActiveRecord::Base
  belongs_to :event_producer, :polymorphic => true
end

I want to find all of the events for a particular group. I figure this is a has_many :through association, but how do I specify a has_many on Group that finds all events in the projects or personal_blogs of a group? I could, of course, specify two associations and concatenate the results, but then I have to re-sort, limit, condition, etc. in Ruby, which could potentially be a performance nightmare with many events. I'd like to do this within ActiveRecord to avoid such a nightmare.

Upvotes: 3

Views: 1911

Answers (3)

Lief
Lief

Reputation: 593

If you need an association that you can preload you could specify a custom 'from' query that includes the group id with the event.

class Group < ActiveRecord::Base
  has_many :projects
  has_many :personal_blogs
  has_many :events, -> {from(<<~SQL)}
    (
      SELECT DISTINCT e.*, g.id as group_id
      FROM events e
        LEFT JOIN projects p ON p.event_id = e.id
        LEFT JOIN personal_blogs pb ON pb.event_id = e.id
        INNER JOIN groups g ON g.id IN (p.group_id, pb.group_id)
    ) as events
  SQL
end

Upvotes: 0

Midwire
Midwire

Reputation: 1100

Why not just do:

class Group < ActiveRecord::Base
  has_many :projects
  has_many :personal_blogs

  def all_events
    projects.events + personal_blogs.events
  end
end

Upvotes: 0

fjuan
fjuan

Reputation: 2324

You could define a method in the Group class like next:

class Group < ActiveRecord::Base
  has_many :projects
  has_many :personal_blogs

  def events
    Event.find(:all, :conditions => ['(type = ? AND event_producer_id IN (?)) OR (type = ? AND event_producer IN (?))', 'project', project_ids, 'personal_blog', personal_blog_ids])
  end
end

If you don't like SQL like the previous one, it's always possible to use Single Table Inheritance. This solution depends on your classes attributes and behavior, but will allow you to use a "has_many through" association.

Upvotes: 5

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