CODEWITHSUNDEEP
ruby-on-railsactiverecord
kyku
kyku

Reputation: 6042

Eager loading last subordinate record

Suppose that model X has_many Ys and model Y belongs_to X. Each Y has a date when it has been inserted to the database. Now, is it possible to load all Xs and eager load last Y for each of X (only the last because each X has bazzilions of Ys)?

Upvotes: 4

Views: 763

Answers (2)

Tilendor
Tilendor

Reputation: 48923

Yes,

Lets take this example

class Song < ActiveRecord::Base
   has_many :votes
end

class Vote < ActiveRecord::Base
  belongs_to :song
end

Add a new association to the Song

# for Rails version <4:
has_one :last_vote, :class_name => 'Vote', :order => 'created_at DESC'

# for Rails version 4+
has_one :last_vote, -> { order(created_at: :desc) }, class_name: 'Vote'

this new association will always return the most recently created Vote for a Song.

To eager load it

# for Rails version <3:
songs = Song.find(:all, :conditions => 'artist_name = "frank"', :include => :last_vote)

# for Rails version 3+:
songs = Song.includes(:last_vote).where(artist_name: 'frank')

Upvotes: 9

kyku
kyku

Reputation: 6042

The previous solution is really great! However, I'm looking for one more feature: is it possible to eager load last subordinate record before a specific date? This is needed so that my users can browse a history a song's votes (with links to previous and next days).

Upvotes: 0

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