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Diffstat (limited to 'activerecord/lib/active_record/associations.rb')
| -rwxr-xr-x | activerecord/lib/active_record/associations.rb | 35 |
1 files changed, 35 insertions, 0 deletions
diff --git a/activerecord/lib/active_record/associations.rb b/activerecord/lib/active_record/associations.rb index 9c753f0514..40eeb481e5 100755 --- a/activerecord/lib/active_record/associations.rb +++ b/activerecord/lib/active_record/associations.rb @@ -108,6 +108,41 @@ module ActiveRecord # project.milestones(true).size # fetches milestones from the database # project.milestones # uses the milestone cache # + # == Eager loading of associations + # + # Eager loading is a way to find objects of a certain class and a number of named associations along with it in a single SQL call. This is + # one of the easiest ways of to prevent the dreaded 1+N problem in which fetching 100 posts that each needs to display their author + # triggers 101 database queries. Through the use of eager loading, the 101 queries can be reduced to 1. Example: + # + # class Post < ActiveRecord::Base + # belongs_to :author + # has_many :comments + # end + # + # Consider the following loop using the class above: + # + # for post in Post.find(:all, :limit => 100) + # puts "Post: " + post.title + # puts "Written by: " + post.author.name + # puts "Last comment on: " + post.comments.first.created_on + # end + # + # To iterate over these one hundred posts, we'll generate 201 database queries. Let's first just optimize it for retrieving the author: + # + # for post in Post.find(:all, :limit => 100, :include => :author) + # + # This references the name of the belongs_to association that also used the :author symbol, so the find will now weave in a join something + # like this: LEFT OUTER JOIN authors ON authors.id = posts.author_id. Doing so will cut down the number of queries from 201 to 101. + # + # We can improve upon the situation further by referencing both associations in the finder with: + # + # for post in Post.find(:all, :limit => 100, :include => [ :author, :comments ]) + # + # That'll add another join along the lines of: LEFT OUTER JOIN comments ON comments.post_id = posts.id. And we'll be down to 1 query. + # But that shouldn't fool you to think that you can pull out huge amounts of data with no performance penalty just because you've reduced + # the number of queries. The database still needs to send all the data to Active Record and it still needs to be processed. So its no + # catch-all for performance problems, but its a great way to cut down on the number of queries in a situation as the one described above. + # # == Modules # # By default, associations will look for objects within the current module scope. Consider: |