From c24e5548fd372e4d0058ab417230ebc2fa3bebde Mon Sep 17 00:00:00 2001 From: Ryan Bigg Date: Tue, 22 Mar 2011 12:16:13 +1100 Subject: Querying guide: mention that performing a where on an relation that contains an includes statement will generate a LEFT OUTER JOIN rather than an INNER JOIN or another query --- railties/guides/source/active_record_querying.textile | 14 ++++++++++++++ 1 file changed, 14 insertions(+) (limited to 'railties') diff --git a/railties/guides/source/active_record_querying.textile b/railties/guides/source/active_record_querying.textile index ed3968e226..e0fab46db9 100644 --- a/railties/guides/source/active_record_querying.textile +++ b/railties/guides/source/active_record_querying.textile @@ -747,6 +747,20 @@ h4. Specifying Conditions on Eager Loaded Associations Even though Active Record lets you specify conditions on the eager loaded associations just like +joins+, the recommended way is to use "joins":#joining-tables instead. +However if you must do this, you may use +where+ as you would normally. + + +Post.includes(:comments).where("comments.visible", true) + + +This would generate a query which contains a +LEFT OUTER JOIN+ whereas the +joins+ method would generate one using the +INNER JOIN+ function instead. + + + SELECT "posts"."id" AS t0_r0, ... "comments"."updated_at" AS t1_r5 FROM "posts" LEFT OUTER JOIN "comments" ON "comments"."post_id" = "posts"."id" WHERE (comments.visible) + + +If in this case there were no comments for any posts, all the posts would still be loaded. By using +joins+ (an INNER JOIN), the join conditions *must* match, otherwise no records will be returned. + h3. Scopes Scoping allows you to specify commonly-used ARel queries which can be referenced as method calls on the association objects or models. With these scopes, you can use every method previously covered such as +where+, +joins+ and +includes+. All scope methods will return an +ActiveRecord::Relation+ object which will allow for further methods (such as other scopes) to be called on it. -- cgit v1.2.3