Add example for ActiveRecord::Base.joins using a string as argument

activerecord/lib/active_record/relation/query_methods.rb

@@ -305,6 +305,11 @@ module ActiveRecord
     #
     #   User.joins(:posts)
     #   => SELECT "users".* FROM "users" INNER JOIN "posts" ON "posts"."user_id" = "users"."id"
+    #
+    # You can use strings in order to customize your joins:
+    #
+    #   User.joins("LEFT JOIN bookmarks ON bookmarks.bookmarkable_type = 'Post' AND bookmarks.user_id = users.id")
+    #   => SELECT "users".* FROM "users" LEFT JOIN bookmarks ON bookmarks.bookmarkable_type = 'Post' AND bookmarks.user_id = users.id
     def joins(*args)
       args.compact.blank? ? self : spawn.joins!(*args.flatten)
     end