From 532f915037ab65873c433e30399d11f93df9f1f8 Mon Sep 17 00:00:00 2001
From: Jon Leighton <j@jonathanleighton.com>
Date: Mon, 7 Mar 2011 00:04:06 +0000
Subject: Referencing a table via the ON condition in a join should force that
 table to be eager-loaded via a JOIN rather than via subsequent queries.

---
 activerecord/lib/active_record/relation.rb | 13 ++++++++++++-
 activerecord/test/cases/relations_test.rb  | 15 +++++++++++++++
 2 files changed, 27 insertions(+), 1 deletion(-)

diff --git a/activerecord/lib/active_record/relation.rb b/activerecord/lib/active_record/relation.rb
index f939bedc81..5af20bf38b 100644
--- a/activerecord/lib/active_record/relation.rb
+++ b/activerecord/lib/active_record/relation.rb
@@ -407,8 +407,19 @@ module ActiveRecord
     private
 
     def references_eager_loaded_tables?
+      joined_tables = arel.join_sources.map do |join|
+        if join.is_a?(Arel::Nodes::StringJoin)
+          tables_in_string(join.left)
+        else
+          [join.left.table_name, join.left.table_alias]
+        end
+      end
+
+      joined_tables += [table.name, table.table_alias]
+
       # always convert table names to downcase as in Oracle quoted table names are in uppercase
-      joined_tables = (tables_in_string(arel.join_sql) + [table.name, table.table_alias]).compact.map{ |t| t.downcase }.uniq
+      joined_tables = joined_tables.flatten.compact.map { |t| t.downcase }.uniq
+
       (tables_in_string(to_sql) - joined_tables).any?
     end
 
diff --git a/activerecord/test/cases/relations_test.rb b/activerecord/test/cases/relations_test.rb
index 37bbb17e74..948fcf7012 100644
--- a/activerecord/test/cases/relations_test.rb
+++ b/activerecord/test/cases/relations_test.rb
@@ -850,4 +850,19 @@ class RelationTest < ActiveRecord::TestCase
   def test_primary_key
     assert_equal "id", Post.scoped.primary_key
   end
+
+  def test_eager_loading_with_conditions_on_joins
+    scope = Post.includes(:comments)
+
+    # This references the comments table, and so it should cause the comments to be eager
+    # loaded via a JOIN, rather than by subsequent queries.
+    scope = scope.joins(
+      Post.arel_table.create_join(
+        Post.arel_table,
+        Post.arel_table.create_on(Comment.arel_table[:id].eq(3))
+      )
+    )
+
+    assert scope.eager_loading?
+  end
 end
-- 
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