Fix #3271.

Building the conditions of a nested through association could
potentially modify the conditions of the through and/or source
association.

This is a Bad Thing.

activerecord/CHANGELOG

@@ -39,6 +39,13 @@
 
 *Rails 3.1.2 (unreleased)*
 
+* Fix bug where building the conditions of a nested through association could potentially
+  modify the conditions of the through and/or source association. If you have experienced
+  bugs with conditions appearing in the wrong queries when using nested through associations,
+  this probably solves your problems. [GH #3271]
+
+  [Jon Leighton]
+
 * If a record is removed from a has_many :through, all of the join records relating to that
   record should also be removed from the through association's target.
 

activerecord/lib/active_record/reflection.rb

@@ -433,7 +433,7 @@ def chain
       # of relevant reflections, plus any :source_type or polymorphic :as constraints.
       def conditions
         @conditions ||= begin
-          conditions = source_reflection.conditions
+          conditions = source_reflection.conditions.map { |c| c.dup }
 
           # Add to it the conditions from this reflection if necessary.
           conditions.first << options[:conditions] if options[:conditions]

activerecord/test/cases/reflection_test.rb

@@ -319,6 +319,12 @@ def test_foreign_key
     assert_equal "category_id", Post.reflect_on_association(:categorizations).foreign_key.to_s
   end
 
+  def test_through_reflection_conditions_do_not_modify_other_reflections
+    orig_conds = Post.reflect_on_association(:first_blue_tags_2).conditions.inspect
+    Author.reflect_on_association(:misc_post_first_blue_tags_2).conditions
+    assert_equal orig_conds, Post.reflect_on_association(:first_blue_tags_2).conditions.inspect
+  end
+
   private
     def assert_reflection(klass, association, options)
       assert reflection = klass.reflect_on_association(association)