Fix merge error when Equality LHS is non-attribute.

This is reworking of rails/rails/pull/7380 made for rails 3.

activerecord/CHANGELOG.md

@@ -1,5 +1,10 @@
 ## Rails 3.2.14 (Jul 22, 2013) ##
 
+*   Fix merge error when Equality LHS is non-attribute.
+    Backport of #7380.
+
+    *Karmes Alexander*
+
 *   Do not re-create destroyed association when saving the parent object.
 
     Fixes #11450.

activerecord/lib/active_record/relation/spawn_methods.rb

@@ -32,11 +32,12 @@ module ActiveRecord
       merged_wheres = @where_values + r.where_values
 
       unless @where_values.empty?
-        # Remove duplicates, last one wins.
+        # Remove duplicate ARel attributes. Last one wins.
         seen = Hash.new { |h,table| h[table] = {} }
         merged_wheres = merged_wheres.reverse.reject { |w|
           nuke = false
-          if w.respond_to?(:operator) && w.operator == :==
+          if w.respond_to?(:operator) && w.operator == :== &&
+            w.left.respond_to?(:relation)
             name              = w.left.name
             table             = w.left.relation.name
             nuke              = seen[table][name]

activerecord/test/cases/relations_test.rb

@@ -668,6 +668,21 @@ class RelationTest < ActiveRecord::TestCase
     assert_equal [developers(:poor_jamis)], dev_with_count.to_a
   end
 
+  def test_relation_merging_with_arel_equalities_keeps_last_equality
+   devs = Developer.where(Developer.arel_table[:salary].eq(80000)).merge(
+     Developer.where(Developer.arel_table[:salary].eq(9000))
+   )
+   assert_equal [developers(:poor_jamis)], devs.to_a
+ end
+
+  def test_relation_merging_with_arel_equalities_with_a_non_attribute_left_hand_ignores_non_attributes_when_discarding_equalities
+    salary_attr = Developer.arel_table[:salary]
+    devs = Developer.where(salary_attr.eq(80000)).merge(
+      Developer.where(salary_attr.eq(9000)).where(Arel::Nodes::NamedFunction.new('abs', [salary_attr]).eq(9000))
+    )
+    assert_equal [developers(:poor_jamis)], devs.to_a
+ end
+
   def test_relation_merging_with_eager_load
     relations = []
     relations << Post.order('comments.id DESC').merge(Post.eager_load(:last_comment)).merge(Post.scoped)