Fix "last equality wins" logic in relation merge

This is a real fix (as compared to the band-aid in b127d86), which uses
the recently-added equality methods for ARel nodes. It has the side
benefit of simplifying the merge code a bit.

activerecord/lib/active_record/relation/merger.rb

@@ -97,18 +97,13 @@ def merged_wheres
           merged_wheres = relation.where_values + values[:where]
 
           unless relation.where_values.empty?
-            # Remove duplicate ARel attributes. Last one wins.
-            seen = Hash.new { |h,table| h[table] = {} }
+            # Remove equalities with duplicated left-hand. Last one wins.
+            seen = {}
             merged_wheres = merged_wheres.reverse.reject { |w|
               nuke = false
-              # We might have non-attributes on the left side of equality nodes,
-              # so we need to make sure they quack like an attribute.
-              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]
-                seen[table][name] = true
+              if w.respond_to?(:operator) && w.operator == :==
+                nuke         = seen[w.left]
+                seen[w.left] = true
               end
               nuke
             }.reverse

activerecord/test/cases/relations_test.rb

@@ -675,11 +675,14 @@ def test_relation_merging_with_arel_equalities_keeps_last_equality
     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
+  def test_relation_merging_with_arel_equalities_keeps_last_equality_with_non_attribute_left_hand
     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))
+    devs = Developer.where(
+      Arel::Nodes::NamedFunction.new('abs', [salary_attr]).eq(80000)
+    ).merge(
+      Developer.where(
+        Arel::Nodes::NamedFunction.new('abs', [salary_attr]).eq(9000)
+      )
     )
     assert_equal [developers(:poor_jamis)], devs.to_a
   end