Fix merge error when Equality LHS is non-attribute

This is at best a band-aid for a more proper fix, since it won't truly
handle the removal of the previous equality condition of these other
nodes. I'm planning to put in some work on ARel toward supporting that
goal.

Related: rails/arel#130, ernie/squeel#153, ernie/squeel#156

2 files changed, 21 insertions, 2 deletions

diff --git a/activerecord/lib/active_record/relation/merger.rb b/activerecord/lib/active_record/relation/merger.rb
index 71aaedee1e..7531f22494 100644
--- a/activerecord/lib/active_record/relation/merger.rb
+++ b/activerecord/lib/active_record/relation/merger.rb
@@ -97,11 +97,14 @@ module ActiveRecord
           merged_wheres = relation.where_values + values[:where]
 
           unless relation.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 == :==
+              # 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]
diff --git a/activerecord/test/cases/relations_test.rb b/activerecord/test/cases/relations_test.rb
index 84027ea5ae..4ef11590a9 100644
--- a/activerecord/test/cases/relations_test.rb
+++ b/activerecord/test/cases/relations_test.rb
@@ -668,6 +668,22 @@ 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.all)