merge doesn't allow to overwrite partially matching nodes

This makes `merge rewhere: true` follow the original `merge`'s behavior.

activerecord/lib/active_record/relation/where_clause.rb

@@ -101,13 +101,12 @@ def contradiction?
       end
 
       def extract_attributes
-        attrs = []
-        predicates.each do |node|
-          Arel.fetch_attribute(node) { |attr| attrs << attr } || begin
-            attrs << node.left if node.equality? && node.left.is_a?(Arel::Predications)
+        predicates.each_with_object([]) do |node, attrs|
+          attr = extract_attribute(node) || begin
+            node.left if node.equality? && node.left.is_a?(Arel::Predications)
           end
+          attrs << attr if attr
         end
-        attrs
       end
 
       protected
@@ -121,6 +120,15 @@ def referenced_columns
         end
 
       private
+        def extract_attribute(node)
+          attr_node = nil
+          Arel.fetch_attribute(node) do |attr|
+            return if attr_node&.!= attr # all attr nodes should be the same
+            attr_node = attr
+          end
+          attr_node
+        end
+
         def equalities(predicates)
           equalities = []
 

activerecord/test/cases/relation/merging_test.rb

@@ -36,6 +36,11 @@ def test_merge_in_clause
 
     assert_equal [mary, bob], david_and_mary.merge(mary_and_bob, rewhere: true)
     assert_equal [david, mary], mary_and_bob.merge(david_and_mary, rewhere: true)
+
+    david_and_bob = Author.where(id: david).or(Author.where(name: "Bob"))
+
+    assert_equal [david], david_and_mary.merge(david_and_bob)
+    assert_equal [david], david_and_mary.merge(david_and_bob, rewhere: true)
   end
 
   def test_merge_between_clause
@@ -61,6 +66,11 @@ def test_merge_between_clause
 
     assert_equal [mary, bob], david_and_mary.merge(mary_and_bob, rewhere: true)
     assert_equal [david, mary], mary_and_bob.merge(david_and_mary, rewhere: true)
+
+    david_and_bob = Author.where(id: david).or(Author.where(name: "Bob"))
+
+    assert_equal [david], david_and_mary.merge(david_and_bob)
+    assert_equal [david], david_and_mary.merge(david_and_bob, rewhere: true)
   end
 
   def test_merge_or_clause
@@ -86,6 +96,11 @@ def test_merge_or_clause
 
     assert_equal [mary, bob], david_and_mary.merge(mary_and_bob, rewhere: true)
     assert_equal [david, mary], mary_and_bob.merge(david_and_mary, rewhere: true)
+
+    david_and_bob = Author.where(id: david).or(Author.where(name: "Bob"))
+
+    assert_equal [david], david_and_mary.merge(david_and_bob)
+    assert_equal [david], david_and_mary.merge(david_and_bob, rewhere: true)
   end
 
   def test_merge_not_in_clause