Preserve duplicate columns in ActiveRecord::Base#select

Author: fatkodima

activerecord/lib/active_record/relation/merger.rb

@@ -85,9 +85,9 @@ def merge_select_values
           return if other.select_values.empty?
 
           if other.model == relation.model
-            relation.select_values |= other.select_values
+            relation.select_values += other.select_values
           else
-            relation.select_values |= other.instance_eval do
+            relation.select_values += other.instance_eval do
               arel_columns(select_values)
             end
           end

activerecord/lib/active_record/relation/query_methods.rb

@@ -426,7 +426,7 @@ def select(*fields)
     end
 
     def _select!(*fields) # :nodoc:
-      self.select_values |= fields
+      self.select_values += fields
       self
     end
 

activerecord/test/cases/relation/select_test.rb

@@ -115,6 +115,14 @@ def test_select_with_hash_argument_with_few_tables
       assert_not_nil post.post_title
     end
 
+    def test_select_preserves_duplicate_columns
+      quoted_posts_id = Regexp.escape(quote_table_name("posts.id"))
+      quoted_posts = Regexp.escape(quote_table_name("posts"))
+      assert_queries_match(/SELECT #{quoted_posts_id}, #{quoted_posts_id} FROM #{quoted_posts}/i) do
+        Post.select(:id, :id).to_a
+      end
+    end
+
     def test_reselect
       expected = Post.select(:title).to_sql
       assert_equal expected, Post.select(:title, :body).reselect(:title).to_sql

activerecord/test/cases/relation_test.rb

@@ -50,7 +50,7 @@ def test_multi_values_deduplication_with_merge
       }
       expected.default = [ Object.new ]
 
-      Relation::MULTI_VALUE_METHODS.each do |method|
+      (Relation::MULTI_VALUE_METHODS - [:select]).each do |method|
         getter, setter = "#{method}_values", "#{method}_values="
         values = expected[method]
         relation = Relation.new(FakeKlass)
@@ -293,6 +293,15 @@ def test_relation_merging_with_joins_as_join_dependency_pick_proper_parent
       assert_equal 3, relation.where(id: post.id).pluck(:id).size
     end
 
+    def test_merge_preserves_duplicate_columns
+      quoted_posts_id = Regexp.escape(quote_table_name("posts.id"))
+      quoted_posts = Regexp.escape(quote_table_name("posts"))
+      posts = Post.select(:id)
+      assert_queries_match(/SELECT #{quoted_posts_id}, #{quoted_posts_id} FROM #{quoted_posts}/i) do
+        posts.merge(posts).to_a
+      end
+    end
+
     def test_merge_raises_with_invalid_argument
       assert_raises ArgumentError do
         relation = Relation.new(FakeKlass)