Rails 6.1: Fix "ActiveRecord::StatementInvalid: A column has been specified more than once in the order by list" (#894)

* add spec showing adapters behaviour

* coerce test avoiding duplicate columns in order list

Author: mgrunberg

test/cases/coerced_tests.rb

@@ -1667,3 +1667,29 @@ def marshal_fixture_path(file_name)
     )
   end
 end
+
+class NestedThroughAssociationsTest < ActiveRecord::TestCase
+  # Same as original but replace order with "order(:id)" to ensure that assert_includes_and_joins_equal doesn't raise
+  # "A column has been specified more than once in the order by list"
+  # Example: original test generate queries like "ORDER BY authors.id, [authors].[id]". We don't support duplicate columns in the order list
+  coerce_tests! :test_has_many_through_has_many_with_has_many_through_habtm_source_reflection_preload_via_joins, :test_has_many_through_has_and_belongs_to_many_with_has_many_source_reflection_preload_via_joins
+  def test_has_many_through_has_many_with_has_many_through_habtm_source_reflection_preload_via_joins_coerced
+    # preload table schemas
+    Author.joins(:category_post_comments).first
+
+    assert_includes_and_joins_equal(
+      Author.where("comments.id" => comments(:does_it_hurt).id).order(:id),
+      [authors(:david), authors(:mary)], :category_post_comments
+    )
+  end
+
+  def test_has_many_through_has_and_belongs_to_many_with_has_many_source_reflection_preload_via_joins_coerced
+    # preload table schemas
+    Category.joins(:post_comments).first
+
+    assert_includes_and_joins_equal(
+      Category.where("comments.id" => comments(:more_greetings).id).order(:id),
+      [categories(:general), categories(:technology)], :post_comments
+    )
+  end
+end

test/cases/order_test_sqlserver.rb

@@ -143,4 +143,11 @@ class OrderTestSQLServer < ActiveRecord::TestCase
     assert_equal post1, Post.order(Arel.sql(order)).first
     assert_equal post2, Post.order(Arel.sql(order)).second
   end
+
+  # Executing this kind of queries will raise "A column has been specified more than once in the order by list"
+  # This test shows that we don't do anything to prevent this
+  it "doesn't deduplicate semantically equal orders" do
+    sql = Post.order(:id).order("posts.id ASC").to_sql
+    assert_equal "SELECT [posts].* FROM [posts] ORDER BY [posts].[id] ASC, posts.id ASC", sql
+  end
 end