Merge pull request #45586 from eileencodes/check-for-cte-support

Fix cases where CTE's are not supported

activerecord/lib/active_record/relation/query_methods.rb

@@ -326,6 +326,9 @@ module ActiveRecord
 
     # Add a Common Table Expression (CTE) that you can then reference within another SELECT statement.
     #
+    # Note: CTE's are only supported in MySQL for versions 8.0 and above. You will not be able to
+    # use CTE's with MySQL 5.7.
+    #
     #   Post.with(posts_with_tags: Post.where("tags_count > ?", 0))
     #   # => ActiveRecord::Relation
     #   # WITH posts_with_tags AS (

activerecord/test/cases/relation/merging_test.rb

@@ -394,6 +394,7 @@ class MergingDifferentRelationsTest < ActiveRecord::TestCase
     assert_equal dev.ratings, [rating_1]
   end
 
+  if ActiveRecord::Base.connection.supports_common_table_expressions?
     test "merging relation with common table expression" do
       posts_with_tags = Post.with(posts_with_tags: Post.where("tags_count > 0")).from("posts_with_tags AS posts")
       posts_with_comments = Post.where("legacy_comments_count > 0")
@@ -421,3 +422,4 @@ class MergingDifferentRelationsTest < ActiveRecord::TestCase
       end
     end
   end
+end

activerecord/test/cases/relation/with_test.rb

@@ -15,6 +15,7 @@ module ActiveRecord
     POSTS_WITH_TAGS_AND_COMMENTS = (POSTS_WITH_COMMENTS & POSTS_WITH_TAGS).sort.freeze
     POSTS_WITH_TAGS_AND_MULTIPLE_COMMENTS = (POSTS_WITH_MULTIPLE_COMMENTS & POSTS_WITH_TAGS).sort.freeze
 
+    if ActiveRecord::Base.connection.supports_common_table_expressions?
       def test_with_when_hash_is_passed_as_an_argument
         relation = Post
           .with(posts_with_comments: Post.where("legacy_comments_count > 0"))
@@ -60,5 +61,12 @@ module ActiveRecord
         assert_raise(ArgumentError) { Post.with(posts_with_tags: nil).load }
         assert_raise(ArgumentError) { Post.with(posts_with_tags: [Post.where("tags_count > 0")]).load }
       end
+    else
+      def test_common_table_expressions_are_unsupported
+        assert_raises ActiveRecord::StatementInvalid do
+          Post.with_tags_cte.order(:id).pluck(:id)
+        end
+      end
+    end
   end
 end