Fix cases where CTE's are not supported

CTE's are not supported for MySQL versions below 8.0, so these tests
were failing locally for me on MySQL 5.7. While 5.7 is quite old, it's
still supported and used by many major Rails applications and we need a
check for support in our tests.

In addition to this PR I'll need to get a buildkite build running for
5.7. This wasn't caught earlier because our buildkite upgraded to 8.0.

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")
@@ -420,4 +421,5 @@ class MergingDifferentRelationsTest < ActiveRecord::TestCase
         relation.load
       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