Joins with custom dataset

1

Hey,
I have following use-case:

I want to create a UserMentions-relation, which is derived from a mentions-table by using group by. It should only provide one entry per issue, user and read flag.

    class UserMentions < ROM::Relation[:sql]
      dataset {
        select { [issue_id, user_id, read, max(:created).as(:created)] }
          .group(:issue_id, :user_id, :read)
      }
      schema(:mentions, infer: true, as: :user_mentions) do
        associations do
          belongs_to :issue
          belongs_to :user
        end
      end
    end

Querying from UserMentions works as expected, but if I want to join UserMentions, i.e.

issues.join(:user_mentions)

the resulting SQL looks like this:

INNER JOIN "mentions" ON ("issues"."id" = "mentions"."issue_id")

instead of

INNER JOIN (
  SELECT "issue_id", "user_id", "read", max("created") AS "created"
  FROM "mentions"
  GROUP BY "issue_id", "user_id", "read"
) as "mentions" ON ("issues"."id" = "mentions"."issue_id")

Do you have a hint, how I could solve my issue with ROM?

Thanks!