ruby-on-rails - 你能加入另一个在 ARel 中不是关联的表吗？

Question

当我想查找分配有特定方案的特定页面时(全部通过 scheme_assignment)，我有一些正在运行的 SQL:

Page.find_by_sql("
    SELECT pages.id
    FROM pages
    INNER JOIN scheme_assignments ON (scheme_assignments.schemable_type = 'Page' AND
                                      scheme_assignments.schemable_id = pages.sid)
    INNER JOIN schemes ON (schemes.sid = scheme_assignments.scheme_id AND
                           schemes.sid IN (4,6,7))
    GROUP BY pages.id")

当我尝试将其转换为 ARel 时:

Page.select("pages.id")
    .joins("scheme_assignments")
    .on("scheme_assignments.schemable_type = 'Page' AND scheme_assignments.schemable_id = pages.sid")
    .joins("schemes")
    .on("schemes.sid = scheme_assignments.scheme_id AND schemes.sid IN (1)")
    .group("pages.id")

我会得到这个错误:NoMethodError: undefined methodjoin_sql' for "schemes":String`

怎么会？是因为它不是我的 Page 模型中的关联吗？是否可以在不在模型中创建关联的情况下将该 SQL 转换为 ARel？

Answer 1

Page.joins("INNER JOIN scheme_assignments 
        ON (scheme_assignments.schemable_type = 'Page' AND scheme_assignments.schemable_id = pages.sid) 
        INNER JOIN schemes 
        ON (schemes.sid = scheme_assignments.scheme_id AND schemes.sid IN (4,6,7))").
    select("DISTINCT pages.*");