你能加入另一个不是ARel协会的桌子吗？

Question

当我想查找具有分配给它的特定方案的特定页面时(通过scheme_assignment),我有一些SQL正在工作:

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")

当我试图将其转换为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Page模型中的关联吗？可以在没有在模型中创建关联的情况下将SQL转换为ARel吗？

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.*");