DataMapper的有n個通過和相關查詢

Question

這裏是我的數據模型：

class A 
    has n, :b 
    has n, :c, through => :b 

def active_c 
    c.active 
end 
end 

class C 
    class << self 
    def active 
     all(self.end_on => nil) + all(:conditions => [ "cs.end_on > applied_to" ]) 
    end 
    end 
end 

的SQL後的樣子：

SELECT `cs`.`id` FROM `cs` INNER JOIN `bs` ON `cs`.`id` = `bs`.`c_id` INNER JOIN `as` ON `bs`.`a_id` = `as`.`id` WHERE (`bs`.`a_id` = 1672 AND `cs`.`end_on` IS NULL) 
SELECT `cs`.`id` FROM `cs` INNER JOIN `bs` ON `cs`.`id` = `bs`.`c_id` INNER JOIN `as` ON `bs`.`a_id` = `as`.`id` WHERE (`bs`.`a_id` = 1672 AND (cs > applied_to)) 
SELECT `id`, `created_at`, `updated_at`, `applied_to`, `end_on` FROM `cs` WHERE (1 = 0 OR 1 = 0) GROUP BY `id`, `created_at`, `updated_at`, `applied_to`, `end_on` ORDER BY `id` 

如果我改變active_c這個IMPL：

def active_c 
    C.all(:bs => {:a_id => self.id}).active 
end 

調用a.active_c的結果SQL是：

SELECT `c_id` FROM `bs` WHERE `a_id` = 1670 
SELECT `c_id` FROM `bs` WHERE `a_id` = 1670 
SELECT `id`, `created_at`, `updated_at`, `applied_to`, `end_on` FROM `cs` WHERE ((1 = 0 AND `end_on` IS NULL) OR (1 = 0 AND (cs.end_on > applied_to))) ORDER BY `id` 

現在兩個問題：

1. 我必須使用cs.end_on在第二個狀態爲爲表也有一欄叫end_on和DM會混淆
2. 爲什麼有3個疑問並且其中1 = 0從

感謝

Answer 1

我試着來設置工作模式，應該在-線W第i個你想達到和這裏有什麼結果：

class A 
    include DataMapper::Resource 

    property :id, Serial 

    has n, :bs 
    has n, :cs, :through => :bs, :via => :c 

    def active_cs 
    cs.active 
    end 
end 

class B 
    include DataMapper::Resource 

    property :id, Serial 

    belongs_to :a 
    belongs_to :c 
end 

class C 
    include DataMapper::Resource 

    property :id, Serial 
    property :end_on, Date 
    property :applied_to, Date 

    has n, :bs 

    def active 
    all(:end_on => nil) + all(:conditions => [ "cs.end_on > applied_to" ]) 
    end 
end 

a = A.create 
b = B.create(:a => a) 
c = C.create(:b => b) 

puts a.active_cs.inspect 

這裏的SQL查詢：

SELECT "id", "end_on", "applied_to" FROM "cs" WHERE ("id" IN (SELECT "cs"."id" FROM "cs" INNER JOIN "bs" ON "cs"."id" = "bs"."c_id" INNER JOIN "as" ON "bs"."a_id" = "as"."id" WHERE ("bs"."a_id" = 1 AND "cs"."end_on" IS NULL)) OR "id" IN (SELECT "cs"."id" FROM "cs" INNER JOIN "bs" ON "cs"."id" = "bs"."c_id" INNER JOIN "as" ON "bs"."a_id" = "as"."id" WHERE ("bs"."a_id" = 1 AND (cs.end_on > applied_to)))) GROUP BY "id", "end_on", "applied_to" ORDER BY "id" 

我不知道如果這正是你所需要的 - 但我希望它能幫助。

這裏有一個工作腳本要點的鏈接：https://gist.github.com/916164