如何在DataMapper中爲同一模型創建兩個多對多關係？

Question

編輯：已更新的問題顯示我使用的:child_key => [:comparison_id]正如評論中所建議的。

我有兩個模型，如下所示：

class Comparison 
    include DataMapper::Resource 
    property :id, Serial 
end 

class Msrun 
    include DataMapper::Resource 
    property :id, Serial 
    property :name, String 
end 

比較來自比較兩組Msruns的。我認爲我會通過比較到Msrun的兩個多對多關係來表示這種情況，但是我在DataMapper中如何做到這一點，正在打破我的觀點。我知道很多一對多的關係都可以通過添加這樣的事情：

has n, :whatevers, :through => Resource 

然而，這樣只會讓兩個模型之間的一個多到多的關係。我也曾嘗試創建兩個連接模式和手動指定的關係，以及手工指定的，像這樣每個關係的子鍵：

# Join model for the comparison-msrun many-to-many relationship. 
class First 
    include DataMapper::Resource 
    belongs_to :msrun, :key => true 
    belongs_to :comparison, :key => true 
end 


# Join model for the comparison-msrun many-to-many relationship. 
class Second 
    include DataMapper::Resource 
    belongs_to :msrun, :key => true 
    belongs_to :comparison, :key => true 
end 

class Comparison 
    include DataMapper::Resource 
    property :id, Serial 

    has n, :firsts 
    has n, :msrun_firsts, 'Msrun', :through => :firsts, :child_key => [:msrun_id] 

    has n, :seconds 
    has n, :msruns_seconds, 'Msrun', :through => :seconds, :child_key => [:msrun_id] 
end 

class Msrun 
    include DataMapper::Resource 
    property :id, Serial 
    property :name, String 

    has n, :firsts 
    has n, :comparison_firsts, 'Comparison', :through => :firsts, :child_key => [:comparison_id] 

    has n, :seconds 
    has n, :comparison_seconds, 'Comparison', :through => :seconds, :child_key => [:comparison_id] 
end 

運行automigrate導致以下錯誤：

rake aborted! 
No relationships named msrun_firsts or msrun_first in First 

什麼時我在這裏做錯了嗎？我該如何做這項工作？

Answer 1

由於每DataMapper的docs

我相信你能做到：

class Msrun 
    include DataMapper::Resource 
    property :id, Serial 
    property :name, String 

    has n, :firsts #This line could probably be omitted 
    has n, :first_comparisons, 'Comparison', :through => :firsts 

    has n, :seconds #This line could probably be omitted 
    has n, :second_comparisons, 'Comparison', :through => :seconds 
end 

Answer 2

什麼您看到的，是關係存儲在一組類似物體的引擎蓋下，更具體的事實，一個使用關係名稱作爲鑑別器的集合。所以在你的情況下會發生什麼，後一個定義會覆蓋前者，因爲集合不允許重複條目（在我們的例子中，爲了集合的目的，將新條目替換爲舊條目）。

這有實際的原因。在一個模型上聲明兩個假定不同的關係是沒有意義的，但是將它們命名爲相同。如何在嘗試訪問它們時區別它們？這體現在DM的實現中，其中由關係名稱命名的方法在資源上被定義。那麼，在您嘗試向該集合添加副本的情況下，DM最終要做的是僅使用後面的選項來生成該方法的實現。即使它接受重複的關係名稱，後一種關係也會導致相同方法的重寫/重新定義版本，從而使您獲得相同的淨效果。

因此，您需要在模型上定義不同名稱的關係。當你考慮它時，它確實很有意義。爲了幫助DM與推斷模型中使用的，可以作爲第三個參數傳遞的型號名稱（或常量本身）的has方法，或作爲第二個參數爲belongs_to

class Comparison 
    include DataMapper::Resource 
    property :id, Serial 

    has n, :firsts 
    has n, :first_msruns, 'Msrun', :through => :firsts 

    has n, :seconds 
    has n, :second_msruns, 'Msrun', :through => :seconds 
end 

class Msrun 
    include DataMapper::Resource 
    property :id, Serial 
    property :name, String 

    has n, :firsts 
    has n, :first_comparisons, 'Comparison', :through => :firsts 

    has n, :seconds 
    has n, :second_comparisons, 'Comparison', :through => :seconds 

end 

希望幫助！