如何定义运行时source_type里面“有很多通过”的多态关联？

Question

有没有什么办法来定义source_type里面的rails运行时间has_manythroughpolymorphic关联。

下面的代码应该给你简要的想法..但它不工作..任何建议？

class A < ... 
    has_many :message_messagables, :foreign_key => :message_id 

    has_many :messagables, :through => :message_messagables, :source => :messagable, :source_type => lambda { |a| a.custom_type } 

def custom_type 
    raise "needs to be defined inside subclass" 
end 

end 


class MessageMessagable < ... 
belongs_to :messagable, :polymorphic => true #[C, D] 
belongs_to :message 
end 

class B < A 

def custom_type 
    "C" 
end 
end 

class E < A 
def custom_type 
    "D" 
end 

end 

Answer 1

我遇到了同样的问题。我通过列出所有可用的源类型并添加了一个调度方法来解决它。

class A < ... 
    has_many :message_messagables, :foreign_key => :message_id 

    # list all source types as association 
    %w(email file).each do |source_type| 
    has_many source_type.pluralize.to_sym, :through => :message_messagables, :source => :messagable, :source_type => source_type 
    end 

    # dispatch methodk 
    def messagables 
    public_send custom_type.pluralize.to_sym 
    end 
end