如何获取关联组中的最新记录

Question

我有以下模型。

class Candidate < ActiveRecord::Base 
    belongs_to :accommodation 
    has_many :rates, :foreign_key => :candidate_id 
end 

class Accommodation < ActiveRecord::Base 
    has_many :rates, :through => :candidates 
end 

class Rate < ActiveRecord::Base 
    belongs_to :candidate 
    delegate :accommodation, :to => :candidate 
end 

class City < Location 
    has_and_belongs_to_many :accommodations, :foreign_key => 'location_id' 
end 

现在我得到所有位置，以迭代它们并获得所有位置的住宿和价格。问题是，我想只有一个住宿的最新价格，而是由一个名为列分组：supplier_id 笔者认为：

<% acc.rates.each do |rate| %> 
        <div class="rate"> 
        <div class=""> 
         <h3> 
         <%= rate.promo %> 
         <small><%= rate.supplier %></small> 
         <span class="rate-price text-right"><strong><%= rate.try(:nightly_rate).try(:ceil) %></strong></span> 
         <small>€</small> 
         </h3> 
        </div> 
        </div> 

       <% end %> 

所以，我怎么能获得最新的价格为每个供应商（按供应商分组），以一种我可以加载的方式，否则性能会太慢。

Answer 1

我认为你需要在2个独立的查询来划分这个查询：

locations = Location.includes(:accomodations) 
accomodation_ids = locations.map(&:accomodation_ids).flatten 
rates = Rate.by_accomodation_and_supplier(accomodation_ids) 

class Rate  
    def self.by_accomodation_and_supplier(accomodation_ids) 
    joins(candidate: { accomodation: accomodation_ids}). 
     order(created_at: :desc).group(:supplier_id) 
    end 
end