具有特定屬性值

Question

查詢HAS_ONE模型我有兩個型號：

class Tree < ActiveRecord::Base             
    has_many :leafs                
    has_one :latest_leaf, -> { order(created_at: :desc) }, class_name: "Leaf"  

    def self.with_connected_latest_leafs           

    end                   
end 

class Leaf < ActiveRecord::Base             
    belongs_to :tree                
end                    

class CreateTrees < ActiveRecord::Migration          
    def change                  
    create_table :trees do |t|             
     t.timestamps                
    end                   
    end                   
end 

class CreateLeafs < ActiveRecord::Migration          
    def change                  
    create_table :leafs do |t|             
     t.integer :tree_id               
     t.string :state               
     t.timestamps                
    end                   
    end                   
end                    

我希望所有樹木的列表，以latest_leaf具有「連接」的狀態。

Answer 1

如果您使用Rails 4，你可以使用這個：

Three.joins(:leafs).where(latest_leaf: { state: "connected" }) 

Answer 2

我可以試試子查詢來獲取最後一片葉子在每個樹。所以：

Tree.joins("INNER JOIN leafs l ON l.id = (SELECT id FROM leafs WHERE tree_id = trees.id ORDER BY created_at DESC LIMIT 1)") 
.where("leafs.state = ?", "connected") 

我希望這對你有所幫助。