Rails Complex連接查詢

Question

這是我的基本模型層次結構：

class Product
  has_many :inventories
end
class Inventory
  belongs_to :product
  has_many :inventory_events
end
class InventoryEvent
  belongs_to :inventory
end

InventoryEvent實例存儲狀態更改+那些更改的時間戳，因此inventory.inventory_events.last顯示當前狀態。

我在針對Product模型創建查詢時遇到了問題，該查詢將為我提供received其當前狀態的所有清單。

我現在所擁有的是：

p = Product.first
p.inventories.joins(:inventory_events).where(inventory_events: {state: 'received'}).all

=> # Here I get back all Inventory that ever had the state 'received' but may 
not currently be 'received'.

我的SQL知識是非常有限的，但是似乎對inventory_events: {}某種限制inventory_events: {}選項可能有效，但是還沒有找到一種方法。

---

編輯：這是我目前的解決方法，只是為了顯示我的最終目標。 希望有一種方法可以對這樣的查詢建模。

class Inventory
  def self.received_items
    includes(:inventory_events).select {|i| i.current_state == 'received'}
  end
  def current_state
    inventory_events.last.state
  end
end

Product.first.inventories.received_items
=> # Here I get the correct array of inventories

Answer 1

您可以通過使用范圍和合並方法來完成此操作。 范圍將使您可以將where條件保持模塊化。 通過merge方法，您可以選擇接收到InventoryEvent的Inventory。

# call this to get the inventories for the product that have been recieved
product.inventories.received

class InventoryEvent
  def self.received
    where("state = ?", "received")
  end

  def self.most_recent
    order("inventory_events.created_at desc").first
  end
end

class Inventory
  def self.received
    joins(:inventory_events).
    merge(InventoryEvent.received).
    merge(InventoryEvent.most_recent)
  end
end

Answer 2

我發現了這段SQL，它一直在為我工作：

joins(:inventory_events).where("inventory_events.id IN (SELECT MAX(id) FROM inventory_events GROUP BY inventory_id) AND state = 'received'")