Rails Complex连接查询

Question

这是我的基本模型层次结构：

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

InventoryEvent实例存储状态更改+那些更改的时间戳，因此inventory.inventory_events.last显示当前状态。

我在针对Product模型创建查询时遇到了问题，该查询将为我提供received其当前状态的所有清单。

我现在所拥有的是：

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知识是非常有限的，但是似乎对inventory_events: {}某种限制inventory_events: {}选项可能有效，但是还没有找到一种方法。

---

编辑：这是我目前的解决方法，只是为了显示我的最终目标。 希望有一种方法可以对这样的查询建模。

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

您可以通过使用范围和合并方法来完成此操作。 范围将使您可以将where条件保持模块化。 通过merge方法，您可以选择接收到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

我发现了这段SQL，它一直在为我工作：

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