mysql - 来自多个模型的包含、选择、排序、限制(单个查询)

Question

我需要创建一个包含下表数据的查询:

*Conversation:一种在用户之间对消息进行分组的模型

class Conversation < ActiveRecord::Base
  # Associations
  has_many :messages, dependent: :destroy
  has_many :conversation_participants
  has_many :users, :through => :conversation_participants
  ## Attributes title, created_at, updated_at
end

---

* ConversationParticipant:一个跟踪对话用户的模型

class ConversationParticipant < ActiveRecord::Base
  ## Associations
  belongs_to :conversation
  belongs_to :user
  ## Attributes conversation_id, user_id, seen, created_at, updated_at
end

---

* Message:一种跟踪内容和发件人的模型

class Message < ActiveRecord::Base
  belongs_to :conversation
  belongs_to :sender, :class_name => "User"
  ## Attributes sender_id, content, conversation_id, created_at, updated_at
end

---

*User:具有属性 name

的模型

---

如何在单个查询中获取以下内容？

1. limit of (5) recent messages from Message of uniq Conversation
2. where user_id = current_user.id from ConversationParticipant
3. order seen = false, then updated_at DESC from ConversationParticipant
4. includes Conversation
5. includes (Message sender) => User
6. includes the other participant from ConversationParticipant => User

---

注意:includes和select很重要，因为这个问题是为了减少查询次数.

Answer 1

这里是我如何包含所有需要的模型，这个查询被翻译成 5 个 sql 查询，因为预加载没有加入(在单独的查询中运行)。

Message.joins("LEFT JOIN messages AS m ON messages.id != m.id 
              AND m.conversation_id = messages.conversation_id 
              AND messages.created_at < m.created_at")
       .where('m.id IS NULL')
       .joins("INNER JOIN conversation_participants AS cp 
              ON cp.conversation_id = messages.conversation_id 
              AND cp.user_id = #{user_id}")
       .order("cp.seen, cp.updated_at DESC")
       .limit(5)
       .includes(:sender)
       .includes(conversation: [{conversation_participants: :user}])