Rails 4对话模型JOIN表与条件

Question

我有一个与

的对话模型

recipient_id
sender_id

获取当前用户的对话看起来像

conversations = Conversation.where("conversations.sender_id =? OR conversations.recipient_id =?",current_user.id,current_user.id)

让我们调用 other_user 对话中不是 current_user 的用户。

由于当前用户既可以是发件人也可以是收件人，因此无法在 users 表上使用 JOIN 或 Include 来获取对话中涉及的“其他用户”。因此，呈现对话会产生 N+1 的情况，为每个其他用户发出 1 个查询。

有没有办法使用 include 或 JOIN 获取 other_user，条件基本上是：

if current_user_id == recipient_id JOIN on recipient_id
if current_user_id == sender_id JOIN on sender_id

Answer 1

如果您只需要 other_user 的某些属性，例如用户名，那么这样的东西应该可以工作：

conversations = Conversation.find_by_sql([<<-SQL, current_user.id, current_user.id, current_user.id])
  SELECT
    convos_involving_user.*
  FROM
    -- We'll use UNION to create a table that joins together the users and
    -- conversations tables based on both sender and recipient. Each
    -- conversation will be represented by two rows in this joined table:
    -- one row joining that conversation to the sender's user info, another
    -- row joining that conversation to the recipient's user info.
    (
    -- join conversations and users based on sender
    SELECT
      conversations.*, users.id AS user_id, users.username AS other_user
    FROM
      conversations
    INNER JOIN
      users ON users.id = conversations.sender_id
    UNION
    -- join conversations and users based on recipient
    SELECT
      conversations.*, users.id AS user_id, users.username AS other_user
    FROM
      conversations
    INNER JOIN
      users ON users.id = conversations.recipient_id
    ) AS convos_involving_user
  -- Now that we conceptually have that big join table with two rows for
  -- each conversation, we filter that table. First, we are only interested
  -- in the conversations where either the recipient or sender is the
  -- current user. But we still have duplicate rows for each message, so we
  -- also filter out the rows that join a conversation rows to the
  -- current_user's row. This leaves, for each conversation involving the
  -- current user, the row that links the conversation to the other user.
  WHERE
    (recipient_id = ? OR sender_id = ?) AND user_id != ?
SQL

然后每个conversations 都会有一个other_user 方法，您可以调用它来获取其他用户的用户名。您可以在您的视图中使用它或其他。为了说明使用other_user 方法：

conversations.each { |convo| puts convo.other_user }

希望这会有所帮助。