Mongoose：find() 忽略重复值

我有一只“聊天”猫鼬Schema它具有以下属性：

const schema = mongoose.Schema({
    ...
    recipient: {
        type: mongoose.Types.ObjectId,
        required: true,
        ref: 'User',
    },
    sender: {
        type: mongoose.Types.ObjectId,
        required: true,
        ref: 'User',
    },
    content: {
        type: String,
    },
    ...
}, {
    timestamps: true,
});

一般来说，我想获取用户每次对话的最后一条消息。这意味着我需要提供一个用户 ID（可以存储在sender or recipient字段）并返回最后一条消息（由createdAt）用户与其他每个用户的关系。

Example:假设我有以下内容documents:

[
  {
    recipient: "One",
    sender: "Two",
    createdAt: ISODate("2014-01-01T08:00:00Z"),

  },
  {
    recipient: "One",
    sender: "Three",
    createdAt: ISODate("2014-02-15T08:00:00Z")
  },
  {
    recipient: "Two",
    sender: "One",
    createdAt: ISODate("2014-02-16T12:05:10Z")
  }
]

插入“One”作为输入 - 所需的结果Model.find(...) is:

[
  {
    recipient: "One",
    sender: "Three",
    createdAt: ISODate("2014-02-15T08:00:00Z")
  },
  {
    recipient: "Two",
    sender: "One",
    createdAt: ISODate("2014-02-16T12:05:10Z")
  }
]

使用示例数据：

[
  {
    recipient: "One",
    sender: "Two",
    createdAt: ISODate("2014-01-01T08:00:00Z"),
    content: "Hi Mr. One! - Two"
  },
  {
    recipient: "One",
    sender: "Three",
    createdAt: ISODate("2014-02-15T08:00:00Z"),
    content: "Hello One! - Three"
  },
  {
    recipient: "Two",
    sender: "One",
    createdAt: ISODate("2014-02-16T12:05:10Z"),
    content: "Whats up, Two? - One"
  }
]

看看下面的聚合：https://mongoplayground.net/p/DTSDWX3aLWe https://mongoplayground.net/p/DTSDWX3aLWe

It...

• Uses $match按收件人或发件人过滤所有邮件。返回与当前用户匹配的那些（One)
• Adds a conversationWith现场使用$addFields其中包含recipient如果它是一条消息给用户一 or the sender如果它是一条消息用户一发送
• 使用按日期对消息进行排序$sort
• 使用以下方式对所有消息进行分组$group由新的conversationWith字段并返回最新消息：firstMessage

完整的聚合管道：

db.collection.aggregate([
  {
    $match: {
      $and: [
        {
          $or: [
            {
              recipient: "One"
            },
            {
              sender: "One"
            }
          ],

        },

      ]
    }
  },
  {
    $addFields: {
      conversationWith: {
        $cond: {
          if: {
            $eq: [
              "$sender",
              "One"
            ]
          },
          then: "$recipient",
          else: "$sender"
        }
      }
    }
  },
  {
    $sort: {
      createdAt: -1
    }
  },
  {
    $group: {
      _id: "$conversationWith",
      firstMessage: {
        $first: "$$ROOT"
      }
    }
  }
])

使用 mongoplayground，您可以逐一删除聚合步骤以查看每个步骤的作用。

Try:

• 只有$match step
• $match + $addFields
• $match + $addFields + $sort
• [..]

为了更好的理解。