两个集合之间的MongoDb查询

[
  { "_id": 1, "name": "Alice" },
  { "_id": 2, "name": "Bob" },
  { "_id": 3, "name": "Charlie" }
]

[
  { "_id": 101, "userId": 1, "product": "Laptop" },
  { "_id": 102, "userId": 2, "product": "Phone" },
  { "_id": 103, "userId": 1, "product": "Tablet" }
]

db.users.aggregate([
  {
    $lookup: {
      from: "orders",           // 目标集合
      localField: "_id",        // 本地集合中的字段
      foreignField: "userId",   // 目标集合中的字段
      as: "orders"              // 输出数组的字段名
    }
  }
])

[
  {
    "_id": 1,
    "name": "Alice",
    "orders": [
      { "_id": 101, "userId": 1, "product": "Laptop" },
      { "_id": 103, "userId": 1, "product": "Tablet" }
    ]
  },
  {
    "_id": 2,
    "name": "Bob",
    "orders": [
      { "_id": 102, "userId": 2, "product": "Phone" }
    ]
  },
  {
    "_id": 3,
    "name": "Charlie",
    "orders": []
  }
]

db.users.aggregate([
  {
    $lookup: {
      from: "orders",
      localField: "_id",
      foreignField: "userId",
      as: "orders"
    }
  },
  {
    $match: {
      "orders": { $ne: [] }  // 过滤掉没有订单的用户
    }
  }
])

[
  {
    "_id": 1,
    "name": "Alice",
    "orders": [
      { "_id": 101, "userId": 1, "product": "Laptop" },
      { "_id": 103, "userId": 1, "product": "Tablet" }
    ]
  },
  {
    "_id": 2,
    "name": "Bob",
    "orders": [
      { "_id": 102, "userId": 2, "product": "Phone" }
    ]
  }
]

db.users.aggregate([
  {
    $lookup: {
      from: "orders",
      localField: "_id",
      foreignField: "userId",
      as: "orders"
    }
  },
  {
    $project: {
      _id: 0,
      name: 1,
      "orders.product": 1
    }
  }
])

[
  {
    "name": "Alice",
    "orders": [
      { "product": "Laptop" },
      { "product": "Tablet" }
    ]
  },
  {
    "name": "Bob",
    "orders": [
      { "product": "Phone" }
    ]
  },
  {
    "name": "Charlie",
    "orders": []
  }
]