如何计算 mongo db 中的 $lookup 字段？

Question

我在 mongodb 中很新，现在我需要计算一个 $lookup 字段，这可能吗？

我有这样的事情：

result = await company.aggregate([
  {
    $lookup: {
      from: 'userFocus',
      localField: '_id',
      foreignField: 'value',
      as: 'focusUsers'
    }
  },
  {
    $project:{
      name: 1,
      focusUsers: {userId: 1}
    }
  }
])

结果如下所示：

[
  {_id: 'xxxx', name: 'first company', focusUsers: [user1, user2, user3...]},
  {_id: 'yyyy', name: 'second company', focusUsers: []},
  {_id: 'zzzz', name: 'third company', focusUsers: []}
]

现在我想要一个额外的列显示 focusUsers 计数，换句话说，我想要一个如下所示的结果：

[
  {_id: 'xxxx', name: 'first company', focusUsers: [user1, user2, user3], focusCount: 3},
  {_id: 'yyyy', name: 'second company', focusUsers: [], focusCount: 0},
  {_id: 'zzzz', name: 'third company', focusUsers: [], focusCount: 0}
]

是否可以？怎么做？请高手指点一下，先谢过了！

Answer 1

您可以使用$size 聚合运算符来查找数组的长度。

company.aggregate([
  { "$lookup": {
    "from": "userFocus",
    "localField": "_id",
    "foreignField": "value",
    "as": "focusUsers"
  }},
  { "$project": {
    "name": 1,
    "focusUsers": 1,
    "focusCount": { "$size": "$focusUsers" }
  }}
])