how to count a $lookup fields in mongo db?

Question

I'm quite new in mongodb, now I need to count a $lookup field, is it possible?

I had something like this:

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

and the result looks like this:

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

Now I want an extra column shows the focusUsers count, in other words, I want a result like the following:

[
  {_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}
]

Is it possible? How to do that? Please some experts advice, thanks in advance!

Answer 1

You can use $size aggregation operator to find the length of an array.

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