MongoDB Aggregate ObjectId inside Array of embedded Documents

Question

I have seen a ton of question about the $lookup aggregator for arrays of ObjectIds but I can't seem to find anything about when the ObjectId is inside an array of embedded documents.

I have the follow document inside a mongodb database:

_id: ObjectId('...')
Alarms: [
  {
    Gateway: ObjectId('...')
    Port: 1
  },
  {
    Gateway: ObjectId('...')
    Port: 2
  }
]

I would like to have the following:

_id: ObjectId('...')
Alarms [
  {
    Gateway: ...(Gateway Object),
    Port: 1
  },
  {
    Gateway: ...(Gateway Object),
    Port: 2
  }
]

I have tried the following with no success:

$lookup: {
  from: 'Gateway',
  localField: 'Alarms.Gateway',
  foreignField: '_id',
  as: 'Alarms.Gateway'
}

But this gives me the following result:

_id: ObjectId('...')
Alarms [
  {
    Gateway: {
      ...(Gateway Object)
    }
    Port: 1
  }
]

What happens if you've third object like { Gateway: ObjectId('...'), Port: 3 } in Alarms that doesn't have a matching doc in Gateway? Do you still want to keep that in Alarms array as is in final result or remove that & keep only the objects that have matches? — whoami - fakeFaceTrueSoul
– whoami - fakeFaceTrueSoul, Commented Jan 24, 2020 at 21:07

whoami - fakeFaceTrueSoul · Accepted Answer · 2020-01-24 21:36:42Z

Please try the below queries :

If you don't want the object which doesn't have match in Gateway collection exist in Alarms array in final result :

db.Alarms.aggregate([{ $unwind: '$Alarms' }, {
    $lookup: {
        from: 'Gateway',
        localField: 'Alarms.Gateway',
        foreignField: '_id',
        as: 'Alarms.Gateway'
    }
}, { $match: { 'Alarms.Gateway': { $ne: [] } } },
{ $addFields: { 'Alarms.Gateway': { $arrayElemAt: ['$Alarms.Gateway', 0] } } },
{ $group: { _id: '$_id', Alarms: { $push: '$Alarms' } } }
])

Test : MongoDB-Playground

Otherwise, if you want all objects in Alarms array to be returned irrespective of whether there is a match in Gateway or not :

db.Alarms.aggregate([{ $unwind: '$Alarms' }, {
    $lookup: {
        from: 'Gateway',
        localField: 'Alarms.Gateway',
        foreignField: '_id',
        as: 'Alarms.GatewayObj'
    }
}, { $addFields: { 'Alarms.Gateway': { $cond: [{ $ne: ['$Alarms.GatewayObj', []] }, { $arrayElemAt: ['$Alarms.GatewayObj', 0] }, '$Alarms.Gateway'] } } },
{ $project: { 'Alarms.GatewayObj': 0 } },
{ $group: { _id: '$_id', Alarms: { $push: '$Alarms' } } }
])

Test : MongoDB-Playground

Difference between two queries would be one will return below object in Alarms array (Vs) one don't.

{
    "Gateway": ObjectId("5e2b5425d02e05b6940de2fb"),
    "Port": 2
 }

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