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
}
]
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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