MongoDb aggregation query with $group and $push into subdocument - mongodb

Question

I have a question regarding the $group argument of MongoDb aggregations. My data structure looks as follows:
My "Event" collection contains this single document:
{
"_id": ObjectId("mongodbobjectid..."),
"name": "Some Event",
"attendeeContainer": {
"min": 0,
"max": 10,
"attendees": [
{
"type": 1,
"status": 2,
"contact": ObjectId("mongodbobjectidHEX1")
},
{
"type": 7,
"status": 4,
"contact": ObjectId("mongodbobjectidHEX2")
}
]
}
}
My "Contact" collection contains these documents:
{
"_id": ObjectId("mongodbobjectidHEX1"),
"name": "John Doe",
"age": 35
},
{
"_id": ObjectId("mongodbobjectidHEX2"),
"name": "Peter Pan",
"age": 60
}
What I want to do is perform an aggregate query on the "Event" collection and get the following result with full "contact" data:
{
"_id": ObjectId("mongodbobjectid..."),
"name": "Some Event",
"attendeeContainer": {
"min": 0,
"max": 10,
"attendees": [
{
"type": 1,
"status": 2,
"contact": {
"_id": ObjectId("mongodbobjectidHEX1"),
"name": "John Doe",
"age": 35
}
},
{
"type": 7,
"status": 4,
"contact": {
"_id": ObjectId("mongodbobjectidHEX2"),
"name": "Peter Pan",
"age": 60
}
}
]
}
}
The arguments I am using right now look as follows (shortened version):
"$unwind" : "$attendeeContainer.attendees",
"$lookup" : { "from" : "contactinfo", "localField" : "attendeeContainer.attendees.contact","foreignField" : "_id", "as" : "contactInfo" },
"$unwind" : "$contactInfo",
"$group" : { "_id": "$_id",
"name": { "$first" : "$name" },
...
"contact": { "$push": { "contact": "$contactInfo"} }
}
However, this leads to the "contact" array being on "Event" level (because of the grouping) instead of one document of the array being at each "attendeeContainer.attendees". How can I push the "contact" array to be at "attendeeContainer.attendees"? (as shown in the desired output above)
I tried things like:
"attendeeContainer.attendees.contact": { "$push": { "contact": "$contactInfo"} }
But mongodb apparently does not allow "." at $group stage.

Answer 1

Try running the following aggregation pipeline, the key is using a final $project pipeline to create the attendeeContainer subdocument:
db.event.aggregate([
{ "$unwind": "$attendeeContainer.attendees" },
{
"$lookup" : {
"from" : "contactinfo",
"localField" : "attendeeContainer.attendees.contact",
"foreignField" : "_id",
"as" : "attendeeContainer.attendees.contactInfo"
}
},
{ "$unwind": "$attendeeContainer.attendees.contactInfo" },
{
"$group": {
"_id" : "$_id",
"name": { "$first": "$name" },
"min" : { "$first": "$attendeeContainer.min" },
"max" : { "$first": "$attendeeContainer.max" },
"attendees": { "$push": "$attendeeContainer.attendees" }
}
},
{
"$project": {
"name": 1,
"attendeeContainer.min": "$min",
"attendeeContainer.max": "$min",
"attendeeContainer.attendees": "$attendees"
}
}
])
Debugging Tips
Debugging the pipeline at the 4th stage, you would get the result
db.event.aggregate([
{ "$unwind": "$attendeeContainer.attendees" },
{
"$lookup" : {
"from" : "contactinfo",
"localField" : "attendeeContainer.attendees.contact",
"foreignField" : "_id",
"as" : "attendeeContainer.attendees.contactInfo"
}
},
{ "$unwind": "$attendeeContainer.attendees.contactInfo" },
{
"$group": {
"_id": "$_id",
"name": { "$first": "$name" },
"min" : { "$first": "$attendeeContainer.min" },
"max" : { "$first": "$attendeeContainer.max" },
"attendees": { "$push": "$attendeeContainer.attendees" }
}
}/*,
{
"$project": {
"name": 1,
"attendeeContainer.min": "$min",
"attendeeContainer.max": "$min",
"attendeeContainer.attendees": "$attendees"
}
}*/
])
Pipeline result
{
"_id" : ObjectId("582c789282a9183adc0b53f5"),
"name" : "Some Event",
"min" : 0,
"max" : 10,
"attendees" : [
{
"type" : 1,
"status" : 2,
"contact" : ObjectId("582c787682a9183adc0b53f3"),
"contactInfo" : {
"_id" : ObjectId("582c787682a9183adc0b53f3"),
"name" : "John Doe",
"age" : 35
}
},
{
"type" : 7,
"status" : 4,
"contact" : ObjectId("582c787682a9183adc0b53f4"),
"contactInfo" : {
"_id" : ObjectId("582c787682a9183adc0b53f4"),
"name" : "Peter Pan",
"age" : 60
}
}
]
}
and the final $project pipeline will give you the desired result:
db.event.aggregate([
{ "$unwind": "$attendeeContainer.attendees" },
{
"$lookup" : {
"from" : "contactinfo",
"localField" : "attendeeContainer.attendees.contact",
"foreignField" : "_id",
"as" : "attendeeContainer.attendees.contactInfo"
}
},
{ "$unwind": "$attendeeContainer.attendees.contactInfo" },
{
"$group": {
"_id": "$_id",
"name": { "$first": "$name" },
"min" : { "$first": "$attendeeContainer.min" },
"max" : { "$first": "$attendeeContainer.max" },
"attendees": { "$push": "$attendeeContainer.attendees" }
}
},
{
"$project": {
"name": 1,
"attendeeContainer.min": "$min",
"attendeeContainer.max": "$min",
"attendeeContainer.attendees": "$attendees"
}
}/**/
])
Desired/Actual Output
{
"_id" : ObjectId("582c789282a9183adc0b53f5"),
"name" : "Some Event",
"attendeeContainer" : {
"min" : 0,
"max" : 10,
"attendees" : [
{
"type" : 1,
"status" : 2,
"contact" : ObjectId("582c787682a9183adc0b53f3"),
"contactInfo" : {
"_id" : ObjectId("582c787682a9183adc0b53f3"),
"name" : "John Doe",
"age" : 35
}
},
{
"type" : 7,
"status" : 4,
"contact" : ObjectId("582c787682a9183adc0b53f4"),
"contactInfo" : {
"_id" : ObjectId("582c787682a9183adc0b53f4"),
"name" : "Peter Pan",
"age" : 60
}
}
]
}
}