$lookup when localField is nested - mongodb

Question

MongoDB version 3.4.10 (Application is using Meteor framework)
Objective: Aggregate documents that are referenced by _id into the containing document as required at runtime.
I have Materials, Models, and Catalog collections with the following documents:
Materials
{ "_id" : "cf4KgXw7ZK6ukdzR7", "name" : "parquet_wood_mahogany" }
Models
{
"_id" : "Mwp5eYYZ4GZzvZuoK",
"name" : "top_square_chamfered",
"type" : "top"
}
{
"_id" : "CqhS2m2RcLZ2Bm4eb",
"name" : "skirt_square",
"type" : "skirt"
}
{
"_id" : "dYP22ajALnWBwpBj2",
"name" : "leg_square",
"type" : "leg"
}
Catalog
{
"_id" : "EcRGzPAq79giYKrbY",
...,
"specs" : {
...,
"models" : [
{
"mesh" : "Mwp5eYYZ4GZzvZuoK",
"material" : "cf4KgXw7ZK6ukdzR7"
},
{
"mesh" : "CqhS2m2RcLZ2Bm4eb",
"material" : "cf4KgXw7ZK6ukdzR7"
},
{
"mesh" : "dYP22ajALnWBwpBj2",
"material" : "cf4KgXw7ZK6ukdzR7"
}
]
}
}
Desired returned document format after aggregation:
{
"_id" : "EcRGzPAq79giYKrbY",
...,
"specs" : {
"dimensions" : {
...,
},
"models" : [
{
"mesh" : {
"_id" : "Mwp5eYYZ4GZzvZuoK",
"name" : "top_square_chamfered",
"type" : "top"
},
"material" : {
"_id" : "cf4KgXw7ZK6ukdzR7",
"name" : "parquet_wood_mahogany"
}
},
{
"mesh" : {
"_id" : "CqhS2m2RcLZ2Bm4eb",
"name" : "skirt_square",
"type" : "skirt"
},
"material" : {
"_id" : "cf4KgXw7ZK6ukdzR7",
"name" : "parquet_wood_mahogany"
}
},
{
"mesh" : {
"_id" : "dYP22ajALnWBwpBj2",
"name" : "leg_square",
"type" : "leg"
},
"material" : {
"_id" : "cf4KgXw7ZK6ukdzR7",
"name" : "parquet_wood_mahogany"
}
}
]
}
}
I haven't included any of my query code because it is so far off the mark as to just be noise. I've been trying to use aggregate, with $lookup combinations, but I'm not getting anywhere close to what I'm after. The MongoDB v3.6 pipeline syntax would make this much easier... but I'm at a complete loss in v3.4.
I would like to avoid using multiple database requests to combine this information if at all possible. Any assistance of advice would be greatly appreciated!
EDIT: Working solution -
db.catalog.aggregate([
{ "$lookup": {
"from": 'models',
"localField": "specs.models.mesh",
"foreignField": "_id",
"as": "models.mesh"
}},
{ "$lookup": {
"from": 'materials',
"localField": "specs.models.material",
"foreignField": "_id",
"as": "models.material"
}},
{ "$unwind": "$models.mesh" },
{ "$unwind": "$models.material" },
{ "$group":{
"_id": "$_id",
"title": { "$first": "$title" },
"desc": { "$first": "$desc" },
"thumbnail": { "$first": "$thumbnail" },
"createdBy": { "$first": "$createdBy" },
"createdAt": { "$first": "$createdAt" },
"specs": { "$first": "$specs" },
"models": { "$push": "$models" }
}},
{ "$project": {
"_id": "$_id",
"title": "$title",
"desc": "$desc",
"thumbnail": "$thumbnail",
"createdBy": "$createdBy",
"createdAt": "$createdAt",
"specs.dimensions": "$specs.dimensions",
"specs.models": "$models",
}}
])

Answer 1

You can try below aggregation
db.catalog.aggregate([
{ "$lookup": {
"from": 'models',
"localField": "specs.models.mesh",
"foreignField": "_id",
"as": "models.mesh"
}},
{ "$lookup": {
"from": 'materials',
"localField": "specs.models.material",
"foreignField": "_id",
"as": "models.material"
}},
{ "$unwind": "$models.mesh" },
{ "$unwind": "$models.material" },
{ "$group":{
"_id": "$_id",
"title": { "$first": "$title" },
"desc": { "$first": "$desc" },
"thumbnail": { "$first": "$thumbnail" },
"createdBy": { "$first": "$createdBy" },
"createdAt": { "$first": "$createdAt" },
"specs": { "$first": "$specs" },
"models": { "$push": "$models" }
}},
{ "$project": {
"title": "$title",
"desc": "$desc",
"thumbnail": "$thumbnail",
"createdBy": "$createdBy",
"createdAt": "$createdAt",
"specs.dimensions": "$specs.dimensions",
"specs.models": "$models",
}}
])