Mongo Aggregate Combine Two Documents - mongodb

Question

Once I've unwound a sub-document array, how do I put it back together with all the original root fields?
Consider the following Tasks data set:
[
{
"_id": "5e95bb1cf36c0ab3247036bd",
"name": "Task A",
"org": "5e95b9894a0aa0b30dfcbc0b",
"creator": "5e117e5cd90de7187b000d87"
},
{
"_id": "5e95bb30f36c0ab3247036be",
"name": "Task B1",
"org": "5e95b9894a0aa0b30dfcbc0b",
"creator": "5e117e5cd90de7187b000d87",
"parent": "5e95bb1cf36c0ab3247036bd"
},
{
"_id": "5e95bb35f36c0ab3247036bf",
"name": "Task B2",
"org": "5e95b9894a0aa0b30dfcbc0b",
"creator": "5e117e5cd90de7187b000d87",
"parent": "5e95bb1cf36c0ab3247036bd"
}
]
So, then I run $graphLookup to get the parent task and populate it's children and then $unwind it and populate the creator field:
[
{
"$match": {
"parent": {
"$exists": false
}
}
},
{
"$graphLookup": {
"from": "tasks",
"startWith": "$_id",
"connectFromField": "_id",
"connectToField": "parent",
"as": "children"
}
},
{
"$unwind": {
"path": "$children"
}
},
{
"$lookup": {
"from": "users",
"localField": "children.creator",
"foreignField": "_id",
"as": "children.creator"
}
},
{
"$unwind": {
"path": "$children.creator"
}
}
]
Which returns the following documents:
[
{
"_id": "5e95bb1cf36c0ab3247036bd",
"name": "Task A",
"org": "5e95b9894a0aa0b30dfcbc0b",
"creator": "5e117e5cd90de7187b000d87",
"children": [
{
"_id": "5e95bb30f36c0ab3247036be",
"name": "Task B1",
"org": "5e95b9894a0aa0b30dfcbc0b",
"creator": {
"name": "Jack Frost"
},
"parent": "5e95bb1cf36c0ab3247036bd"
}
]
},
{
"_id": "5e95bb1cf36c0ab3247036bd",
"name": "Task A",
"org": "5e95b9894a0aa0b30dfcbc0b",
"creator": "5e117e5cd90de7187b000d87",
"children": [
{
"_id": "5e95bb35f36c0ab3247036bf",
"name": "Task B2",
"org": "5e95b9894a0aa0b30dfcbc0b",
"creator": {
"name": "Bill Nye"
},
"parent": "5e95bb1cf36c0ab3247036bd"
}
]
},
]
Lastly, I need to merge all of these duplicate documents back together and join the $children. This is the part I can't figure out. Below is some junk I'm trying but it seems messy to have to specifically list every property.
Is there a better way to combine multiple (mostly) matching docs?
[
...
{
"$group": {
"_id": "$_id",
"name": {
"$mergeObjects": "$properties"
},
"watchers": {
"$addToSet": "$watchers"
},
"assignees": {
"$addToSet": "$assignees"
},
"org": {
"$addToSet": "$$ROOT.org"
},
"children": {
"$push": "$children"
}
}
}
]

Answer 1

Answering my own question here, the best solution I can find is to specify each property but pass it the $first operator. This will ensure that the original value will be passed through.
{
$group: {
_id: '$_id',
name: {$first: '$name'},
org: {$first: '$org'},
creator: {$first: '$creator'},
children: {$push: '$children'}
}
}