MongoDB grouping aggregate query with relationship - mongodb

Question

Let's say that I have the following documents in the association collection:
{
"id" : 1,
"parentId" : 1,
"position" : {
"x" : 1,
"y" : 1
},
"tag" : "Beta"
},
{
"id" : 2,
"parentId" : 2,
"position" : {
"x" : 2,
"y" : 2
},
"tag" : "Alpha"
},
{
"id" : 3,
"parentId" : 1,
"position" : {
"x" : 3,
"y" : 3
},
"tag" : "Delta"
},
{
"id" : 4,
"parentId" : 1,
"position" : {
"x" : 4,
"y" : 4
},
"tag" : "Gamma"
},
{
"id" : 5,
"parentId" : 2,
"position" : {
"x" : 5,
"y" : 6
},
"tag" : "Epsilon"
}
I would like to create an aggregate query to produce the following result:
{
"_id" : 2,
"position" : {
"x" : 2,
"y" : 2
},
"tag" : "Alpha",
"children" : [
{
"position" : {
"x" : 5,
"y" : 6
},
"tag" : "Epsilon"
}
]
},
{
"_id" : 1,
"position" : {
"x" : 1,
"y" : 1
},
"tag" : "Beta"
"children" : [
{
"position" : {
"x" : 3,
"y" : 3
},
"tag" : "Delta"
},
{
"position" : {
"x" : 4,
"y" : 4
},
"tag" : "Gamma"
}
]
}
However, I was able only to create the following grouping query which puts "all-the-related" documents in children array:
db.association.aggregate([{
$group : {
_id : "$parentId",
children : {
$push : {
position : "$position",
tag :"$tag"
}
}
}
}])
I don't know how to filter out "position" and "tag" specific to "parent" points and put them at the top level.

Answer 1

Although Valijon's answer is working, it needs to be sorted before.
Here's a solution without the need of sorting, but using graphLookup stage (which is perfect to achieve what you need)
db.collection.aggregate([
{
$graphLookup: {
from: "collection",
startWith: "$id",
connectFromField: "id",
connectToField: "parentId",
as: "children",
}
},
{
$match: {
$expr: {
$gt: [
{
$size: "$children"
},
0
]
}
}
},
{
$addFields: {
children: {
$filter: {
input: "$children",
as: "child",
cond: {
$ne: [
"$id",
"$$child.id"
]
}
}
}
}
}
])
The first stage is doing the job.
The second one is here to filter documents that don't have any child.
The third is present only to remove parent from children array. But if you can remove self-reference in the parent, this last stage will not be needed anymore.
You can try it here

Answer 2

By making sure the documents are sorted (parent - children 1 - children 2 ... - children n), we can merge grouped document with the 1st child (which is parent). In the last step, we need to remove parent from children array.
Try this one:
db.association.aggregate([
{
$sort: {
parentId: 1,
id: 1
}
},
{
$group: {
_id: "$parentId",
children: {
$push: {
position: "$position",
tag: "$tag"
}
}
}
},
{
$replaceRoot: {
newRoot: {
$mergeObjects: [
"$$ROOT",
{
$arrayElemAt: [
"$children",
0
]
}
]
}
}
},
{
$addFields: {
children: {
$slice: [
"$children",
1,
{
$size: "$children"
}
]
}
}
}
])
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