MongoDB add field with keys from object - mongodb

Question

I have the following two collections:
{
"organizations": [
{
"_id": "1",
"name": "foo",
"users": { "1": "admin", "2": "member" }
},
{
"_id": "2",
"name": "bar",
"users": { "1": "admin" }
}
],
"users": [
{
"_id": "1",
"name": "john smith"
},
{
"_id": "2",
"name": "bob johnson"
}
]
}
The following query works to merge the users into members when I just use an array of the user ids to match, however, the users prop is an object.
{
"collection": "organizations",
"command": "aggregate",
"query": [
{
"$lookup": {
"from": "users",
"localField": "users",
"foreignField": "_id",
"as": "members"
}
}
]
}
What I'm hoping to do is lookup by id then create a members array from the results with the user object including the role (value of the users objects:
{
"_id": "1",
"name": "foo",
"users": {
"1": "admin",
"2": "member"
},
"members": [
{
"_id": "1",
"name": "john smith",
"role": "admin"
},
{
"_id": "2",
"name": "bob johnson",
"role": "user"
}
]
}
Here's the sandbox I have setup: https://mongoplayground.net/p/yhRpeRvJf3u

Answer 1

You really need to change your schema design, this will cause the performance on retrieving data,
$addFields to add new field usersArray convert users object to array using $objectToArray, the format will be k(key) and v(value),
$lookup to join users collection, set localField name to usersArray.k
$addFields, remove usersArray field using $$REMOVE,
$map iterate loop of members array and $reduce to iterate loop of usersArray and get matching role as per _id and merge current fields and role field using $mergeObjects
db.organizations.aggregate([
{
$addFields: {
usersArray: {
$objectToArray: "$users"
}
}
},
{
"$lookup": {
"from": "users",
"localField": "usersArray.k",
"foreignField": "_id",
"as": "members"
}
},
{
$addFields: {
usersArray: "$$REMOVE",
members: {
$map: {
input: "$members",
as: "m",
in: {
$mergeObjects: [
"$$m",
{
role: {
$reduce: {
input: "$usersArray",
initialValue: "",
in: { $cond: [{ $eq: ["$$this.k", "$$m._id"] }, "$$this.v", "$$value"] }
}
}
}
]
}
}
}
}
}
])
Playground

Answer 2

First of all, the problem with your query is you want to use a KEY to do the $lookup, then the members field always gonna be empty.
You are trying to use users as local field, but users is an object, so you need the key (users.1, users.2, ... )
To do this you need to use $objectToArray, which create an object array with two fields: k and v for key and value. So now, you can $lookup with the field users.k.
To get the query you need $unwind before $lookup because you also want the users filed into the new document.
With the new object created using $objectToArray, you can do $unwind to get the values in differents documents. And then $lookup to get the "join".
Here, localField uses the value k created by $objectToArray (the object key).
After that, $set to add the field with the role and $group again into one document.
Ive used _id to get the values without changes between stages, and into members push the members in each collection.
And then, $project to output the values you want. In this case, tha calues "stored" into _id and the array members in "one level" using $reduce.
So, the query you need I think is this:
db.organizations.aggregate([
{
"$match": {
"_id": "1"
}
},
{
"$set": {
"usersArray": {
"$objectToArray": "$users"
}
}
},
{
"$unwind": "$usersArray"
},
{
"$lookup": {
"from": "users",
"localField": "usersArray.k",
"foreignField": "_id",
"as": "members"
}
},
{
"$set": {
"members.role": "$usersArray.v"
}
},
{
"$group": {
"_id": {
"_id": "$_id",
"users": "$users",
"name": "$name"
},
"members": {
"$push": "$members"
}
}
},
{
"$project": {
"members": {
"$reduce": {
"input": "$members",
"initialValue": [],
"in": {
"$concatArrays": [
"$$value",
"$$this"
]
}
}
},
"users": "$_id.users",
"name": "$_id.name",
"_id": "$_id._id"
}
}
])
Example here