Looping through array to count in mongodb/mongoose - arrays

Question

I have a user schema that contains a value called amputationInfo:
amputationInfo: [
{
type: String,
},
],
Here is an example of what that might look like in the database:
amputationInfo: [
"Double Symes/Boyd",
"Single Above-Elbow"
]
I have a review Schema that allows a user to leave a review, it contains a reference to the user who left it:
user: {
type: mongoose.Schema.ObjectId,
ref: 'User',
require: [true, 'Each review must have an associated user!'],
},
When a user leaves a review, I want to create an aggregate function that looks up the user on the review, finds their amputationInfo, loops through the array and adds up the total amount of users that contain "Double Symes/Boyd", "Single Above-Elbow"
So if we have 3 users and their amputationInfo is as follows:
amputationInfo: [
"Double Symes/Boyd",
"Single Above-Elbow"
]
amputationInfo: [
"Single Above-Elbow"
]
amputationInfo: []
The return from the aggregate function will count each term and add one to the corresponding value and look something like this:
[
{
doubleSymesBoyd: 1,
singleAboveElbow: 2
}
]
Here is what I have tried, but I just don't know enough about mongoDB to solve the issue:
[
{
'$match': {
'prosthetistID': new ObjectId('6126ca6148f34c00189f86f5')
}
}, {
'$lookup': {
'from': 'users',
'localField': 'user',
'foreignField': '_id',
'as': 'userInfo'
}
}, {
'$unwind': {
'path': '$userInfo'
}
}
]
After the $unwind, the resulting object has a userInfo key, that contains an amputationInfo array nested:

Answer 1

You can have following stages
$unwind to deconstruct the array
first $group to get the sum of each category
second $group to push into one document and make it as key value pair
$arrayToObject to get the desired output
$replaceRoot to make the data output into root
Here is the code
db.collection.aggregate([
{ "$unwind": "$userInfo.amputationInfo" },
{
"$group": {
"_id": "$userInfo.amputationInfo",
"count": { "$sum": 1 }
}
},
{
$group: {
_id: null,
data: { $push: {
k: "$_id",
v: "$count"
}
}
}
},
{ $project: { data: { "$arrayToObject": "$data" } } },
{ "$replaceRoot": { "newRoot": "$data" } }
])
Working Mongo playground