How I can compile $cond with $ne on mongoose aggregate - mongodb

Question

This is the sample about my data:
[{
_id: ObjectId('5e982040227ddfb12bf43e39'),
classId: 'class-1',
state: 'active',
logs: []
},
{
_id: ObjectId('5e982040227ddfb12bf43e38'),
classId: 'class-1',
state: 'unactive',
logs: []
},
{
_id: ObjectId('5e982040227ddfb12bf43e40'),
classId: 'class-1',
state: 'graduated',
logs: []
}]
I want to change state for all students in class class-1 and state not equal unactive.
This is my query:
db.getCollection('student').aggregate([{
$addFields: {
state: {
$cond: [{ classId: 'class-1', $ne: ['$state', 'unactive']}, 'unactive', '$state']
},
logs: {
$cond: [{
{ classId: 'class-1', $ne: ['$state', 'unactive']},
{
$concatArrays: ['$logs', [{ from: '$state', to: 'unactive', at: new Date() }],
'$logs'
]}
}]
}
}
}, {
$out: 'student'
}]);
But it's throw an error: FieldPath field names may not start with '$'.
Here is document I'm following: https://docs.mongodb.com/manual/reference/operator/aggregation/cond/
If I remove $ne condition and keep only filter by classId, It's working well, but I don't want to unactive users who guys unactived.
Here my expected resuult:
[{
_id: ObjectId('5e982040227ddfb12bf43e39'),
classId: 'class-1',
state: 'unactive',
logs: [{ from: 'active', to: 'unactive', at: ... }]
},
{
_id: ObjectId('5e982040227ddfb12bf43e38'),
classId: 'class-1',
state: 'unactive',
logs: []
},
{
_id: ObjectId('5e982040227ddfb12bf43e40'),
classId: 'class-1',
state: 'unactive',
logs: [{ from: 'graduated', to: 'unactive', at: ...}]
}]
Please give me an idea if you know the way to resolve this problem

Answer 1

I believe you are trying to update. If that's not the case then, let me know.
You are using the $cond operator in a wrong way. The first argument in it, expects some logical condition like $and, $or, or any combination of logical operators.
The below query will be helpful:
db.student.aggregate([
{ // Unwind the 'logs' array field, along with preserving empty arrays.
$unwind: {
path: "$logs",
preserveNullAndEmptyArrays: true
}
},
{ // Group the documents by '_id' field.
$group: {
_id: "$_id",
classId: {
$first: "$classId"
},
oldState: {
$first: "$state"
},
logs: { // Conditionally push the object.
$push: {
$cond: [
{
$and: [
{
$eq: [
"$classId",
"class-1"
]
},
{
$ne: [
"$state",
"unactive"
]
}
]
},
{
"from": "$state",
"to": "unactive",
"at": "20200522"
},
"$$REMOVE" // If the above condition results false, then don't push.
]
}
}
}
},
{ // Project the required fields.
$project: {
classId: 1,
logs: 1,
state: { // conditionally set the 'state' field.
$cond: [
{
$and: [
{
$eq: [
"$classId",
"class-1"
]
},
{
$ne: [
"$oldState",
"unactive"
]
}
]
},
"unactive",
"$oldState"
]
}
}
},
{ // overwrite the output of this aggregation to 'student' collection.
$out : 'student'
}
])