I have a questions collection with _id and name and other fields(..), and a tests collection with _id, name and array of questions.
I'm trying to get all the questions with their fields and adding a field "usedIn" which counts the number of tests that the specific questions is present in.
questions = await Question.aggregate([
/*{
"$match": {
params
}
},*/
{
"$lookup": {
"from": "tests",
"let": {"questionId": "$_id"},
pipeline: [
{
"$match": {
"$expr": {
"$in": ["$$questionId", "$questions"],
},
},
},
],
as: "tests"
}
},
{
"$addFields": {
"usedIn": {
"$size": "tests"
}
}
},
{
"$project": fieldsObject
},
])
This code is giving me this error:
Error: Failed to optimize pipeline :: caused by :: The argument to $size must be an array, but was of type: string
What Am I doing wrong ?
You can do it like this:
db.questions.aggregate([
{
"$lookup": {
"from": "tests",
"localField": "_id",
"foreignField": "questions",
"as": "usedIn"
}
},
{
"$project": {
"usedIn": {
"$size": "$usedIn"
},
"name": 1
}
}
])