MongoDB insert object into array if not exists - mongodb

Question

I am working on an app that manages the voting on polls by users. I use MongoDB as DB. (I am a newbie)
My DB is structured as follows:
[
{
"question_id": "0001",
"text": "What's your favourite color ? ",
"answers": [
{
"_id": "872ffaskdfba23jdafs",
"text": "Blue",
"user": {
"user_id": "u0001",
"nickname": "Paul"
}
},
{
"_id": "7832ab28b879293fabb2",
"text": "Red",
"user": {
"user_id": "u0002",
"nickname": "Eric"
}
},
{
"_id": "664oahh229s0f829323av",
"text": "Red",
"user": {
"user_id": "u0003",
"nickname": "Johhny"
}
}
]
}
]
Users can leave a maximum of answer for each question; so, for example, users Paul,Eric and Johhny can't vote anymore since their Id's are already present inside 'answers' array.
How could achieve this kind of control with MongoDB ?

Answer 1

You can use $reduce to achieve the behaviour.
create a union of the original array and the element you want to add
iterate the created union through $reduce, append the current element if the user_id is not already exists in our accumulator result; otherwise keep the accumulator result
db.collection.update({}, [
{
"$addFields": {
"answers": {
"$reduce": {
"input": {
"$concatArrays": [
"$answers",
[
<answer object that you want to insert>
]
]
},
"initialValue": [],
"in": {
"$cond": {
"if": {
"$not": {
"$in": [
"$$this.user.user_id",
"$$value.user.user_id"
]
}
},
"then": {
"$concatArrays": [
"$$value",
[
"$$this"
]
]
},
"else": "$$value"
}
}
}
}
}
}
])
Here is the Mongo playground for your reference