Conditional Projection if element exists in Array in mongodb - node.js

Question

Is there a direct way to project a new field if a value matches one in a huge sub array. I know i can use the $elemMatch or $ in the $match condition, but that would not allow me to get the rest of the non matching values (users in my case).
Basically i want to list all type 1 items and show all the users while highlighting the subscribed user. The reason i want to do this through mongodb is to avoid iterating over multiple thousand users for every item. Infact that is the part 2 of my question, can i limit the number of user's array that would be returned, i just need around 10 array values to be returned not thousands.
The collection structure is
{
name: "Coke",
type: 2,
users:[{user: 13, type:1},{ user:2: type:2}]
},
{
name: "Adidas",
type: 1,
users:[{user:31, type:3},{user: 51, type:1}]
},
{
name: "Nike",
type: 1,
users:[{user:21, type:3},{user: 31, type:1}]
}
Total documents are 200,000+ and growing...
Every document has 10,000~50,000 users..
expected return
{
isUser: true,
name: "Adidas",
type: 1,
users:[{user:31, type:3},{user: 51, type:1}]
},
{
isUser: false,
name: "Nike",
type: 1,
users:[{user:21, type:3},{user: 31, type:1}]
}
and i've been trying this
.aggregate([
{$match:{type:1}},
{$project:
{
isUser:{$elemMatch:["users.user",51]},
users: 1,
type:1,
name: 1
}
}
])
this fails, i get an error "Maximum Stack size exceeded". Ive tried alot of combinations and none seem to work. I really want to avoid running multiple calls to mongodb. Can this be done in a single call?
I've been told to use unwind, but i am bit worried that it might lead to memory issues.
If i was using mysql, a simple subquery would have done the job... i hope i am overlooking a similar simple solution in mongodb.

Answer 1

Process the conditions for the array elements and match the result by using a combination of the $anyElementTrue which evaluates an array as a set and returns true if any of the elements are true and false otherwise, the $ifNull operator will act as a safety net that evaluates the following $map expression and returns the value of the expression if the expression evaluates to a non-null value. The $map in the $ifNull operator is meant to apply the conditional statement expression to each item in the users array and returns an array with the applied results. The resulting array will then be used evaluated by the $anyElementTrue and this will ultimately calculate and return the isUser field for each document:
db.collection.aggregate([
{ "$match": { "type": 1} },
{
"$project": {
"name": 1, "type": 1,
"isUser": {
"$anyElementTrue": [
{
'$ifNull': [
{
"$map": {
"input": "$users",
"as": "el",
"in": { "$eq": [ "$$el.user",51] }
}
},
[false]
]
}
]
}
}
}
])