query to retrieve multiple objects in an array in mongodb - mongodb

Question

Suppose I have a array of objects as below.
"array" : [
{
"id" : 1
},
{
"id" : 2
},
{
"id" : 2
},
{
"id" : 4
}
]
If I want to retrieve multiple objects ({id : 2}) from this array, the aggregation query goes like this.
db.coll.aggregate([{ $match : {"_id" : ObjectId("5492690f72ae469b0e37b61c")}}, { $unwind : "$array"}, { $match : { "array.id" : 2}}, { $group : { _id : "$_id", array : { $push : { id : "$array.id"}}}} ])
The output of above aggregation is
{
"_id" : ObjectId("5492690f72ae469b0e37b61c"),
"array" : [
{
"id" : 2
},
{
"id" : 2
}
]
}
Now the question is:
1) Is retrieving of multiple objects from an array possible using find() in MongoDB?
2) With respect to performance, is aggregation is the correct way to do? (Because we need to use four pipeline operators) ?
3) Can we use Java manipulation (looping the array and only keep {id : 2} objects) to do this after
find({"_id" : ObjectId("5492690f72ae469b0e37b61c")}) query? Because find will once retrieve the document and keeps it in RAM. But if we use aggregation four operations need to be performed in RAM to get the output.
Why I asked the 3) question is: Suppose if thousands of clients accessing at the same time, then RAM memory will be overloaded. If it is done using Java, less task on RAM.
4) For how long the workingSet will be in RAM??
Is my understanding correct???
Please correct me if I am wrong.
Please suggest me to have right insight on this..

Answer 1

No. You project the first matching one with $, you project all of them, or you project none of them.
No-ish. If you have to work with this array, aggregation is what will allow you to extract multiple matching elements, but the correct solution, conceptually and for performance, is to design your document structure so this problem does not arise, or arises only for rare queries whose performance is not particularly important.
Yes.
We have no information that would allow us to give a reasonable answer to this question. This is also out of scope relative to the rest of the question and should be a separate question.