MongoDB fields limitation in array [duplicate] - mongodb

Question

This question already has answers here:
Retrieve only the queried element in an object array in MongoDB collection
(18 answers)
Closed 8 years ago.
I am looking for a way - and dont even now if this is possible - just to return a part of a list saved in mongodb.
Lets have a look in my currently document:
{
_id : 'MyId',
name : 'a string',
conversations : [
{
user : 'Mike',
input : 'Some input'
},
{
user : 'Stephano',
input : 'some other input'
}
]
}
What I now want to do is smth like this:
var myOutput;
myOutput = db.my_collection.find(
{
_id : 'MyId',
'conversations.user' : 'Mike'
}, {
_id : 1,
name : 1,
conversations : {
$where : {
user : 'Mike'
}
}
});
Goal is it just to get back the conversation array item where user has the value Mike.
Is this still possible in MongoDB ? didn't found any reference in the documentation for the field limitations in mongoDB.

Answer 1

Use the $ positional operator in a projection:
> db.my_collection.find({ "_id" : "MyId", "conversations.user" : "Mike" },
{ "_id" : 1, "name" : 1, "conversations.$" : 1 })
{
"_id" : 'MyId',
"name" : 'a string',
"conversations" : [
{ "user" : 'Mike', "input" : 'Some input' }
]
}
This projects only first matching array element.

Answer 2

Are you aware of the aggregation pipeline?
db.my_collection.aggregate([
{ "$match": { "_id": "MyId"}}, { "$unwind": "$conversations"},
{ "$match": {"conversations.user": "Mike"}}
])
Output
{
"_id" : "MyId",
"name" : "a string",
"conversations" :
{
"user" : "Mike",
"input" : "Some input"
}
}