finding document in mongodb with specific id and username - json

Question

{
"data": [
{
"_id": 555,
"username": "jackson",
"status": "i am coding",
"comments": [
{
"user": "bob",
"comment": "bob me "
},
{
"user": "daniel",
"comment": "bob the builder"
},
{
"user": "jesus",
"comment": "bob the builder"
},
{
"user": "hunter",
"comment": "bob the builder"
},
{
"user": "jeo",
"comment": "bob the builder"
},
{
"user": "jill",
"comment": "bob the builder"
}
]
}
]
}
so i want to get the result with _id :555 and user:bob i tried with below code but i cant make it work it returns empty array
app.get('/all',function(req , res){
db.facebook.find({_id:555},{comments:[{user:"bob"}]},function(err,docs){res.send(docs,{data:docs});});
} );
i want the result to be like this listed below with the comment with user:bob
{
"_id": 555,
"username": "jackson",
"status": "i am coding",
"comments": [
{
"user": "bob",
"comment": "bob me "
}
]
}

Answer 1

Only aggregate or mapReduce could exclude items from subarray in output. Shortest is to use $redact:
db.facebook.aggregate({
$redact:{
$cond:{
if:{$and:[{$not:"$username"},{$ne:["$user","bob"]}]},
then: "$$PRUNE",
else: "$$DESCEND"
}
}
})
Explanation:
$reduct would be applied to each subdocument starting from whole document. For each subdocument $reduct would either prune it or descend. We want to keep top level document, that is why we have {$not:"$username"} condition. It prevents top level document from pruning. On next level we have comments array. $prune would apply condition to each item of comments array. First condition {$not:"$username"} is true for all comments, and second condition {$ne:["$user","bob"]} is true for all subdocuments where user!="bob", so such documents would be pruned.
Update: in node.js with mongodb native driver
db.facebook.aggregate([{
$redact:{
$cond:{
if:{$and:[{$not:"$username"},{$ne:["$user","bob"]}]},
then: "$$PRUNE",
else: "$$DESCEND"
}
}
}], function(err,docs){})
One more thing: $prune is new operator and available only in MongoDB 2.6