MongoDB get only matching elements from nested array - mongodb

Question

I have a collection like this:
{
_id : 123,
username : "xy",
comments : [
{
text : "hi",
postdate : 123456789
},
{
text : "hi1",
postdate : 555555555
},
{
text : "hi2",
postdate : 666666666
},
{
text : "hi3",
postdate : 987654321
}
]}
Now I want only the comments that have postdate 555555555 or higher and 987654321 or lower. I have this query, but it doesn't work:
db.post.aggregate([
{$match : {$and : [
{"_id": ObjectId("123")},
{"comments.posttime" : {$lte : 987654321}},
{"comments.posttime" : {$gte : 555555555}}
]}}
,{$unwind: "$comments"}]).pretty();
But when I try this it gets me all of the array elements. How should this be done?
Thank you!

Answer 1

Use $redact to Restricts the contents of the document,
db.an.aggregate([{
$redact: {
"$cond": [{
$and: [{
"$gte": [{
"$ifNull": ["$postdate", 555555555]
},
555555555
]
}, {
"$lte": [{
"$ifNull": ["$postdate", 987654321]
},
987654321
]
}]
}, "$$DESCEND", "$$PRUNE"]
}
}]).pretty()

Answer 2

you have to unwind the comments first and then do the match. so that comments array will be flattened and match condition can filter it properly.
[{$unwind: "$comments"},{$match : {$and : [
{"_id": ObjectId("123")},
{"comments.posttime" : {$lte : 987654321}},
{"comments.posttime" : {$gte : 555555555}}
]}}]
this will give one row for each comment, if you want the matching comments inside the array, use aggregate on _id and $push the comments