MongoDB positional operator with nested arrays - mongodb

Question

Sample collection structure:
{
"_id" : ObjectId("57cfd62001ca2dd672cfebb1"),
"name" : "Category",
"parent" : ObjectId("57cfd5d101ca2dd672cfebb0"),
"posts" : [
{
"name" : "Post",
"author" : ObjectId("57cfd09401ca2dd672cfebac"),
"content" : "Some content.",
"comments" : [
{
"author" : ObjectId("57cfd09401ca2dd672cfebab"),
"content" : "First comment",
"rating" : 2
},
{
"author" : ObjectId("57cfd09401ca2dd672cfebac"),
"content" : "Second comment",
"rating" : 5
}
]
}
]
}
I would like to select all comments whose author is ObjectId("57cfd09401ca2dd672cfebab").
This query is working,
db.categories.find({ 'posts.comments.author':ObjectId("57cfd09401ca2dd672cfebab") })
but I would like to return only first matching comment with positional operator. Something like this is not working. Does MongoDB support positional operator with nested arrays?
db.categories.find({ 'posts.comments.author': ObjectId("57cfd09401ca2dd672cfebab") },
{ 'posts.comments.$': 1 })

Answer 1

You are having more than two level of nesting ...find() may not work ,instead try aggregation:-
> db.categories.aggregate([{$unwind:"$posts"},{$unwind:"$posts.comments"},
{$match:{"posts.comments.author":ObjectId("57cfd09401ca2dd672cfebab")}},
{$project:{_id:0,"posts.comments":1}}]).pretty()
Output:
{
"posts" : {
"comments" : {
"author" : ObjectId("57cfd09401ca2dd672cfebab"),
"content" : "First comment",
"rating" : 2
}
}
}