How to project only matched array item in mongodb aggregation? - arrays

Question

I have a collection called shows, with documents as:
{
"url": "http://www.tvmaze.com/shows/167/24",
"name": "24",
"genres": [
"Drama",
"Action"
],
"runtime": 60
},
{
"url": "http://www.tvmaze.com/shows/4/arrow",
"name": "Arrow",
"genres": [
"Drama",
"Action",
"Science-Fiction"
],
"runtime": 60
}
I wanted to search shows with genre 'Action' and project the result array as
{
"url": "http://www.tvmaze.com/shows/167/24",
"name": "24",
"genres": [
"Action" // I want only the matched item in
//my result array
],
"runtime": 60
} , //same for the second doc as well
If I use
db.shows.find({genres:'Action'}, {'genres.$': 1});
It works but the same does not work in aggregate method with $project
Shows.aggregate([
{
$match: { 'genres': 'Action'}
},
{
$project: {
_id: 0,
url: 1,
name: 1,
runtime: 1,
'genres.$': 1
}
}
]);
this is the error I get on this aggregate query
Invalid $project :: caused by :: FieldPath field names may not start with '$'."

Answer 1

db.collection.aggregate([
{
$match: {
"genres": {
$regex: "/^action/",
$options: "im"
}
}
},
{
$project: {
_id: 0,
url: 1,
name: 1,
runtime: 1,
genres: {
$filter: {
input: "$genres",
as: "genre",
cond: {
$regexMatch: {
input: "$$genre",
regex: "/^action/",
options: "im"
}
}
}
}
}
}
])
Here is how I solved it, Thanks #turivishal for the help