Project boolean value if any of the nested array item satisfies a condition in mongoDB - mongodb

Question

An example structure of my collection is below
{
"_id" : ObjectId("5f631d6f3792ae9ce5e35ddd"),
"from" : "kathy",
"content" : "hello",
"to" : [
{
"name" : "david",
"isfavorite" : true
},
{
"name" : "james",
"isfavorite" : false
},
{
"name" : "steve",
"isfavorite" : true
}
]
}
{
"_id" : ObjectId("5f631d6f3792ae9ce5e35dde"),
"from" : "kathy",
"content" : "hey",
"to" : [
{
"name" : "david",
"isfavorite" : false
},
{
"name" : "john",
"isfavorite" : false
},
{
"name" : "roy",
"isfavorite" : true
}
]
}
I am trying to get messages received by a particular person. In this case if we show steve's messages I need to know if it is set as favorite by him. What I need is to project the favorite flag on the outer document itself if it matches some condition
I tried this but not working
db.getCollection('Messages_Collection').aggregate([
{
$match:
{
"to":
{
$elemMatch:
{
"name":"steve"
}
}
}
},
{
$project:
{
"_id":1,
"from":1,
"to":1,
"isfavorite":
{
$cond:
{
if:
{
"$to":
{
$elemMatch:
{
"isfavorite":true,
"name":"david"
}
},
then:true,
else:false
}
}
}
}
}])
Mongo version :4.0.13

Answer 1

You can simply use $filter to filter the particular person and get.
[
{
$match: {
"to.name": "steve"
}
},
{
$addFields: {
isfavorite: {
"$arrayElemAt": [
{
$filter: {
input: "$to",
cond: {
$eq: [
"$$this.name",
"steve"
]
}
}
},
0
]
}
}
},
{
$addFields: {
isfavorite: "$isfavorite.isfavorite"
}
}
]
Working Mongo playground