MongoDB If condition as the second field in gt - mongodb

Question

I want to select the $user_a_seen_at field if $user_a_id == socket.user_id otherwise select the $user_b_seen_at field. But my query isn't working.
$gt: ["$$this.created_at", IF CONDITION TO SELECT A FIELD ]
$project: {
unread_messages: {
$size: {
$filter: {
input: "$messages",
cond: {
$and: [
{ $eq: ["$$this.to_id", socket.user_id] },
{
$gt: [
"$$this.created_at", {
if: { $eq: ["$user_a_id", socket.user_id] },
then: "$user_a_seen_at",
else: "$user_b_seen_at"
}
]
}
]
}
}
}
}
}
Sample data
{
"_id" : ObjectId("5e4c57649fad2e2cac9f8cd5"),
"user_a_id" : 1,
"user_b_id" : 2,
"user_a_seen_at" : ISODate("2020-02-18T21:30:12.418Z"),
"user_b_seen_at" : ISODate("2020-02-18T15:30:12.418Z"),
"messages" : [
{
"text" : "Hello",
"_id" : ObjectId("5e4c57649fad2e2cac9f8cd4"),
"from_id" : 1,
"to_id" : 2,
"created_at" : ISODate("2020-02-18T21:30:12.409Z")
}
],
"created_at" : ISODate("2020-02-18T21:30:12.418Z"),
"last_activity" : ISODate("2020-02-18T21:30:12.418Z"),
"__v" : 0
}

Answer 1

You need to use $cond operator to evaluate a boolean expression to return one of the two specified return expressions
{ $cond: { if: <boolean-expression>, then: <true-case>, else: <false-case> } }
//or simplified
{ $cond: [ <boolean-expression>, <true-case>, <false-case> ] }
db.collection.aggregate([
{
$project: {
unread_messages: {
$size: {
$filter: {
input: "$messages",
cond: {
$and: [
{
$eq: [
"$$this.to_id",
socket.user_id
]
},
{
$gt: [
"$$this.created_at",
{
$cond: [
{
$eq: [
"$user_a_id",
socket.user_id
]
},
"$user_a_seen_at",
"$user_b_seen_at"
]
}
]
}
]
}
}
}
}
}
}
])
MongoPlayground