Mongoose - Add 'is_self' field in aggregate - node.js

Question

I'm building a chat app, that should retrieve all new messages from MongoDB, grouped in to conversations. But each message should have a new 'is_self' field
Edit:
The 'is_self' field contains a boolean for if the message if from the user.
so pseudo:
is_self: {$cond: {if: {message.sender == MYID)}, then: true, else: false}
So lets say I have Message model
var MessageSchema = new Schema({
conversation_id:{type: mongoose.Schema.ObjectId, ref: 'Conversation', required: true},
message: {type: String, required: true},
sender: {type: mongoose.Schema.ObjectId, ref: 'User'},
created: {type: Date, default: Date.now},
read: {type: Boolean, default: false}
});
And a Conversation model
var ConversationSchema = new Schema({
from: {type: mongoose.Schema.ObjectId, ref: 'User', required: true},
to: {type: mongoose.Schema.ObjectId, ref: 'User', required: true},
last_changed: {type: Date, default: Date.now},
created: {type: Date, default: Date.now}
});
Now I try to do an Aggregate, to load all messages that are inside a conversation_id array and are created > last_checked date...
So it looks like this:
mongoose.model("Message").aggregate([
// First find all messages
{
$match: {
$and: [{conversation_id: {$in: idArray}}, {created: {$gt: lastChecked}}]
}
},
// Add is self field
{
$group: {
_id: $_id,
$is_self: {
$cond: {'if(message.sender == MYID then true else false': '??'}
}
}
},
// Sort by date
{$sort: {created: -1}},
// Then group by conversation
{
$group: {
_id: '$conversation_id',
messages: {
$push: '$$ROOT'
},
}
}
// TODO: find users for unknown conversation
/*,
{
$project: {
user: {
$or: [{conversation_id: {$in: knownConversations}}]
}
}
}*/
])
I tried with $cond and if / else statement but Mongo doesn't allow that..
Thanks!

Answer 1

Simple usage of the $eq operator which returns boolean. Also $push will take any object format you throw at it:
var senderId = // whatever;
mongooose.model("Message").aggregate([
{ "$match": {
"conversation_id": { "$in": idArray },
"created": { "$gt": lastChecked }
}},
{ "$group": {
"_id": "$conversation_id",
"messages": {
"$push": {
"message": "$message",
"is_self": {
"$eq": [ "$sender", senderId ]
}
}
}
}}
// whatever else
],function(err,results) {
})
If you want, then combine with $cond to alternately add "is_self" only when detected:
{ "$group": {
"_id": "$conversation_id",
"messages": {
"$push": {
"$cond": [
{ "$eq": [ "$sender", senderId] },
{
"message": "$message",
"is_self": true
},
{
"messsage": "$message"
}
]
}
}
}}