mongoose populate field using _id of its own document - mongodb

Question

I'm creating an appliction using MEAN stack in which i've one to one chat feature. I'm using mongoose and have 3 schemas :
Users
{
name: {
type: String,
required: true
},
email: {
type: String
},
phone: {
type: Number,
required: true
},
profile_pic: {
type: String,
default: null
},
password: {
type: String
},
salt: {
type: String
}
}
Messages
{
message : {
type : String,
required : true
},
timestamp : {
type : Number,
required : true
},
from : {
type : String,
required : true
},
conversation_id : {
type : mongoose.Schema.Types.ObjectId,
required : true
}
}
Conversations
{
users : [ {
type: type : mongoose.Schema.Types.ObjectId,
ref : User
}]
}
Now, when user goes to his messenger i want to show him all his past conversations with latest messages so right now i'm exceuting find command on Conversation and populating User
then after getting all conversations i'm executing find command for each conversation to get their latest message, so by this there are so many find operations are done on database in single request. i.e. why i need some solution like populate latest message for each conversation ( like i can populate using _id of conversation )
Edit
Here is the code i'm using right now :
let completeConversations = [];
conversation.find({ "users": req.body.token_data.id }).populate({ path: 'users', select: 'name profile_pic' }).exec((err, conversations) => {
if (err) throw err
if (conversations.length == 0) {
res.status(200);
res.json({ message: "No Conversations Found" })
} else {
conversations.forEach(element => {
messages.find({ conversation_id: element._id }).sort('-timestamp').limit(1).exec((err, message) => {
if (err) {
res.status(200);
res.json({ message: "Latest Message Not Found", conversations })
} else {
element = JSON.parse(JSON.stringify(element));
element.latest = message[0];
completeConversations.push(element);
}
if (completeConversations.length === conversations.length) {
res.json(completeConversations)
}
})
});
}
})
thanks in Advance