Mongoose - REST API - Schema With Query to different model

In this case, the best way to handle it would be to add a post save hook to your Activity schema to store the most recent _id in the latest_activity path of your User schema. That way you'd always have access to the id without having to do the extra query.

ActivitySchema.post('save', function(doc) {    UserSchema.findOne({username: doc.owner}).exec(function(err, user){        if (err)            console.log(err); //do something with the error        else if (user) {            user.latest_activity = doc._id;            user.save(function(err) {                if (err)                    console.log(err); //do something with the error            });        }    });});

Inspired by @BrianShambien's answer you could go with the post save, but instead of just storing the _id on the user you store a sub doc of only the last activity. Then when you grab that user it has the last activity right there.

User Model

username :     {type: String, unique: true},age :          {type: Number},last_activity: ActivitySchema

Then you do a post save hook on your ActivitySchema

ActivitySchema.post('save', function(doc) {    UserSchema.findOne({username: doc.owner}).exec(function(err, user){        if (err) errHandler(err);        user.last_activity = doc;        user.save(function(err) {            if (err) errHandler(err);        });    });});

**********UPDATE************

This is to include the update to the user if they are not an owner, but a particpant of the the activity.

ActivitySchema.post('save', function(doc) {    findAndUpdateUser(doc.owner, doc);    if (doc.participants) {        for (var i in doc.participants) {            findAndUpdateUser(doc.participants[i], doc);        }    }});var findAndUpdateUser = function (username, doc) {    UserSchema.findOne({username: username}).exec(function (err, user) {        if (err) errHandler(err);        user.last_activity = doc;        user.save(function (err) {            if (err) errHandler(err);        });    });});