Mongoose 'reversed' population, i.e. populating a parent object based on the reference defined in child schema

If you want to get a ProjectGroup object, which contains all the projects under that group. You can use Populate Virtuals. (Mongoose version > 4.5.0)

create a virtual schema in your schema file.

ProjectGroupSchema.virtual('projects', {  ref: 'Project', // The model to use  localField: 'projectGroupId', // Your local field, like a `FOREIGN KEY` in RDS  foreignField: 'group', // Your foreign field which `localField` linked to. Like `REFERENCES` in RDS  // If `justOne` is true, 'members' will be a single doc as opposed to  // an array. `justOne` is false by default.  justOne: false});

and query in following:

ProjectGroup.find().populate('projects').exec(function(error, results) {  /* `results.projects` is now an array of instances of `Project` */});

If you cannot see virtuals part, plese set { toJSON: { virtuals: true } } to your model.

var ProjectGroupSchema = new Schema({    projectGroupId    : String,    title             : String}, { toJSON: { virtuals: true } });

You can achieve this by using aggregate function. First group projects by "projectGroup" and then populate result.

project.aggregate([   {$group: {_id: "$group", projects: {$push: "$$ROOT"}}}],  function(err,results) {    user.populate( results, { "path": "projects.subscribers" }, function(err,results) {        if (err)         console.log(err);        res.send(results);    });});