mongoose conditional populate on subdocument

Question

i have a schema that looks like this:

var UserSchema = new Schema({
  name              : { type: String, required: true },
  email             : { type: String, lowercase: true },
  fences            : [{type: Schema.Types.ObjectId, ref: 'Group'}]
});

var GroupMemberSchema = new Schema({ 
    user  : { type: Schema.Types.ObjectId, ref: 'User', required: true },
  status  : { type: String, default : 'Invited' }
});

var GroupSchema = new Schema({
  name          : String,
  members       : [GroupMemberSchema],
  type          : String
});

Group and User are exported as their own collections. I have an endpoint api/users/me in which I want to get my user and all my groups. Within the group I want to populate the users on my members. I have this working properly with this code:

User.findOne({
      _id: userId
    })
    .populate('groups')
    .exec(function(err, user) { 
      if (err) return next(err);
      if (!user) return res.json(401);

      var options = {
        path: 'groups.members.user',
        model: 'User'
      };

      User.populate(user, options, function (err, user) {
        return res.json(user);
      });

    });

However, I do not want to populate each member's associated user IF the group type == 'Special'. How would I put on the options do this?

Answer 1

You can use the "match" property inside populate.

Something like this:

User.findOne({
      _id: userId
    })
    .populate('groups')
    .exec(function(err, user) { 
      if (err) return next(err);
      if (!user) return res.json(401);

      var options = {
        path: 'groups.members.user',
        model: 'User',
        match: { 'group.type': { $ne: 'Special' } }
      };

      User.populate(user, options, function (err, user) {
        return res.json(user);
      });

    });

Answer 2

You are populating subdocument, but it's no differences in populate mechanics

Just select all groups with type !== 'Special' and run populate on filtered array

var options = {
  path: 'members.user',
  model: 'User'
};
var specialGroups = _.filter(user.groups, function(group){return group.type !== 'Special'})

User.populate(specialGroups, options, function (err, user) {
  return res.json(user);
});

so inside groups array you have some documents populated and some not populated. It's strange, but you can use specialGroups instead of user.groups