Sorting Mongoose populate() with a condition

I’m currently using populate() like so:

Schemas:

  var DefinitionSchema = mongoose.Schema({
    title: String, 
    slug: String, 
    description: String
  });
  DefinitionSchema.index({description: 'text'});
  DefinitionSchema.plugin(textSearch);

  var SectionSchema = mongoose.Schema({
      heading: String,
      intro: String,
      alpha: false,
      definitions: [{ type: Schema.Types.ObjectId, ref: 'Definition' }]
  });

  var PageSchema = mongoose.Schema({
    time : { type : Date, default: Date.now },
    title: String, 
    slug: String, 
    intro: String,
    body: String,  
    sections: [SectionSchema]
  });

Page rendering:

exports.edit = function(req, res){
    Page.findOne({ slug: req.param('page') }, function(err, page){
        res.render('admin/page_edit', { page: page});
    })
    .populate({path: 'sections.definitions', options: {sort: {slug: 'asc'}}});
};

This means that sections.definitions are sorted alphabetically for every section.

I was wondering whether it was possible to pass a condition to the sort within populate, so that I can change the sorting based on the value of (the parent document) section.alpha.

I figured it would look like this:

function sorting(section) {
    var sort = {}
    if (section.alpha) {
        sort = {slug: 'ace'}
    }
    return sort;
}

query
...
.populate({path: 'sections.definitions', options: {sort: sorting(sections)}});

Any help would be much appreciated.

Thanks, Samuel