Ember Serializer is not serializing embbed records

I am trying to serialize my payload from server, but it is not working.

Here is an example of my payload:

events:[{
  id: "57f358856c616cf434fd0500"
  annotations:[{_id: "57f358856c616cf434ff0500", desc: "hello world"}]
}]

I want to change annotations _id to id.

Here is my serializer:

//event.js
export default ActiveModelSerializer.extend(DS.EmbeddedRecordsMixin, {
  attrs:{
    annotations:{embedded:'always'}
  }
});

//annotation.js
export default ActiveModelSerializer.extend(DS.EmbeddedRecordsMixin,{      
  attrs:{
    id: '_id',
  },
});

Even though I am using DS.EmbeddedRecordsMixin, it still doesn't work. Can anyone help me please? Thank you.