Model save() method not dealing with mappings

[Tegola]

Hello,
I'm working with a model that use a mapping, and I'm seeing a different behavior with GET and POST, with POST not getting all data.

Here's the simplified version of my model:

Ext.define('MyMessage', {
    extend: 'Ext.data.Model',
    config: {
        fields: [
            { name: 'id', type: 'int' },
            { name: 'title', type: 'string' },
            { name: 'body', type: 'string' },
        ],
        associations: [
            { type: 'belongsTo', model: 'MyUser', foreignKey: 'author.id' },
        ]
    }
});

When calling MyMessage.load(<id>), the "author.id" field works fine.
But then if I call:

var newMsg = Ext.create('MyMessage', {
    title: 'MyTitle',
    body: 'MyBody'
});


newMsg.save();

The record is saved, the local client data is updated except for the "author.id" association, which is undefined. Even tough the data returned from the server has the "author" object inside.

Is that normal?