Ember model access to belongsTo property return null

I have a model Person with this property : address_fk: DS.belongsTo('address') refering to an Address model.

In the adress model, there are several properties such as zip_code , building ...

My goal is to create a computed property in Person to display the full address by accessing this.get('address_fk.zip_code') but it returns me undefined

I'm surely missing something here but I really don't know what.