Rules for HasOne and BelongsTo Associations in ExtJS

1. Put the proxy in the model, unless you have a very good reason not to [1]
2. Always use fully qualified model name
3. Always set the getterName
4. Always set the setterName
5. Always set the associationKey, if the foreign object is returned in the same response as this object
6. Always set the foreignKey, if you want to load the foreign object at will
7. Consider changing the instanceName to something shorter
8. The getter behaves differently depending on whether the foreign object is loaded or not. If it's loaded, the foreign object is returned. Otherwise, you need to pass in a callback to get it.
9. You should set the name property if you plan to override this association.
10. You do not need a belongsTo relationship for a hasMany to work
11. Set the primaryKey property if the id field of the parent model is not "id"
12. Sometimes you need to use uses or requires for the belongsTo association. Watch out for circular references though.
13. Calling setter() function does not seem to set the instance. Set object.belongsToInstance = obj  if calling the setter().

Ext.define('Assoc.model.PhoneNumber', {
    extend:'Ext.data.Model',

    fields:[
        'number',
        'contact_id'
    ],

    belongsTo:[
    {
      name:'contact',
      instanceName:'contact',
      model:'Assoc.model.Contact',
      getterName:'getContact',
      setterName:'setContact',
      associationKey:'contacts',
      foreignKey:'contact_id'
    }
    ],

    proxy:{
        type:'ajax',
        url:'assoc/data/phone-numbers.json',
        reader:{
            type:'json',
             root:'phoneNumbers'
        }
    }
});

/*
 * Assuming Contact model uses an AJAX proxy with url 'contacts', and its id field is "id", 
 * the below function call will make an http request like this:
 * /contacts?id=88
 */

var pn = new Assoc.model.PhoneNumber( { contact_id:88 } );

pn.getContact( function(contact, operation){ 
  console.log('tried to load contact. this.contact is now set to the contact');
} );

/* you can call phoneNumber.setContact(contact). This will set contact_id on the phone number, BUT it won't set the contact instance on phonenumber, which is likely a bug: */

var contact = new Assoc.model.Contact({id:77});

var phoneNumber = new Assoc.model.PhoneNumber();

phoneNumber.setContact(contact);

console.log(phoneNumber.get('contact_id')) //77

console.log(phoneNumber.contact) //undefined

phoneNumber.contact = contact; //you need to do this if you want to use that reference in the future.

[1] The store will inherit its model's proxy, and you can always override it if necessary