Database Systems: Design, Implementation, and Management

Theoretically, there are two things you need to think about, one of them being non-Cakephp specific.

A. Relationships: A relationship is always bidirectional. When determining the relationship between two Models/Objects/Tables, you always ask two questions:

1. How many instances of B are related to one instance of A?
2. How many instances of A are related to one instance of B?

You've said, One user has one Role. However, One Role has how many Users related to it? That will tell you the complete relationship between a User and a Role. (Apologies for the digression but this is important and I'm referencing this book.)

B. Difference between hasMany and belongsTo: This is determined based on the direction of traversing a relationship. Based on point A, say you've determined that: One User has one Role but One Role has many Users.

Now when you are in the User's model trying to fetch related Role data, you need to define the following in the User model:

class CoreUser extends AppModel {
  public $belongsTo = array(
    'Role' => array(
       'className' => 'CoreRole',
       'foreignKey' => 'role_id'
    )
  );
}

But when you are in the Role's model and trying to fetch related User data, you will need to define the following in the Role model:

class CoreRole extends AppModel {
  public $hasMany = array(
    'User' => array(
       'className' => 'CoreUser',
       'foreignKey' => 'role_id'
    )
  );
}

For a full discussion refer to this answer.