The common pattern in CakePHP is to give each table a Primary, Auto Incrementing `id` column, but when utilizing HasOne and BelongsTo relationships between two models this column is superflous and can make tracing downstream relationships more difficult. For example, when splitting data between User and Profile models, giving the profile it's own `id` column adds no useful information since the `user_id` column will by definition be a unique reference to a user. The auto increment values may remain in sync and give you two columns of duplicate data, but if they diverge models related to the profile (e.g. ProfilePhoto) now lose their direct association with the user model. It is relatively easy though to modify the models and clean things up.