User-Employee relationship missing constraints

[eilst]

Is your feature request related to a problem? Please describe.

There are at least two behaviors of the system that depend on the following missing constraint: Each hr.employee must have one res.user (required). And one res.user can only have one hr.employee.

1. At least one record rule has this missing constraint as a dependency. (a traceback will be generated if not respected)
2. In [ADD] Add check double bookable partners #395, each hr.employee is expected to have a res.partner.since it has this related field: related='user_id.partner_id.allow_double_book',

Describe the solution you'd like
Each hr.employee must have one res.user (required). And one res.user can only have one hr.employee.