Re: [boost] [multi_index] Relationship Table

From: JOAQUIN LOPEZ MU?Z (joaquin_at_[hidden])
Date: 2006-10-13 06:48:50

Hello Joe,

----- Mensaje original -----
De: Joe Gottman <jgottman_at_[hidden]>
Fecha: Jueves, Octubre 12, 2006 3:19 am
Asunto: [boost] [multi_index] Relationship Table
Para: boost_at_[hidden]

>   One pattern that seems to come up pretty often is the 
> relationship table. 
> Suppose I have 2 classes, Employee and Skill.  An Employee can 
> have any number of Skills, and a Skill can be possessed by any
> number of Employees. But an Employee can have a given Skill
> only once.  The user would need to be able to search for which
> Skills a given Employee has, and which Employees have a given
> Skill.  What is the best way of using a multi_index_container 
> to model this use case?

For instance, you can have the following (warning: uncompiled,
beware typos):

typedef std::pair<
  boost::reference_wrapper<const Employee>,
  boost::reference_wrapper<const Skill>
> EmployeeSkillPair;

typedef member<
  EmployeeSkillPair,
  EmployeeSkillPair::first_type,
  &EmployeeSkillPair::first
> EmployeeExtractor;

typedef member<
  EmployeeSkillPair,
  EmployeeSkillPair::second_type,
  &EmployeeSkillPair::second
> SkillExtractor;

multi_index_container<
  EmployeeSkillPair,
  indexed_by<
    ordered_unique<
      composite_key<
        EmployeeSkillPair,
        EmployeeExtractor,
        SkillExtractor
      >
    >,
    ordered_non_unique<
      EmployeeExtractor
    >
  >
> EmployeeSkillRelations;

The first index enforces the uniqueness of a given
Employee-Skill link and lets you obtain the Skills
associated to an Employee (use equal_range passing
the Employee of interest --inside a tuple due to
the usage of composite keys, please consult the docs.)
The second index lets you retrieve the Employees
having a particular Skill.
 
>   Suppose, in addition, that every Employee who has a Skill has
> a numerical rating at that Skill.  How would you do a
> multi_index_container in that case?

Augment EmployeeSkillPair to store this rating too.

> Joe Gottman 

Joaquín M López Muñoz
Telefónica, Investigación y Desarrollo