relational_types

Modeling the relations between objects. See documentation to know more.

lib.rs:

Modeling the relations between objects.

By default, feature relational_types_procmacro is enabled, exposing macros to help build relations. See documentation of the crate relational_types_procmacro for more information.

This module defines types for modeling the relations between objects, and use them thanks to the GetCorresponding custom derive.

Let's clarify that with an example. Suppose that Bikes have a Brand. Bikes also have an Owner, and these Owners have a Job. Bikes also have a Kind.

Brand - Bike - Owner - Job
         |
        Kind

Let's defines these relations and use them a bit:

#[derive(Default, GetCorresponding)]
pub struct World {
    brands_to_bikes: OneToMany<Brand, Bike>,
    owners_to_bikes: OneToMany<Owner, Bike>,
    jobs_to_owners: OneToMany<Job, Owner>,
    kinds_to_bikes: OneToMany<Kind, Bike>,
}
let world = World::default();
let mbk: Idx<Brand> = get_mbk_brand();
let owners_with_mbk: IdxSet<Owner> = world.get_corresponding_from_idx(mbk);
let jobs_with_mbk: IdxSet<Job> = world.get_corresponding(&owners_with_mbk);
println!(
   "{} owners with {} different jobs own a bike of the brand MBK.",
   owners_with_mbk.len(),
   jobs_with_mbk.len()
);

First, we want to model the relations between the object. One bike has a brand, and a brand has several bikes (hopefully). Thus, we use a OneToMany<Bike, Brand> to model this relation.

We repeat this process to model every relation. We obtain without too much effort the World struct.

The GetCorresponding derive looks at each field of the World struct, keeping the fields containing _to_ with a type with 2 generics, and interpret that as a relation. For example, bikes_to_brands: OneToMany<Bike, Brand> is a relation between Bike and Brand. Using all the relations, it generates a graph, compute the shortest path between all the types, and generate an impl GetCorresponding for each feasible path.

These impl GetCorresponding are used by World::get_corresponding_from_idx and World::get_corresponding that are helpers to explore the World.

Thus, when we call world.get_corresponding_from_idx(mbk) for Owner, we will use the generated code that, basically, gets all the Bikes corresponding to the Brand MBK, and then gets all the Owners corresponding to these Bikes.

Imagine that, in our application, we use a lot the Owner->Kind and Brand->Kind search. To do these searches, we pass by Bike, and there is a lot of Bikes in our model. Thus, as an optimization, we want to precompute these relations.

Brand - Bike - Owner - Job
   \     |      /
    `-- Kind --'

The shortcuts Brand - Kind and Kind - Owner allow our optimization, but we now have a problem for the Owner->Brand search: we can do Owner->Kind->Brand and Owner->Bike->Brand with a cost of 2. The first solution is clearly wrong, introduced by our shortcuts. To fix this problem, we can put a weight of 1.9 on Brand - Kind and Kind - Owner. The path Owner->Kind->Brand now cost 3.8 and is discarded.

Let's implement that:

#[derive(GetCorresponding)]
pub struct World {
    brands_to_bikes: OneToMany<Brand, Bike>,
    owners_to_bikes: OneToMany<Owner, Bike>,
    jobs_to_owners: OneToMany<Job, Owner>,
    kinds_to_bikes: OneToMany<Kind, Bike>,

    // shortcuts
    #[get_corresponding(weight = "1.9")]
    brands_to_kinds: ManyToMany<Brand, Kind>,
    #[get_corresponding(weight = "1.9")]
    kinds_to_owners: ManyToMany<Kind, Owner>,
}
impl World {
    fn new() -> World {
        let brands_to_bikes = create_brands_to_bikes();
        let owners_to_bikes = create_owners_to_bikes();
        let jobs_to_owners = create_jobs_to_owners();
        let kinds_to_bikes = create_kinds_to_bikes();
        World {
            brands_to_kinds: ManyToMany::from_relations_sink(
                &brands_to_bikes,
                &kinds_to_bikes,
            ),
            kinds_to_owners: ManyToMany::from_relations_sink(
                &kinds_to_bikes,
                &owners_to_bikes,
            ),
            brands_to_bikes,
            owners_to_bikes,
            jobs_to_owners,
            kinds_to_bikes,
        }
    }
}