I'm starting to use Grakn and I'm having a hard time with how roles and relationships are "linked". Specifically, I am having some difficulty modeling role hierarchies.
Considering a fictional example with two scenarios, first suppose I have only two types: person and object; and a purchasing relationship between a customer role hierarchy and a product role hierarchy.
So, I would have something like:
type: person
role: customer level 3 (sub customer level 2)
...
type: object
The relationship is unique (buys) and is used to relate roles considering the respective levels in the hierarchy:
customer level 1 --- buys --- product level 1
customer level 2 --- buys --- product level 2
customer level 3 --- buys --- product level 3
In a second scenario, I would have only one role hierarchy, the customer one and the object type would have only one role: product. So, the relationships would be something like:
customer level 1 --- buys --- product
customer level 2 --- buys --- product
customer level 3 --- buys --- product
How can I model this kind of problem with Grakn (Graql)? I would appreciate it if someone could show how this (or similar) problem would be modeled with Graql.
To construct a role hierarchy in Graql you need to override a parent role with a child by defining a child relation.
I think that for your first case there are two possible options, depending upon what you want to restrict:
define
buys sub relation,
abstract,
relates product,
relates customer;
buys-level-1 sub buys,
relates product-level-1 as product,
relates customer-level-1 as customer;
buys-level-2 sub buys-level-1,
relates product-level-2 as product-level-1,
relates customer-level-2 as customer-level-1;
buys-level-3 sub buys-level-2,
relates product-level-3 as product-level-2,
relates customer-level-3 as customer-level-2;
person sub entity,
plays customer-level-1,
plays customer-level-2,
plays customer-level-3;
object sub entity,
plays product-level-1,
plays product-level-2,
plays product-level-3;
In this scenario, if you inserted a buys-level-3 relation:
insert
$per isa person; $obj isa object;
$b(product-level-3: $obj, customer-level-3: $per) isa buys-level-3;
You will be able to match for it using buys-level-2, buys-level-1 and buys. So this query will get back the person and the relation we just inserted:
match $b(customer-level-1: $per) isa buys-level-1; get;
Alternatively, you can change the restriction like this:
buys-level-3 sub buys,
relates product-level-3 as product,
relates customer-level-3 as customer;
In this case the match we made would not return the inserted relation or person. Let the restrictions that you want to impose on your queries/data guide you here for the best model choice.
The second scenario is then easy to see, we have:
define
buys sub relation,
abstract,
relates product,
relates customer;
buys-level-1 sub buys,
relates product,
relates customer-level-1 as customer;
buys-level-2 sub buys-level-1,
relates product,
relates customer-level-2 as customer-level-1;
buys-level-3 sub buys-level-2,
relates product,
relates customer-level-3 as customer-level-2;
person sub entity,
plays customer-level-1,
plays customer-level-2,
plays customer-level-3;
object sub entity,
plays product;
In any of these scenarios, you can retrieve all buys relations and their roleplayers with:
match $b(customer: $per, product: $obj) isa buys; get;