Relationships

Relationships in Red are defined by the is relationship trait. It should receive (at least) a Str named parameter called :model that represents the related model, a Callable positional parameter that will return one (or more) referencing columns, and it must have a sigil. The sigil can be:

• $ - Represents a belongs to relationship that “stores” only 1 element. Red will run the callable passing the model itself as the only parameter.

• @ - Represents a has many relationship that “stores” a ResultSeq of elements. Red will run the callable passing the model defined by the :model named parameter.

(for the experimental has-one, please take a look at experimental doc)

So, for example:

# Person.rakumod
use Red:ver<2>;

model Person {
  has UInt $!id    is serial;
  has Str  $.name  is column;
  has      @.books is relationship(*.author-id, :model<Book>);
}

# Book.rakumod
use Red:ver<2>;

model Book {
  has UInt $!id        is serial;
  has Str  $.name      is column;
  has UInt $!author-id is referencing(*.id, :model<Person>);
  has      $.author    is relationship(*.author-id, :model<Person>);
}

Book has a foreign key ($!author-id) that references Person.id (this is defined by the is referencing trait). And it also has a relationship ($.author) that’s auto pre-fetched using a join based on book.author_id = person.id.

Person has no foreign key but it does have a relationship (@.books) that isn’t pre-fetched because it’s a has many relationship. So when its value is accessed a new query is run.

On that example, to create a new book related to (written by) a given person, one could do something like:

my $author = Person.^all.first: *.name eq "Chico Buarque";
$author.books.create: :name<Benjamin>;

As mentioned, @.books is a ResultSeq, so it accepts any of its methods, eg:

$author.books.grep(*.starts-with: "Ben").elems

For getting the number of books written by “Chico Buarque” that starts with “Ben”.