How to relationship

The way to represent a foreign key is using is referencing that has the same meaning as is column{ :references(...) }

A relationship is expressed with the trait is relationship and it's not a column. A relationship must receive a reference to a foreign key (a column with is referencing) (it will change in the future), and should pass a block that will return it.

If the attribute is Scalar (has $.attr is relationship{...}) the block will receive it's own model as a parameter to return the foreign key.

If the attribute is Positional (has @.attr is relationship{...}) the block will receive the target model as a parameter to return the foreign key.

model Person {...}

model Post is rw {
    has Int         $.id        is serial;
    has Str         $.title     is column{ :unique };
    has Str         $.body      is column;
    has Int         $!author-id is referencing{ Person.id };
    has Person      $.author    is relationship{ .author-id };
}

model Person is rw {
    has Int  $.id            is serial;
    has Str  $.name          is column;
    has Post @.posts         is relationship{ .author-id };
}

or

model Post is rw {
    has Int         $.id        is serial;
    has Str         $.title     is column{ :unique };
    has Str         $.body      is column;
    has Int         $!author-id is referencing{ :model<Person>, :column<id> };
    has             $.author    is relationship( *.author-id, :model<Person> );
}

model Person is rw {
    has Int  $.id            is serial;
    has Str  $.name          is column;
    has      @.posts         is relationship( *.author-id, :model<Post> );
}