Collection AB {
  model B is table<bbb> { ... }
  model A is table<aaa> { has UInt $.id is serial; has B @.bs is relationship{ .a-id } }
  model B { has UInt $.id is serial; has UInt $.a-id is referencing{ A.id }; has A $.a is relationship{ .a-id } }

  has A $.a;
  has B $.b;
}

----------------------
# If multiple files.

use Model1;
use Model2;
use Model3;

Collection AB {
  has Model1 $.model1;
  has Model2 $.model2;
  has Model3 $.model3;
}