code: protected boolean shouldBindCollectionWithForeignKey(ToMany property) {
          return ((property instanceof org.grails.datastore.mapping.model.types.OneToMany) && property.isBidirectional() ||
                  !shouldCollectionBindWithJoinColumn(property)) &&
                  !Map.class.isAssignableFrom(property.getType()) &&
                  !(property instanceof ManyToMany) &&
                  !(property instanceof Basic);
      }
docs: /*
  We bind collections with foreign keys if specified in the mapping and only if
  it is a unidirectional one-to-many that is.