createRelation method
Implementation
ModelRelation createRelation(String name, [int uid = 0]) {
final id = model.lastRelationId.id + 1;
if (uid != 0 && model.containsUid(uid)) {
throw StateError('uid already exists: $uid');
}
final uniqueUid = uid == 0 ? model.generateUid() : uid;
final relation = ModelRelation.create(IdUid(id, uniqueUid), name);
relations.add(relation);
model.lastRelationId = relation.id;
return relation;
}