Batch entity creation in axon - domain-driven-design

Question

Disclaimer: I'm completely new to Event Sourcing, Axon Framework and DDD, so there is quite big possibility I'm doing something wrong.
I'm doing an application to manage events - conferences, workshops etc.
I have one aggregate root, which is EventProposal. There should be a possibility to assign a TodoList to given EventProposal. TodoList is another aggregate root, consisting of TodoItems - entities. TodoItem can be edited, mark as done etc.
There is a requirement of assigning TodoItem to TodoList, which I've implemented this way:
public class TodoList extends AbstractAnnotatedAggregateRoot {
#AggregateIdentifier
private TodoListId id;
#EventSourcedMember
private List<TodoItem> todoItems = Lists.newArrayList();
...
public void assignTodoItem(TodoItemId todoItemId, String content, LocalDate creationDate) {
if (alreadyHasTodoItemWith(todoItemId)) {
apply(new TodoItemNotAssignedToTodoList(
id, todoItemId)
);
return;
}
apply(new TodoItemAssignedToTodoListEvent(
id, todoItemId, content, creationDate)
);
}
#EventSourcingHandler
public void on(TodoItemAssignedToTodoListEvent event) {
final TodoItem item = TodoItemFactory.create(
event.todoItemId(),
event.description(),
event.createdAt()
);
todoItems.add(item);
}
Corresponding success-path command and event:
#Value
#Accessors(fluent = true)
public class AssignTodoItemToTodoListCommand {
#TargetAggregateIdentifier
private final TodoListId todoListId;
private final TodoItemId todoItemId;
private final String description;
private final LocalDate createdAt;
}
#Value
#Accessors(fluent = true)
public class TodoItemAssignedToTodoListEvent {
private final TodoListId todoListId;
private final TodoItemId todoItemId;
private final String description;
private final LocalDate createdAt;
}
This could be easly tested with Axon's BDD-way. (GivenThenFixture)
But now - there is another requirement: one should be able to create TodoList upon existing TodoListTemplate. Template is just an aggregate that wrapps a collection of TodoItemTemplates.
And there comes my implementation problem. I tried something like (in TodoList class):
public void fulfillWith(TodoListTemplate todoListTemplate, LocalDate creationDate) {
if (alreadyHasAnyTodoItem()) {
apply(new TodoListNotFulfilledWithTemplateEvent(
id,
todoListTemplate.id()
)
);
return;
}
apply(new TodoListFulfilledWithTemplateEvent(
id,
todoListTemplate.id(),
todoListTemplate.todoItemDescriptions(),
creationDate
)
);
}
#EventSourcingHandler
public void on(TodoListFulfilledWithTemplateEvent event) {
todoItems.addAll(
fromDescriptions(event.todoItemDescriptions(), event.fulfilledAt())
);
}
private Collection<TodoItem> fromDescriptions(Collection<String> descriptions, LocalDate creationDate) {
return descriptions.stream()
.map(description -> TodoItemFactory.create(description, creationDate))
.collect(Collectors.toList());
}
Again, commands and events:
#Value
#Accessors(fluent = true)
public class FulfillTodoListWithTemplateCommand {
private final TodoListId todoListId;
private final TodoListTemplateId todoListTemplateId;
private final LocalDate creationDate;
}
#Value
#Accessors(fluent = true)
public class TodoListFulfilledWithTemplateEvent {
private final TodoListId todoListId;
private final TodoListTemplateId todoListTemplateId;
private final List<String> todoItemDescriptions;
private final LocalDate fulfilledAt;
}
Problem:
As you can see - there is TodoItemFactory class involved, generating an unique ID:
public static TodoItem create(String content, LocalDate createdAt) {
return TodoItemFactory.create(nextId(), content, createdAt);
}
This way it can't be tested with axon - it gives me error org.axonframework.test.AxonAssertionError: Illegal state change detected!
This is quite obvious - working aggregate's ID is different than the one's constructed after aplying events.
Finally, my question arrives:
How should I solve this?
Generate those IDs somewhere and include them in outcoming TodoListFulfilledWithTemplateEvent? This leads to having two collections in an event - one for IDs, one for item contents/descriptions.
Assume that I got those IDs generated before and include them not only in outcoming Event, but also in incoming Command. This leads to same ugliness as before, but twice.
Do it in the way that I invoke 'assingTodoItemToTodoList' method multiple times, which will generate a lot of events and could produce reordering as events are async.
Sorry for verboseness, I tried to be as specific as possible.