public class BacklogItem extends ConcurrencySafeEntity {
  ...
  private Product product;
  ...
}

public class BacklogItem extends ConcurrencySafeEntity {
  ...
  private ProductId productId;
  ...
}

public class ProductBacklogItemService ... {
    ...
    @Transactional
    public void assignTeamMemberToTask( String aTenantId,
        String aBacklogItemId, String aTaskId,
        String aTeamMemberId) {
        BacklogItem backlogItem = backlogItemRepository.backlogItemOfId(
          new TenantId(aTenantId),
          new BacklogItemId(aBacklogItemId)
        );
        Team ofTeam =
            teamRepository.teamOfId( backlogItem.tenantId(), backlogItem.teamId());
        backlogItem.assignTeamMemberToTask(
          new TeamMemberId(aTeamMemberId), ofTeam,
          new TaskId(aTaskId)
        );
      }
      ...
}

public class BacklogItem extends ConcurrencySafeEntity {
  ...
  public void commitTo(Sprint aSprint) {
    ...
    DomainEventPublisher
      .instance()
      .publish(
          new BacklogItemCommitted(
            this.tenantId(),
            this.backlogItemId(),
            this.sprintId()
          )
      );
  }
  ...
}

public class ProductBacklogItemService ... {
    ...
    @Transactional
    public void planBatchOfProductBacklogItems(
      String aTenantId, String productId,
      BacklogItemDescription[] aDescriptions) {
        Product product = productRepository.productOfId(
          new TenantId(aTenantId), new ProductId(productId)
        );
        for (BacklogItemDescription desc : aDescriptions) {
          BacklogItem plannedBacklogItem = product.planBacklogItem(
            desc.summary(), desc.category(),
            BacklogItemType.valueOf(desc.backlogItemType()),
            StoryPoints.valueOf(desc.storyPoints())
          );
            backlogItemRepository.add(plannedBacklogItem);
        }
    }
    ...
}