简单的汇总根和存储库 [英] Simple aggregate root and repository

问题描述

我是试图理解聚合根概念的众多人之一，我想我已经理解了！
但是，当我开始为这个示例项目建模时，我很快陷入了困境。

I'm one of many trying to understand the concept of aggregate roots, and I think that I've got it! However, when I started modeling this sample project, I quickly ran into a dilemma.

我有两个实体 ProcessType 和 Process 。没有 ProcessType 和 ProcessType 的 Process 是不存在的有许多进程。因此，进程拥有对类型的引用，没有它就不能存在。

I have the two entities ProcessType and Process. A Process cannot exist without a ProcessType, and a ProcessType has many Processes. So a process holds a reference to a type, and cannot exist without it.

ProcessType 应该是一个集合根？新进程将通过调用 processType.AddProcess（new Process（））创建。
但是，我还有其他实体，它们仅持有对 Process 的引用，并通过 Process.Type 。在这种情况下，先通过 ProcessType 是没有意义的。

So should ProcessType be an aggregate root? New processes would be created by calling processType.AddProcess(new Process()); However, I have other entities that only holds a reference to the Process, and accesses its type through Process.Type. In this case it makes no sense going through ProcessType first.

但是，聚合之外的AFAIK实体仅被允许保存对集合根的引用，而不是对集合内实体的引用。那我这里有两个聚合，每个聚合都有自己的存储库吗？

But AFAIK entities outside the aggregate are only allowed to hold references to the root of the aggregate, and not entities inside the aggregate. So do I have two aggregates here, each with their own repository?

推荐答案

我在很大程度上同意西西弗斯所说的话，特别是关于不将自己限制在DDD的规则上可能会导致非常不合逻辑的解决方案。

I largely agree with what Sisyphus has said, particularly the bit about not constricting yourself to the 'rules' of DDD that may lead to a pretty illogical solution.

关于您的问题，我遇到过很多次这种情况，我将 ProcessType称为查找。查找是定义的对象，并且具有否对其他实体的引用；在DDD术语中，它们是价值对象。我称之为查找的其他示例可能是团队成员的 RoleType，例如可以是测试人员，开发人员或项目经理。甚至将一个人的标题都定义为查找-先生，小姐，夫人，博士

In terms of your problem, I have come across the situation many times, and I would term 'ProcessType' as a lookup. Lookups are objects that 'define', and have no references to other entities; in DDD terminology, they are value objects. Other examples of what I would term a lookup may be a team member's 'RoleType', which could be a tester, developer, project manager for example. Even a person's 'Title' I would define as a lookup - Mr, Miss, Mrs, Dr.

我将您的流程汇总建模为：

I would model your process aggregate as:

public class Process
{
     public ProcessType { get; }
}

正如您所说，这些类型的对象通常需要在UI因此需要自己的数据访问机制。但是，我个人并没有为此创建存储库，而是创建了 LookupService。对我来说，这通过严格保留用于聚合根的存储库来保持DDD的优雅。

As you say, these type of objects typically need to populate dropdowns in the UI and therefore need their own data access mechanism. However, I have personally NOT created 'repositories' as such for them, but rather a 'LookupService'. This for me retains the elegance of DDD by keeping 'repositories' strictly for aggregate roots.

以下是我的应用服务器上的命令处理程序以及如何实现的示例此：

Here is an example of a command handler on my app server and how I have implemented this:

团队成员总计：

public class TeamMember : Person
{
    public Guid TeamMemberID
    {
        get { return _teamMemberID; }
    }

    public TeamMemberRoleType RoleType
    {
        get { return _roleType; }
    }

    public IEnumerable<AvailabilityPeriod> Availability
    {
        get { return _availability.AsReadOnly(); }
    }
}

命令处理程序：

public void CreateTeamMember(CreateTeamMemberCommand command)
{
    TeamMemberRoleType role = _lookupService.GetLookupItem<TeamMemberRoleType>(command.RoleTypeID);

    TeamMember member = TeamMemberFactory.CreateTeamMember(command.TeamMemberID,
                                                           role,
                                                           command.DateOfBirth,
                                                           command.FirstName,
                                                           command.Surname);

    using (IUnitOfWork unitOfWork = UnitOfWorkFactory.CreateUnitOfWork())
        _teamMemberRepository.Save(member);
}

客户端还可以使用LookupService填充下拉菜单等：

The client can also make use of the LookupService to populate dropdown's etc:

ILookup<TeamMemberRoleType> roles = _lookupService.GetLookup<TeamMemberRoleType>();

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