Map 表行到 DDD 值 object 带 EF 内核

Question

scenario

I have two aggregates in my microservice Domain model: User and UserNotification.

public class User : Aggreagete
{
    public int Id { get; private set; }
}

public class UserNotification : Aggreagete
{
    public int Id { get; private set; }

    public int UserId { get; private set; }

    public bool EnableSms { get; private set; }

    public bool EnableEmail { get; private set; }

    public NotificationKind NotificationKind { get; private  set; }

    public UserModel User { get;private set }
}

UserValueObject needs to be mapped from the same table as User Aggregate. I use UserValueObject because from Notification bounded Context it needs to be immutable.

I have following EF mappings:

    public void Configure(EntityTypeBuilder<User> builder)
    {
        builder.ToTable("Users", SCHEME);

        builder.HasKey(o => o.Id);
        builder.Property(o => o.Id);

        builder.HasMany<UserNotification>()
            .WithOne()
            .HasForeignKey(entry => entry.UserId)
            .OnDelete(DeleteBehavior.Cascade);
    }

    public void Configure(EntityTypeBuilder<UserNotification> builder)
    {
        builder.ToTable("UserNotifications", SCHEME);

        builder.HasKey(o => o.Id);
        builder.Property(o => o.Id);

        builder.Property(e => e.UserId).IsRequired();
        builder.Property(e => e.EnableSms).IsRequired();
        builder.Property(e => e.EnableEmail).IsRequired();
        builder.Property(e => e.NotificationKind).IsRequired()
            .HasConversion(
                v => v.ToString(),
                strValue => Enum.Parse<NotificationKind>(strValue)
            );
        builder.OwnsOne(e => e.User).ToTable("Users");
    }

problem

I receive error during "Add Migration"

Cannot use table 'dbo.Users' for entity type 'User' since it is being used for entity type 'UserModel' and there is no relationship between their primary keys.

UserModel is:

public class UserModel : ValueObject
{
    public int Id { get; private set; }
    public string Email { get; private set; }
    public string Phone { get; private set; }
    public string Name { get; private set; }
}

question

I would like to map UserModel.Id from database table "Users", but Email, Phone and Name should be injected in repository with using HTTP request to Identity Service. Is is possible to do such operation with EF core? I see that it is not possible to achieve this with owned entity, because Users table is used also to mapped other aggreagate.

Answer 1

You are getting the correct exception due to the name conflict.

For the User entity you ask EF to use the Users table:

builder.ToTable("Users", SCHEME);

but you also ask it to use the same table to store value objects of the UserNotification entity:

builder.OwnsOne(e => e.User).ToTable("Users");

If your plan is to "reuse" the Users aggregate persistence to magically map the value project to it than I can definitely tell that it's not a good idea.

Value objects are typed pieces of state. They belong to entities and managed by entities. You have to update the content of the VO when needed.

You also don't need to keep the user information in the aggregate, unless you use it in any of the methods of your notification aggregate. If you need to be able to query and join - just do that in the query. Don't use EF for queries and write a plain SQL with inner join to get the information you need. Look up CQRS, Jimmy Bogard advocated using EF for the write side and plain SQL queries for the read side some time ago, it makes total sense.