如何在不同DbContext上找到的模型之间创建关系？

Question

I am developing an application following the DDD pattern.

I have the following contexts for employee management and user account management called

EmployeeManagementContext

and

UserAccountManagementContext

Both contexts are on a separate project.
The project for employee management has the following models.

public class Employee  
{  
    public int Id { get; private set; }  
    public string Name { get; private set; }  
    public DateTime DateOfBirth { get; private set; }  
}  

The project for user account management has the following models.

public class Employee  
{  
    public int Id { get; private set; }  
    public string Name { get; private set; }   
}

public class UserAccount  
{  
    public int Id { get; private set; }  
    public string Username { get; private set; }  
    public string Password { get; private set; }  
}  

EmployeeManagementContext

public class EmployeeManagementContext : DbContext  
{  
    public DbSet<Employee> Employees { get; set; }  
}  

UserAccountManagementContext

public class UserAccountManagementContext : DbContext  
{  
    public DbSet<UserAccount> UserAccounts { get; set; }  
}  

I can successfully migrate both context by having different context keys but the problem is I loose the relationship between the Employee and UserAccount models.

Basically, the business rules that I need to implement between the two models are as follow:

An Employee may or may not have a UserAccount.
A UserAccount is owned by exactly one Employee only.

This means that I should have a one to zero-or-one relationship between Employee and UserAccount like the diagram below.

Please disregard the wrong relationship notation, its seems to be a limitation of the tool I am using but it is a one-to-zero-or-one relationship I assure you.

I tried the following configurations in UserAccount project:

public class UserAccountConfiguration : EntityTypeConfiguration<UserAccount>  
{  
    HasKey(x => x.Id);  

    Property(x => x.Id)
        .HasDatabaseGeneratedOption(DatabaseGeneratedOption.None);  
}

public class EmployeeConfiguration : EntityTypeConfiguration<Employee>  
{  
    HasKey(x => x.Id);  

    Property(x => x.Id)
        .HasDatabaseGeneratedOption(DatabaseGeneratedOption.Identity);  

    HasOptional(x => x.UserAccount)
        .WithRequired(x => x.Employee);
}  

public class UserAccountManagementContext : DbContext  
{  
    public DbSet<UserAccount> UserAccounts { get; set; }  

    protected override void OnModelCreating(DbModelBuilder modelBuilder)
    {
        modelBuilder.Configurations.Add(new UserAccountConfiguration());
        modelBuilder.Configurations.Add(new EmployeeConfiguration());
        base.OnModelCreating(modelBuilder);
    }
}  

The above configurations result to an error because Employee table already exist because of the EmployeeManagementContext in employee management project.

If I try to add the following configuration in EmployeeConfiguration of employee management project,

ToTable("Users");

EF doesn't complain anymore and creates a Users table which then creates the relationship I need between Employee and UserAccount. But the problem is, if I try to query Employee/Users in UserAccountManagementContext, it doesn't contain anything and I don't think its good design to keep on creating smaller tables that is derived from the orginal table because it will only clutter the database, right?...

I would greatly appreciate your help, thanks.

Answer 1

You should focus more on the domain and less on the database.

From what I can see, you have two Aggregates ( Employee and UserAccount ), in possible 2 bounded contexts (I can't name them as I don't have enough data). In general it's not recommended to force any invariant in a strongly consistent manner between the two Aggregates but there are exceptions. They may be as well in different databases, having different technologies. Let's now see how you can enforce the two invariants:

An Employee may or may not have a UserAccount.

This can be modeled with a nullable UserAccountId on a Employee , without any low level database references. Depending on the business rules, when an UserAccound is deleted (if this is a valid business operation on it), using a Saga/Process manager, you can set to null the corresponding UserAccountId in the Employee that had this account.

A UserAccount is owned by exactly one Employee only.

The simplest way to enforce this invariant is of technological nature: create an unique index on the UserAccountId . Other solutions imply using Sagas but are not as good as this one, for example would permit for a short period of time for the invariant to be broken.