Create a relation from list of Ids - Entity Framework - entity-framework-core

Question

Is there a way to create a relationship using a list of ids?
I have done this for a single property like below:
builder.HasOne<Employee>()
.WithMany()
.HasForeignKey(x => x.EmployeeId);
But couldn't figure out how to do it if I have a list of EmployeeId.

Answer 1

In a database you don't create a Company table with a column with a list of Employee IDs... You either add a CompanyId to your Employee table for the company they belong to, or you create a joining table CompanyEmployee which links the two.
Entity Framework entities essentially represent relational data using objects. A Company can have a List of Employees (not employee IDs) where EF will work out (or be configured) to expect that the Employee will manage the FK reference back to its company.
public class Company
{
[Key]
public int CompanyId { get; set; }
public virtual ICollection<Employee> Employees { get; set; } = new List<Employee>();
}
public class Employee
{
[Key]
public int EmployeeId { get; set; }
[ForeignKey("CompanyId")]
public virtual Company Company { get; set; }
}
This is assuming you are using EF Core 6 (given .Net Core 6 tag) With EF6 you would either need to define the FK property for CompanyId or Map the shadow property with .Map(x => x.MapKey()) EF Core can manage this a bit better. EF works out the relationships, you don't need anything like trying to have a List<int> on Company for employee IDs.
Now if you happen to ever want to get the Employee IDs for a given company you can fetch that via the company or the employees:
var employeeIds = context.Company
.Where(x => x.CompanyId == companyId)
.SelectMany(x => x.Employees.Select(e => e.EmployeeId).ToList())
.ToList();
or
var employeeIds = context.Employees
.Where(x => x.CompanyId == companyId)
.Select(x => x.EmployeeId)
.ToList();