How to work with Foreign Keys in C # ASP .NET Core?

Good night, guys.

I'm a beginner in C # ASP .NET Core and I'm having problems with Foreign Key. I need to populate a combobox with data from a database table.

The following is my model Seller

using System;
using System.Collections.Generic;
using System.Linq;

namespace SalesWebMvc.Models {
    public class Seller {
        public int id { get; set; }
        public string name { get; set; }
        public string email { get; set; }
        public double basesalary { get; set; }
        public DateTime birthdate { get; set; }
        public Department department { get; set; }
        public int DepartmentId { get; set; }
        public ICollection<SalesRecord> sales { get; set; } = new List<SalesRecord>();

        public Seller() {

        }

        public Seller(int id, string name, string email, double basesalary, DateTime birthdate, Department department) {
            this.id = id;
            this.name = name;
            this.email = email;
            this.basesalary = basesalary;
            this.birthdate = birthdate;
            this.department = department;
        }

        public void AddSales(SalesRecord sr) {
            sales.Add(sr);   
        }

        public void RemoveSales(SalesRecord sr) {
            sales.Remove(sr);
        }

        public double TotalSales(DateTime initial, DateTime final) {
            return sales.Where(sr => sr.date >= initial && sr.date <= final).Sum(sr => sr.amount);
        }
    }
}

The following is my Department model:

using System.Collections.Generic;
using System;
using System.Linq;

namespace SalesWebMvc.Models {
    public class Department {
        public int id { get; set; }
        public string nome { get; set; }
        public ICollection<Seller> sellers { get; set; } = new List<Seller>();

        public Department() {

        }

        public Department(int id, string name) {
            this.id = id;
            this.nome = name;
        }

        public void AddSeller(Seller s) {
            sellers.Add(s);
        }

        public double TotalSales(DateTime initial, DateTime final) {
            return sellers.Sum(seller => seller.TotalSales(initial, final));
        }
    }
}

In the view I put it like this:

<div class="form-group">
<label asp-for="Seller.DepartmentId" class="control-label"></label>
<select asp-for="Seller.DepartmentId" asp-items="@(new SelectList(Model.Departments,"Id",
"nome"))" class="form-control"></select>
</div>

Gives referential integrity error!

  

MySqlException: Can not add or update a child row: a foreign key   constraint fails

I would like to know the simplest way to put all departments within a combobox in the View.

Thank you.