diff --git a/WD7UVN_HFT_2023241.Models/Customer.cs b/WD7UVN_HFT_2023241.Models/Customer.cs
index 1cdd9f2..4001ddd 100644
--- a/WD7UVN_HFT_2023241.Models/Customer.cs
+++ b/WD7UVN_HFT_2023241.Models/Customer.cs
@@ -17,6 +17,6 @@ namespace WD7UVN_HFT_2023241.Models
         public string EMAIL { get; set; }
         public string PHONE { get; set; }
         [ForeignKey(nameof(Service))]
-        public int SERVICE_ID { get; set; }
+        public int? SERVICE_ID { get; set; }
     }
 }
diff --git a/WD7UVN_HFT_2023241.Models/Employee.cs b/WD7UVN_HFT_2023241.Models/Employee.cs
index 78253a6..cfd301b 100644
--- a/WD7UVN_HFT_2023241.Models/Employee.cs
+++ b/WD7UVN_HFT_2023241.Models/Employee.cs
@@ -1,10 +1,5 @@
-using System;
-using System.Collections.Generic;
-using System.ComponentModel.DataAnnotations;
+using System.ComponentModel.DataAnnotations;
 using System.ComponentModel.DataAnnotations.Schema;
-using System.Linq;
-using System.Text;
-using System.Threading.Tasks;
 
 namespace WD7UVN_HFT_2023241.Models
 {
@@ -17,8 +12,8 @@ namespace WD7UVN_HFT_2023241.Models
         public string EMAIL { get; set; }
         public string PHONE { get; set; }
         [ForeignKey(nameof(MaintainerTeam))]
-        public int MAINTAINER_ID { get; set; }
+        public int? MAINTAINER_ID { get; set; }
         [ForeignKey(nameof(Employee))]
-        public int MANAGER_ID { get; set; }
+        public int? MANAGER_ID { get; set; }
     }
 }
diff --git a/WD7UVN_HFT_2023241.Models/MaintainerTeam.cs b/WD7UVN_HFT_2023241.Models/MaintainerTeam.cs
index 585f09c..217393c 100644
--- a/WD7UVN_HFT_2023241.Models/MaintainerTeam.cs
+++ b/WD7UVN_HFT_2023241.Models/MaintainerTeam.cs
@@ -11,8 +11,7 @@ namespace WD7UVN_HFT_2023241.Models
         [Required]
         public string NAME { get; set; }
         public string EMAIL { get; set; }
-        [Required]
         [ForeignKey(nameof(Employee))]
-        public int LEADER_ID { get; set; }
+        public int? LEADER_ID { get; set; }
     }
 }
diff --git a/WD7UVN_HFT_2023241.Models/Service.cs b/WD7UVN_HFT_2023241.Models/Service.cs
index c445502..9da9cf4 100644
--- a/WD7UVN_HFT_2023241.Models/Service.cs
+++ b/WD7UVN_HFT_2023241.Models/Service.cs
@@ -8,8 +8,7 @@ namespace WD7UVN_HFT_2023241.Models
         [Key]
         public int ID { get; set; }
         [ForeignKey(nameof(MaintainerTeam))]
-        [Required]
-        public int MAINTAINER_ID { get; set; }
+        public int? MAINTAINER_ID { get; set; }
         [Required]
         public string NAME { get; set; }
         public string VERSION { get; set; }
diff --git a/WD7UVN_HFT_2023241.Repository/CompanyDbContext.cs b/WD7UVN_HFT_2023241.Repository/CompanyDbContext.cs
index ce38ab2..1a05e90 100644
--- a/WD7UVN_HFT_2023241.Repository/CompanyDbContext.cs
+++ b/WD7UVN_HFT_2023241.Repository/CompanyDbContext.cs
@@ -40,7 +40,7 @@ namespace WD7UVN_HFT_2023241.Repository
                 .HasOne<Service>()
                 .WithMany()
                 .HasForeignKey(customer => customer.SERVICE_ID)
-                .OnDelete(DeleteBehavior.Cascade));
+                .OnDelete(DeleteBehavior.SetNull));
 
             modelBuilder.Entity<Service>(service => service
                 .HasOne<MaintainerTeam>()
@@ -60,6 +60,12 @@ namespace WD7UVN_HFT_2023241.Repository
                 .HasForeignKey(emp => emp.MANAGER_ID)
                 .OnDelete(DeleteBehavior.SetNull));
 
+			modelBuilder.Entity<Employee>(emp => emp
+				.HasOne<MaintainerTeam>()
+				.WithMany()
+				.HasForeignKey(emp => emp.MAINTAINER_ID)
+				.OnDelete(DeleteBehavior.SetNull));
+
             //loading test values
             modelBuilder.Entity<Customer>().HasData(
                 new Customer{