Related data in tables using Entity Framework

I have a question on related data in tables using Entity Framework. I have 2 tables "customer" && "customerpreferences", since both tables have customer, then the primary key is "Id_Cus". I reviewed eager, lazy loading. It was decided to use eager loading. My service implements the REST architecture on WCF. I would like to know whether it is necessary to write (Include (a => a.customerpreferences)) in each of the methods. And another question, in the screenshots attached below, I checked out where I inserted the expression described above, but this did not work, could you tell me where to put this expression. Thank you in advance!

My code for 3rd image

using System;
using System.Collections.Generic;
using System.Linq;
using System.Runtime.Serialization;
using System.ServiceModel;
using System.Text;
using MySql.Data;
using System.Data.Entity;
using WcfRestFullService.Model;
namespace WcfRestFullService
{
// NOTE: You can use the "Rename" command on the "Refactor" menu to change the class name "CustomerSevice" in code, svc and config file together.
// NOTE: In order to launch WCF Test Client for testing this service, please select CustomerSevice.svc or CustomerSevice.svc.cs at the Solution Explorer and start debugging.
public class CustomerSevice : ICustomerSevice
{
MySQLEntities dc;
public CustomerSevice()
{
dc = new MySQLEntities();
}
public List GetAllCustomer()
{
var query = (from a in dc.customers
select a).Distinct().Include(c=>c.customerpreference);
List CustomersList = new List();
query.ToList().ForEach(x =>
{
CustomersList.Add(new customer
{
Id_Cus = x.Id_Cus,
FirstName_Cus = x.FirstName_Cus,
LastName_Cus = x.LastName_Cus,
PhoneNum_Cus = x.PhoneNum_Cus,
Email_Cus = x.Email_Cus,
});
});
return CustomersList;
}
public customer CustomerDetails(string Id_Cus)
{
customer Cust = new customer();
try
{
var query = (from a in dc.customers
where a.Id_Cus.Equals(Id_Cus)
select a).Distinct().FirstOrDefault();
Cust.Id_Cus = query.Id_Cus;
Cust.FirstName_Cus = query.FirstName_Cus;
Cust.LastName_Cus = query.LastName_Cus;
Cust.PhoneNum_Cus = query.PhoneNum_Cus;
Cust.Email_Cus = query.Email_Cus;
}
catch (Exception ex)
{
throw new FaultException<string>(ex.Message);
}
return Cust;
}
// DELETE
public void DeleteCustomer(string Id_Cus)
{
//MySQLEntities Cust = new MySQLEntities(); //check the file Model.edmx->ModelContext.tt->MySQLEntitys
int k = Convert.ToInt32(Id_Cus);
customer cur = (from n in dc.customers
where n.Id_Cus == k
select n).ToList().First();
dc.Configuration.ValidateOnSaveEnabled = false;
dc.customers.Remove(cur);
dc.SaveChanges();
}
//Insert/POST
public void InsertCustomer(customer customerDataContract)
{
//MySQLEntities Cust = new MySQLEntities();
customer cust = new customer();
cust.Id_Cus = Convert.ToInt32(customerDataContract.Id_Cus);
cust.FirstName_Cus = customerDataContract.FirstName_Cus;
cust.LastName_Cus = customerDataContract.LastName_Cus;
cust.PhoneNum_Cus = Convert.ToInt32(customerDataContract.PhoneNum_Cus);
cust.Email_Cus = customerDataContract.Email_Cus;
dc.customers.Add(cust);
dc.SaveChanges();
}
//Update/PUT
public void UpdateCustomer(customer customerDataContract)
{
//using (CustomerDataContract Cust = new CustomerDataContract())
//using (MySQLEntities Cust = new MySQLEntities())
{
int k = Convert.ToInt32(customerDataContract.Id_Cus);
customer cust = dc.customers.Where(n => n.Id_Cus == k).Include(a=>a.customerpreference).FirstOrDefault();
cust.Id_Cus = Convert.ToInt32(customerDataContract.Id_Cus);
cust.FirstName_Cus = customerDataContract.FirstName_Cus;
cust.LastName_Cus = customerDataContract.LastName_Cus;
cust.PhoneNum_Cus = Convert.ToInt32(customerDataContract.PhoneNum_Cus);
cust.Email_Cus = customerDataContract.Email_Cus;
dc.SaveChanges();
}
}
}
}

//------------------------------------------------------------------------------
//
// This code was generated from a template.
//
// Manual changes to this file may cause unexpected behavior in your application.
// Manual changes to this file will be overwritten if the code is regenerated.
//
//------------------------------------------------------------------------------
namespace WcfRestFullService.Model
{
using System;
using System.Collections.Generic;
using System.Linq;
using System.Runtime.Serialization;
using System.Web;
[DataContract]
public partial class customerpreference
{
[DataMember]
public int Id_Cus { get; set; }
[DataMember]
public int Id_Res { get; set; }
[DataMember]
public string Name_Dis { get; set; }
[DataMember]
public int Id_Type { get; set; }
public virtual customer customer { get; set; }
public virtual order order { get; set; }
public virtual type_dishes type_dishes { get; set; }
}
}

//------------------------------------------------------------------------------
//
// This code was generated from a template.
//
// Manual changes to this file may cause unexpected behavior in your application.
// Manual changes to this file will be overwritten if the code is regenerated.
//
//------------------------------------------------------------------------------
namespace WcfRestFullService.Model
{
using System;
using System.Collections.Generic;
using System.Linq;
using System.Runtime.Serialization;
using System.Web;
[DataContract]
public partial class customer
{
[System.Diagnostics.CodeAnalysis.SuppressMessage("Microsoft.Usage", "CA2214:DoNotCallOverridableMethodsInConstructors")]
public customer()
{
this.dishesrankings = new HashSet();
this.orders = new HashSet();
}
[DataMember]
public int Id_Cus { get; set; }
[DataMember]
public string FirstName_Cus { get; set; }
[DataMember]
public string LastName_Cus { get; set; }
[DataMember]
public int PhoneNum_Cus { get; set; }
[DataMember]
public string Email_Cus { get; set; }
[System.Diagnostics.CodeAnalysis.SuppressMessage("Microsoft.Usage", "CA2227:CollectionPropertiesShouldBeReadOnly")]
public virtual ICollection dishesrankings { get; set; }
public virtual customerpreference customerpreference { get; set; }
[System.Diagnostics.CodeAnalysis.SuppressMessage("Microsoft.Usage", "CA2227:CollectionPropertiesShouldBeReadOnly")]
public virtual ICollection orders { get; set; }
}
}