What is the right way to write a class model in .NET Core-CodePudding

I want to have a table of Books, a table of Customers and a table of Loans. The Customers table should have a one to many relationship with the Loans table. The Books table should also have a one to many relationship with the Loans table.
In order to simulate this behavior, in my C# code I have declared the class Loan which has a field of type Customer object and another field of type Book object.
Because of this, in the database the Loan entity has (among others) two fields: one called CustomerID (of type int) and one called BookId(of type int). There are cases in the code where I need to retrieve a list of all the loans that belong to a customer. When I do this, a list of Loans objects is returned indeed, but for each object, the fields CustomerID and BookId are null. That is because in the database, EF Core stores them as int and in my C# code classes they are objects of type Customer and Book.

Is my approach of creating the tables wrong? And also what should be a solution to my problem? Thanks in advance, I am new to EF Core so any help would be appreciated.

Customer.cs

public class Customer : Person
{
    public DateTime DateJoined { get; set; }                            
    public bool Status { get; set; }                                    
    
    [Required(ErrorMessage = "Please enter a valid email!")]
    public string Email { get; set; } = string.Empty;
    [Required(ErrorMessage = "Please enter a valid password")]
    [StringLength(20,ErrorMessage = "The password can contain up to 20 characters")]
    [MinLength(5,ErrorMessage = "The password can be at least 5 characters long")]
    public string Password { get; set; } = string.Empty;                
}

Book.cs

public class Book
{
    public int Id { get; set; }
    public string Title { get; set; } = string.Empty;
    public string? ImageUrl { get; set; }
    public string? Cathegory { get; set; }
    public int PublicationYear { get; set; }
    public int CopiesAvailable { get; set; }
    public string Description { get; set; } = string.Empty;
    public bool IsBookOfTheMonth { get; set; }
    public Author Author { get; set; } = default!;
}

Loan.cs

public class Loan
{
    public int Id { get; set; }
    public DateTime DateBorrowed { get; set; }
    public int Status { get; set; }
    public Customer Customer { get; set; } = default!;
    public Book Book { get; set; } = default!;
}

CodePudding user response：

You need to use Include extension method to load related data:

var loans = context.Loans
    .Include(loan => loan.Customer)
    .Include(loan => loan.Book)
    .ToList();

Use ThenInclude extension method to include multiple levels of related data.

var loans = context.Loans
    .Include(loan => loan.Customer)
    .Include(loan => loan.Book)
    .ThenInclude(book => book.Author)
    .ToList();

These methods are located in the Microsoft.EntityFrameworkCore namespace so to use them you need to add:

using Microsoft.EntityFrameworkCore;

https://learn.microsoft.com/en-us/ef/core/querying/related-data/eager

Also it is recommended to have a foreign key property defined in your entities:

public class Loan
{
    public int Id { get; set; }
    public DateTime DateBorrowed { get; set; }
    public int Status { get; set; }
    
    public int CustomerId { get; set; }
    public Customer Customer { get; set; } = default!;
    
    public int BookId { get; set; }
    public Book Book { get; set; } = default!;
}

https://learn.microsoft.com/en-us/ef/core/modeling/relationships?tabs=fluent-api,fluent-api-simple-key,simple-key#fully-defined-relationships