如何使用C＃.net核心实体框架执行联接

Question

I have this relation ship.

In both of this model I have a navigation properties, however I still don't know how to use them.

The Customer Model

 public class Customer
{
    public int Id { get; set; }
    [Required]
    public string FirstName { get; set; }
    [Required]
    public string LastName { get; set; }
    [Required]     
    public int CustomerGroupId { get; set; }
    public CustomerGroup CustomerGroup { get; set; }

}

Customer Group Model

public class CustomerGroup
{
    public int Id { get; set; }
    public string Name { get; set; }
    public List<Customer> Customers { get; set; }
}

I want to display a list of customers along with their customer group name.

I have this query but its only giving the customer groups list but not the customers.

Changes to new query executing.

what im getting as a return

The CustomerGroup is null.

Answer 1

如果您同时希望组及其客户，则需要告诉EF包括导航属性：

db.CustomerGroups.Include(g => g.Customers).ToListAsync()

Answer 2

Get all customers and in the view, when you loop through the list of customers , simply access the CustomerGroup navigational property of each item.

var customers = db.Customers.ToList();
return View(customers);

and in the view

@model List<Customer>
@foreach(var c in Model)
{
  <p>@c.FirstName </p>
  <p>@c.CustomerGroup.Name</p>
}

Make sure your CustomerGroup property is virtual

public class Customer
{
    public int Id { get; set; }
    [Required]
    public string FirstName { get; set; }
    [Required]
    public string LastName { get; set; }
    [Required]     
    public int CustomerGroupId { get; set; }
    public virtual CustomerGroup CustomerGroup { get; set; }    
}

Keep in mind that this will do lazy loading. To do eager loading (a single query to get everything), you may consider using a view model and project to that.

So create a view model

public class CustomerVm
{
  public string FirstName { set;get;}
  public string GroupName { set;get;}
}

and in your GET action project your LINQ query result to a collection of this view model.

 var customerVms = db.Customers
                    .Select(x=> new CustomerVm { FirstName = x.Name, 
                                                 GroupName =x.CustomerGroup.Name})
                    .ToList();
 return View(customerVms );

Make sure your view is now strongly typed to a list of CustomerVm's.

@model List<CustomerVm>
@foreach(var c in Model)
{
  <p>@c.FirstName </p>
  <p>@c.GroupName</p>
}

Add other properties to your view model as needed by the view.

Take a look at this post for understanding the difference between eager loading vs lazy loading