How can i define eager loading for my data structure in asp.net mvc 5?

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I have a very simple data structure: Product has a Review My question is how can i eagerly load all the reviews for a product? I have an Action ViewResult method for a view called ProductDetails as seen below.

C#

public ViewResult ProductDetails(int productId)
{
    Product product = repository.Products
        .FirstOrDefault(p => p.ProductID == productId);
    return View(product);
}

Within the ProductDetails view i defined this simple @HTML.DisplayFor(model => model.ProductName) and also called for a partial view @HTML.Partial("_Review", new app.models.review()); which renders the _Review Partial view into the ProductDetails view to display the related Reviews for the specific product but no luck. I couldn't find anything searching, any help would be much appreciated. Thanks in advance.
Edit:

I am using entity framework model first approach. Below are my models, take a look.

C#

using System;
using System.Collections.Generic;
using System.ComponentModel.DataAnnotations;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Web.Mvc;

namespace EShopperTheme.Domain.Entities
{
    public class Product
    {
        public int ProductID { get; set; }
        public string ProductName { get; set; }
        public string ProductDescription { get; set; }
        public decimal ProductPrice { get; set; }
        public string ProductCategory { get; set; }
        public string MenSubCategory { get; set; }
        public string WomenSubCategory { get; set; }
        public string KidSubCategory { get; set; }
        public string ElectronicSubCategory { get; set; }
        public string FurnitureandHomeSubCategory { get; set; }
        public string BookSubCategory { get; set; }
        public decimal ProductWeight { get; set; }
        public string AvailableSizes { get; set; }
        public string AvailableColors { get; set; }
        public string Size { get; set; }
        public string Color { get; set; }
        public decimal Discount { get; set; }
        public int UnitsInStock { get; set; }
        public Boolean ProductIsUnlimited { get; set; }
        public Boolean ProductAvailable { get; set; }
        public Boolean DiscountAvailable { get; set; }
        public byte[] MainPicture { get; set; }
        public string MainPictureMimeType { get; set; }
        public byte[] SecondPicture { get; set; }
        public string SecondPictureMimeType { get; set; }
        public byte[] ThirdPicture { get; set; }
        public string ThirdPictureMimeType { get; set; }

    }
}

Review model is below:

C#

using System;
using System.Collections.Generic;
using System.ComponentModel.DataAnnotations;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Web.Mvc;

namespace EShopperTheme.Domain.Entities
{
    public class Review
    {
        [HiddenInput(DisplayValue=false)]
        public int ReviewId { get; set; }
        [Required]
        public int ProductID { get; set; }
        [Required]
        public string Name { get; set; }
        [Required]
        public string Subject { get; set; }
        [Required]
        public string Body { get; set; }
    }
}

and finally the ProductViewModel is :

C#

using EShopperTheme.Domain.Entities;
using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;

namespace EShopperTheme.Models
{
    public class ProductsListViewModel
    {
        public IEnumerable<Product> Products { get; set; }
      
        public List<Review> Reviews { get; set; }
        public PagingInfo PagingInfo { get; set; }
        
    }
}

What I have tried:

Didn't Tried anything as far as i am new to programming.

Posted 18-Feb-16 4:54am

Barcelonista Naser

Updated 18-Feb-16 8:25am

v5

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Richard Deeming · Accepted Answer · 2016-02-18T05:08:00

If you're using Entity Framework, then you need to use the Include method:
Entity Framework Loading Related Entities[^]

C#

Product product = repository.Products
    .Include(p => p.Reviews)
    .FirstOrDefault(p => p.ProductID == productId);

If you're using something else, then you need to tell us what you're using, and show us the relevant parts of the code.

EDIT:
First, update your repository to return IQueryable<...> instead of IEnumerable<...>:

C#

public interface IProductsRepository
{
    IQueryable<Product> Products { get; }
    IQueryable<Review> Reviews { get; }
    ...
}

public class EFProductRepository : IProductsRepository
{
    ...
    
    public IQueryable<Product> Products
    {
        get { return context.Products; }
    }
    
    public IQueryable<Review> Reviews
    {
        get { return context.Reviews; }
    }
    
    ...
}

Then, add the navigation properties to define the relationship between the Product and Review classes:

C#

public class Product
{
    ...
    public virtual ICollection<Review> Reviews { get; set; }
}

public class Review
{
    ...
    [Required]
    public int ProductID { get; set; }
    public virtual Product Product { get; set; }
    ...
}

Make sure you have a using System.Data.Entity; statement at the top of your controller file.

Now you can use the Include method to eagerly load the reviews for the product:

C#

Product product = repository.Products
    .Include(p => p.Reviews)
    .FirstOrDefault(p => p.ProductID == productId);

And you can display your reviews in your view:

HTML

@HTML.DisplayFor(model => model.ProductName)

<p>Reviews:</p>
<ul>
@foreach (var review in model.Reviews)
{
    <li>@HTML.Partial("_Review", review)</li>
}
</ul>