Entity Framework 6 Eager Loading

In this Entity Framework Tutorial, we are going to understand about an important concept related to Object Relational Mapping that is “Eager Loading” with the help of a practical example. We already have discussed eager loading at high level, and another related concept “Lazy Loading” that is opposite to Eager Loading while discussing Entity Framework Interview Questions along with more Entity Framework concepts. To see a practical example for Lazy Loading in Entity Framework, follow here.

Eager loading is the process by which a query loads related entities along with it as part of the query. It is achieved by the using the “Include” method. For example, when querying orders, eager-load their product details also in a single query.

More Practical Entity Framework Tutorial

In order to practically grasp Entity Framework, we highly recommend to go through these real time implementations:

We have created 3 entities, Category, Products and Orders. In below diagram, entities with their relationships look as:

The class of Products will look as:

public partial class Product 
{ 
  public Product() { 
      this.Orders = new HashSet<Orders>(); 
  }

  public int ProductId { get; set; } 
  public string ProductName { get; set; } 
  public string Price { get; set; } 
  public int CategoryId { get; set; }
  public virtual Category Category { get; set; } 
  public virtual ICollection<Orders> Orders { get; set; } 
}

public partial class Product

{

public Product() {

this.Orders = new HashSet<Orders>();

}

public int ProductId { get; set; }

public string ProductName { get; set; }

public string Price { get; set; }

public int CategoryId { get; set; }

public virtual Category Category { get; set; }

public virtual ICollection<Orders> Orders { get; set; }

}

The class of order entity will look as:

public partial class Order
{ 
  public int OrderId { get; set; } 
  public string ProductCount { get; set; } 
  public int ProductName { get; set; }
  public virtual Product Product { get; set; } 
}

public partial class Order

{

public int OrderId { get; set; }

public string ProductCount { get; set; }

public int ProductName { get; set; }

public virtual Product Product { get; set; }

}

Following code does the eager loading in case of products and orders:

public ActionResult Index() 
{ 
   var orders = db.Orders.Include(o => o.Product); 
   return View(orders.ToList()); 
} 

<table class="table"> 
  <tr>
    <th> Product Name </th> 
    <th> Product Count </th> 
    <th> Total </th> 
    <th>&nbsp;</th> 
</tr>

@foreach (var item in Model) 
{ 
    <tr> 
       <td> @Html.DisplayFor(modelItem => item.Product.ProductName) </td> 
       <td> @Html.DisplayFor(modelItem => item.ProductCount) </td> 
       <td> @(Convert.ToDouble(item.Product.Price) * Convert.ToDouble(item.ProductCount)) </td>
       <td> 
            @Html.ActionLink("Edit", "Edit", new { id = item.OrderId }) | 
            @Html.ActionLink("Details", "Details", new { id = item.OrderId }) |
            @Html.ActionLink("Delete", "Delete", new { id = item.OrderId }) 
       </td> 
     </tr>
}
</table>

public ActionResult Index()

{

var orders = db.Orders.Include(o => o.Product);

return View(orders.ToList());

}

<tr>

<th> Product Name </th>

<th> Product Count </th>

<th> Total </th>

</tr>

@foreach (var item in Model)

{

<tr>

<td> @Html.DisplayFor(modelItem => item.Product.ProductName) </td>

<td> @Html.DisplayFor(modelItem => item.ProductCount) </td>

<td> @(Convert.ToDouble(item.Product.Price) * Convert.ToDouble(item.ProductCount)) </td>

<td>

@Html.ActionLink("Edit", "Edit", new { id = item.OrderId }) |

@Html.ActionLink("Details", "Details", new { id = item.OrderId }) |

@Html.ActionLink("Delete", "Delete", new { id = item.OrderId })

</td>

</tr>

}

</table>

Resultant screen will be:

In order to create an order, we can do as:

public ActionResult Create() 
{ 
   ViewBag.ProductName = new SelectList(db.Products, "ProductId", "ProductName"); 
   return View(); 
}

div class="form-group"> 
  @Html.LabelFor(model => model.ProductName, "ProductName", htmlAttributes: new { @class = "control-label col-md-2" }) 
     <div class="col-md-10"> 
           @Html.DropDownList("ProductName", null, htmlAttributes: new { @class = "form-control" }) 
           @Html.ValidationMessageFor(model => model.ProductName, "", new { @class = "text-danger" }) 
     </div>
</div>

<div class="form-group"> 
   @Html.LabelFor(model => model.ProductCount, htmlAttributes: new { @class = "control-label col-md-2" }) 
      <div class="col-md-10"> 
            @Html.EditorFor(model => model.ProductCount, new { htmlAttributes = new { @class = "form-control" } }) 
            @Html.ValidationMessageFor(model => model.ProductCount, "", new { @class = "text-danger" })
      </div>
</div>

public ActionResult Create()

{

ViewBag.ProductName = new SelectList(db.Products, "ProductId", "ProductName");

return View();

}

div class="form-group">

@Html.LabelFor(model => model.ProductName, "ProductName", htmlAttributes: new { @class = "control-label col-md-2" })

@Html.DropDownList("ProductName", null, htmlAttributes: new { @class = "form-control" })

@Html.ValidationMessageFor(model => model.ProductName, "", new { @class = "text-danger" })

</div>

@Html.LabelFor(model => model.ProductCount, htmlAttributes: new { @class = "control-label col-md-2" })

@Html.EditorFor(model => model.ProductCount, new { htmlAttributes = new { @class = "form-control" } })

@Html.ValidationMessageFor(model => model.ProductCount, "", new { @class = "text-danger" })

</div>

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The screen will look as:

This screen will place an order of products. As we have added foreign key constraint in orders with products. So with eager loading we can load the related product details while loading orders.

var orders = db.Orders.Include(o => o.Product);

1	var orders = db.Orders.Include(o => o.Product);

After reading this Entity Framework Tutorial, it will be easy to implement eager loading in our application. Follow here to understand that correctly choosing between eager-loading and lazy-loading in practical scenarios.

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Correct Answer: B, D

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