Advanced ASP.NET : The Entity Framework (part 1) - Creating an Entity Data Model

There's a lot to be said for do-it-yourself data access. It allows you to separate the data layer from the rest of your web application—and even hand off the data component to a completely different programmer. And because all the details are there in explicit ADO.NET code, you have the freedom to tweak them, all without disturbing the rest of your web page code. This approach allows you to change your data strategy as your applications evolves—whether you simply want to replace hard-coded queries with stored procedure calls or you have a more ambitious plan in mind, such as implementing a data logging system or using database cache invalidation.

But despite these benefits, the trend of the future is toward increasingly convenient ways to handle data. This shift began with basic data binding and gained steam with rich data-display controls such as the GridView  and the no-code data source controls . And now, ASP.NET developers have the most ambitious high-level data access tool that Microsoft has invented so far: the Entity Framework.

At its simplest, the Entity Framework gives you a way to generate code based on the structure of your database. For example, if you point Visual Studio to the Northwind database, it will use the Entity Framework to create classes that model the various types of records (Customers, Employees, Products, and so on). It will also generate data access code that you can use to manipulate this data—for example, to query it when you want to show it in a page and to update it when you need to commit a user's change. Although the Entity Framework is technically part of ADO.NET (and it uses the familiar ADO.NET classes behind the scenes), when you use the Entity Framework, you don't worry about these details. Instead of writing your own ADO.NET code, you let the Entity Framework take care of all the details for you.

This sort of automatic code-generation system is nothing new. In fact, hundreds of companies and independent developers have spent years inventing their own technologies that automatically build data code. Some of these technologies are quite good (and some even resemble the Entity Framework). However, many suffer from serous limitations. Although they make the programmer's life easier for basic database operations, they fail to deal with more sophisticated scenarios such as calling stored procedures, working with different types of relational database products, translating data into different representations, dealing with errors, and assembling data from multiple tables into a single set of objects. Although the Entity Framework isn't a complete replacement for traditional ADO.NET, it deals with all these issues, which makes it a practical option for developers who need to quickly create data-driven applications.

NOTE

In fact, the Entity Framework is a replacement for a similar Microsoft technology called LINQ to SQL, which was released with .NET 3.5. Although LINQ to SQL is still in use today and has many similarities to the Entity Framework, it also has disadvantages. Most obviously, it doesn't work with databases other than SQL Server, and it doesn't give developers the flexibility to use data objects that don't exactly match the underlying tables.

The Entity Framework isn't for everyone. If you're already comfortable using the traditional ADO.NET objects, you may find that the Entity Framework adds little benefit but introduces additional complexity, because it places a whole new layer between your web page code and your database. The Entity Framework also isn't the right choice if you need to squeeze every possible millisecond of speed out of your web application, because additional layers always add some extra overhead, even when they're designed as carefully and cleverly as possible.

To get a sense of how the Entity Framework works, you can take it for a test spin. In the following sections, you'll see how to create an entity data model and use it to retrieve and update data.

1. Creating an Entity Data Model

The first step to use the Entity Framework is adding an entity data model to your website. Right-click your website in the Solution Explorer and choose Add New Item (or pick Website => Add New Item from the menu). Choose the ADO.NET Entity Data Model template, which is the first choice in the list, and supply a good name (Figure 1). For example, if you're creating a model for the Northwind database, the name Northwind.edmx makes sense. (The extension .edmx stands for Entity Data Model.) When you're finished, click Add.

Figure 1. Creating an Entity Data Model

When you add a model to a projectless website (as opposed to a web project), Visual Studio informs you that you need to place the automatically generated code in the App_Code folder. Click Yes to allow it to do that automatically.

Next, Visual Studio starts the Entity Data Model Wizard. The first step asks you whether you want to generate your model from an existing data model (which is the quickest approach) or define all your entities by hand (choose Empty Model). In most cases, the best bet is to get Visual Studio to do the work for you, because you can always alter the entities it creates or remove ones you don't need. When you've chosen, click Next to move to the next step.

NOTE

An entity is just another term for a data object. Each entity stores information about a particular data item. (In the simplest case, each entity corresponds to a single record in a table.)

In the next step, you pick the connection for your database. If you've already defined the connection in the Server Explorer , it automatically appears in the drop-down connection list (Figure 2). If not, you need to click the New Connection button to create it before you continue.

Figure 2. Specifying your database connection

Visual Studio stores the connection string for your connection in the <connectionStrings> section of the web.config file, which allows you to change it quickly and easily later. By default, it takes the name of your connection and adds the word Entities to name your connection string (as in NorthwindEntities), but you can type in something else if you prefer. When you're finished, click Next to continue.

In the third step, Visual Studio connects to your database and retrieves a complete catalog of all its tables, views, and stored procedures. You can then choose which of these database items you want to include in your model and generate code for. Usually, you'll pick all the tables at once, so you have access to all your data if you need it (and so that all the relationships between your tables are preserved). However, you can also choose to select individual tables, by placing a check mark next to the ones you want to include. Figure 3 shows an example that includes all the tables but tells Visual Studio to generate code for just one of the stored procedures (Ten Most Expensive Products).

Figure 3. Specifying your database connection

This step of the wizard also gives you two additional options:

Pluralize or singularize object names:

This somewhat confusing option tells Visual Studio to use a remarkably intelligent name-generation algorithm when it creates your data classes. If you choose this option (which most people do), Visual Studio will create a Product item to represent each record in a Products table. However, it will add a Products property to represent a collection of products that's referenced by a ProductCategory record. In short, Visual Studio automatically uses the names that make the most logical sense. But Visual Studio's name generating is a lot more sophisticated than simply adding and removing the letter s. For example, Visual Studio can correctly pluralize words like Address, Territory, Person to Addresses, Territories, and People. That's because it relies on a hefty pluralization dictionary that lists the singular and plural forms of English-language nouns.

Include foreign key columns:

This option determines whether Visual Studio will include foreign key columns in your data model. If you don't, each entity will be a completely distinct object with no links to any other entity. If you do (which is the most common approach), you'll be able to navigate the relationships in your data using properties. For example, you'll be able to find all the products in a specific category using an automatically generated navigation property like ProductCategory.Products.

Once you've finished this step, click Finish. Visual Studio generates the model. Once it's finished, you'll have two new files in the App_Code project folder: the model file you specified initially (for example, Northwind.edmx) and a file that contains the automatically generated VB code for the mode (for example, Northwind.Designer.vb ). You'll learn about both files in the next section.