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Fluent API vs Data Annotations- Working with Configuration-Part1

, 15 Oct 2012 CPOL
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Domain Model Layer will be better isolated from infrastructure implementation. Everything what you can configure with DataAnnotations is also possible with the Fluent API. The reverse is not true. So, from the viewpoint of configuration options and flexibility the Fluent API is “better”. Learning th


If you consider the DDD architecture designs, one thing you must fulfill that is isolation our Domain Model Layer. To achieve that you must isolate Domain Entities from any other layers such as infrastructure layers, Data Persistence layers where you have a selected data technology. So you need to satisfy the  Persistence Ignorance Principle for domain Classes (Entities, Value-Objects, Domain Services, etc.). Here comes the question for right choice. This is the reason POCO entities are the right choice for DDD.

If you go further deep into Code First, you’ll see that the initial way to make the mappings from your POCO Domain Entities classes towards your final database tables is based on conventions. I have question ?? what will happen if those conventions are not ok (if you need to adjust to an existing database or whatever) ?? . So you need to customize those conventions. You can do that either in two ways. Let us start describing those two ways:

Code first leverages a programming pattern referred to as convention over configuration. Code first gives you two ways to add these configurations to your classes. One is using simple attributes called DataAnnotations within the ” System.ComponentModel.DataAnnotations “ namespace which is part of the .NET Frameworkand the other is using code first’s Fluent API, which provides you with a way to describe configurations imperatively, in code. Customizing it withFluent API is a way that fits much better with the PI (Persistence Ignorant), because our Domain Model will be ignorant of those mappings which will be defined into our Data Persistence Infrastructure layer, so, our Domain Model Layer will be better isolated from infrastructure implementation. 

Everything what you can configure with DataAnnotations is also possible with the Fluent API. The reverse is not true. So, from the viewpoint of configuration options and flexibility the Fluent API is “better”. Learning the Fluent API is almost a Must , the DataAnnotations are a nice-to-have for simple applications.

We will describe the example for MVC4 Code First application.

public class Author
{
 public int AuthorId { get; set; }
 public string Name { get; set; }
 public string Description { get; set; }
 public byte[] Photo { get; set; }
 public virtual ICollection<Book> Books { get; set; }
}
public class Book
{
 public int BookId { get; set; }
 public string Name { get; set; }
 public DateTime PublishDate { get; set; }
 public Author Author { get; set; }
}
public class LibraryDB:DbContext
{
 public DbSet<Author> Authors { get; set; }
 public DbSet<Book> Books { get; set; }
}

The Author and Book class describes a particular Author might have collection of Books. So Author object can have one or more Books associated with it. Our class LibraryDB, will inherit from DbContext in order to gain all of DbContext’s capabilities. Additionally, it will expose properties that return queryable sets, DbSets, of Author class and Book classe. The last class represents a complete data layer that you can use in applications. For DbContext you will be able to query, change, track and save Author and Book data. Thanks to DbContext :) .

Let’s start with the Author Author class. There are three things I would like to change:

Ensure the Name is provided.

Limit the max length of Description field to 500 characters.

Store the Photo into SQL Server image type.

We need “System.ComponentModel.DataAnnotations” assembly that is part of .NET 4.0. Apply the following code:

public class Author
{
 public int AuthorId { get; set; }
 [Required]
 public string Name { get; set; }
 [MaxLength(500)]
 public string Description { get; set; }
 [Column(TypeName="image")]
 public byte[] Photo { get; set; }
 public virtual ICollection<Book> Books { get; set; }
}
public class Book
{
 public int BookId { get; set; }
 [Required]
 public string Name { get; set; }
 public DateTime PublishDate { get; set; }
 public Author Author { get; set; }
}

The Required annotation needs no additional information, whereas the MaxLength and Column have parameter information that you need to provide. The parameter provided to the Column annotation is specific to the database that you are mapping to. We want to store Photo in a SQL Server image field. So we configured the data type. All three annotations will impact the databse schema. On the Book class we have only placed the required annotaion no character limit, so in the database schema you will find the length is nvarchar(max). Note that, we can also use MinLength which is an interesting annotation. While MaxLength has a database counterpart, MinLength does not. MinLength will be used for EF validation without impacting the database.

MinLength is the only configuration that can be achieved using Data Annotation but has no counterpart in the Fluent API configurations.

If you opened up the database table you will find the changes that we have made in our classes. We will discuss on EF DataBase Migration later.

Configuring with Fluent API:

The concern of a Fluent API isn’t specific to Code First or the EF. The basic is chained method calls to produce code that is easy for the developers to read. The return type of each call then defines the valid methods for the next call. There are more reasons why developers like Fluent API.

public class LibraryDB:DbContext
{
 public DbSet<Author> Authors { get; set; }
 public DbSet<Book> Books { get; set; }
 protected override void OnModelCreating(DbModelBuilder modelBuilder)
 {
   modelBuilder.Entity<Author>()
    .Property(n => n.Name).IsRequired();
   modelBuilder.Entity<Author>()
    .Property(d => d.Description).HasMaxLength(500);
   modelBuilder.Entity<Author>()
    .Property(p => p.Photo).HasColumnType(“image”);
   modelBuilder.Entity<Book>()
    .Property(n => n.Name).IsRequired();
 }
}

Let us describe the mechanism above code, how it works actually. Julia Lerman & Rowan Miller describes nicely in their “Programming Entity Framework Code First” book. “When its time to build the model, the DBContext first looks at the classes and learns what it can from them. At this point, the context is ready to reason out the model, but there is an opportunity for the developers to interrupt the context and perform additional configuration. DbContext.OnModelCreating method called by the context just before the model is built. The method is virtual, so you can override it and insert your own logic. This is where the Fluent API configuration goes”.

Stay Tuned: Be Right Back with more advanced…


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This article, along with any associated source code and files, is licensed under The Code Project Open License (CPOL)

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About the Author

Monjurul Habib
Software Developer (Senior)
Bangladesh Bangladesh
A life-long-learner, maker and soft music fan. Likes building things to solve problems. Lives in Dhaka with wife and wonderful, smart kid and works as a Senior Software Engineer in applications architecture team.
 
He has years of successful records serving mid and large scale .NET applications. Have a wide range of experience working in domestic and international client environment. Expertise in different areas of software development life cycles and Software Architecture.
 
I am always looking for new information and value your feedback (especially where I got something wrong!).
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GeneralMy vote of 5 PinmemberMember 969271918-Jan-13 20:21 
GeneralRe: My vote of 5 PinmemberMonjurul Habib7-Mar-13 7:41 
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GeneralMy vote of 5 Pinmemberhopeful4ever19-Oct-12 9:30 
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GeneralMy vote of 5 PinmemberMarcus Kramer16-Oct-12 5:41 
GeneralRe: My vote of 5 PinmemberMonjurul Habib16-Oct-12 6:40 
thanks Smile | :)
Premature optimization is the root of all evil in programming.
--C.A.R. Hoare

GeneralMy vote of 5 Pinmembercisjackie15-Oct-12 19:03 
GeneralRe: My vote of 5 PinmemberMonjurul Habib15-Oct-12 19:10 
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GeneralRe: My vote of 5 PinmemberMonjurul Habib15-Oct-12 9:48 
GeneralRe: My vote of 5 PinmemberMarcus Kramer15-Oct-12 11:09 
GeneralRe: My vote of 5 PinmemberMonjurul Habib15-Oct-12 11:11 
GeneralRe: My vote of 5 PinmemberMonjurul Habib15-Oct-12 11:19 
GeneralRe: My vote of 5 PinmemberMarcus Kramer15-Oct-12 11:37 
GeneralRe: My vote of 5 PinmemberMonjurul Habib15-Oct-12 11:45 
GeneralRe: My vote of 5 PinmemberMarcus Kramer15-Oct-12 11:49 
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