Using .NET Core Data Annotation

In this article, I will be explaining how to use some properties of data annotation in order to make it easier to model your database and also to save your time with front-end validations.

How to create your database?

• Check it here

What is Data Annotation?

"Data Annotation provides attribute classes that are used to define metadata for ASP.NET MVC and ASP.NET data controls."

Why would I use Data Annotation?

There are 3 main areas where you may use it; two of them are related to your data presentation to your end user and one is to design your database.

1. Front-End: Validation Attributes
   Used to enforce validation rules in your view. It can be used to validate email, data, fields with masks, etc.
   
2. Front-End: Display Attributes
   Used to specify how your properties from your Model will be displayed. It can be used within Resources to display a different value depending on the user language.
   
3. Back-End: Modelling Attributes
   Used to specify the table limitations and the relationship between classes. It can be used to set field type, length, mask, etc.

How to do it?

Let's see it now. In this example, we are going to use a very simple trip manager model, TripManager class.

public class TripManager    
{    
    [Key]    
    public int Id { get; set; }    
 
    [ForeignKey( "HotelID" )]    
    public Hotel Hotel { get; set; }    
 
    public int HotelID { get; set; }    
 
    [ForeignKey( "CarID" )]    
    public Car Car { get; set; }    
 
    public int CarID { get; set; }    
 
    [DataType( DataType.Date )]    
    public DateTime CheckInDate { get; set; }    
 
    [DataType( DataType.Date )]    
    public DateTime CheckOutDate { get; set; }    
 
    [Column( "TodaysPrice" )]    
    [Range( 10.30, 46.60 )]    
    public double Price { get; set; }    
 
    public Person Responsable { get; set; }    
}

Hotel Class

public class Hotel    
{    
    public int Id { get; set; }    
    
    [DisplayFormat( NullDisplayText = "Null name" )]    
    public string Name { get; set; }    
} 

Person Class

public class Person    
{    
    public int Id { get; set; }    
    
    [StringLength( 50, ErrorMessage = "Name cannot be longer than 50 characters." )]    
    public string Name { get; set; }    
    
    [NotMapped]    
    public List<Place> VisitedPlaces { get; set; }    
}

Car Class

public class Car    
{    
    public int Id { get; set; }    
    
    [Required]    
    public string Model { get; set; }    
    
    public string Brand { get; set; }    
}

Place Class

public class Place    
{    
    public int Id { get; set; }    
    public string Location { get; set; }    
}

Now, let's understand the application of each one data annotation attribute used here.

Key

[Key]    
public int Id { get; set; }

Explanation

Sets the key of the table.

Result

Foreign Key

[ForeignKey( "HotelID" )]    
public Hotel Hotel { get; set; }    
    
public int HotelID { get; set; }  

Explanation

Sets the foreign key of this relationship as the property described.

Result in the database