Entity Framework Key Data Annotation Discover How to use Key Attribute

The Key attribute can be applied to a property to make it a key property in an entity class and the corresponding column to a primary key column in the database.

• The default Code First convention look for a property named "Id", or a combination of "class name" and "Id", such as BookId.
• This property will map to a primary key column in the database.

public class Book
{
    public int BookId { get; set; }
    public string Title { get; set; }
}

public class Category
{
    public int CatId { get; set; }
    public string CategoryName { get; set; }
}

The Book class follow the default code first conventions, and the name of the key field in the Category class does not meet the Entity Framework naming convention for entity keys.

• If code first does not find a property that matches the default convention, it will throw an exception because Entity Framework requires a key property.
• The Key attribute overrides this default convention, you can use the key annotation to specify which property is to be used as the EntityKey.

public class Category
{
    [Key]
    public int CatId { get; set; }
    public string CategoryName { get; set; }
}

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Composite keys

A primary key that consists of more than one property. For example, a BookCategory entity class has a primary key which contains BookId and CategoryId.

public class BookCategory
{
    [Key]
    public int BookId { get; set; }
    [Key]
    public int CategoryId { get; set; }
    public Book Book { get; set; }
    public Category Category { get; set; }
}

Using the above example would result in an InvalidOperationException.

System.InvalidOperationException: 'Unable to determine composite primary key ordering for type 'BookCategory'. Use the ColumnAttribute or the HasKey method to specify an order for composite primary

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Entity Framework requires you to define an order for the key properties. You can do this by using the Column annotation to specify an order.

public class BookCategory
{
    [Key, Column(Order = 1)]
    public int BookId { get; set; }
    [Key, Column(Order = 2)]
    public int CategoryId { get; set; }
    public Book Book { get; set; }
    public Category Category { get; set; }
}

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