EF Core Advanced Topics Generated Values

A generated property is a property whose value is generated (either by EF or the database) when the entity is added and/or updated. Three value generation patterns can be used for properties.

• No value generation
• The value generated on add
• The value generated on add or update

No Value Generation

No value generation means that you will always supply a valid value to be saved to the database. This valid value must be assigned to new entities before they are added to the context.

Disabling value generation on a property is typically necessary if a convention configures it for value generation. For example, if you have a primary key of type int, it will be implicitly set configured as the value generated on add.

Data Annotations

You can disable value generation using data annotation as shown below.

public class Author
{
    [DatabaseGenerated(DatabaseGeneratedOption.None)]
    public int AuthorId { get; set; }
    public string Name { get; set; }
}

Fluent API

You can also disable value generation using fluent API as shown below.

protected override void OnModelCreating(ModelBuilder modelBuilder)
{
    modelBuilder.Entity<Author>()
        .Property(a => a.AuthorId)
        .ValueGeneratedNever();
}

Value Generated on Add

The value generated on add means that a value is generated when new entities are inserted. How the value is generated for added entities will depend on the database provider being used.

• Database providers may automatically set up value generation for some property types, but others may require you to manually set up how the value is generated.
• By convention, when entities are added, non-composite primary keys of type short, int, long, or Guid are set up to have values generated, if a value isn't provided by the application.
• All the necessary configurations are handled by the database provider. For example, a numeric primary key in SQL Server is automatically set up to be an IDENTITY column.

Data Annotations

You can configure any property to have its value generated for inserted entities using data annotation as shown below.

public class Author
{
    public int AuthorId { get; set; }
    public string Name { get; set; }
    [DatabaseGenerated(DatabaseGeneratedOption.Identity)]
    public DateTime Created { get; set; }
}

Fluent API

You can configure any property to have its value generated for inserted entities using fluent API as shown below.

protected override void OnModelCreating(ModelBuilder modelBuilder)
{
    modelBuilder.Entity<Author>()
        .Property(a => a.Created)
        .ValueGeneratedOnAdd();
}

Default Values

In relational databases, you can configure a column with a default value, if a row is added and no value is specified for that column, then the default value will be used.

You can configure a default value on a property.

protected override void OnModelCreating(ModelBuilder modelBuilder)
{
    modelBuilder.Entity<Author>()
        .Property(a => a.Rating)
        .HasDefaultValue(3);
}

You can also specify a SQL fragment that is used to calculate the default value.

protected override void OnModelCreating(ModelBuilder modelBuilder)
{
    modelBuilder.Entity<Author>()
        .Property(a => a.Created)
        .HasDefaultValueSql("getdate()");
}

When you specify a default value, it will implicitly configure the property as the value generated on add.

Value Generated on Add or Update

The value generated on add or update means that a new value is generated every time the record is either inserted or updated. Like value generated on add, if you specify a value for the property on a newly added instance of an entity, that value will be inserted rather than a value being generated. It is also possible to set an explicit value when updating.

Data Annotations

You can configure any property to have its value generated for inserted or updated entities using data annotation as shown below.

public class Author
{
    public int AuthorId { get; set; }
    public string Url { get; set; }
    [DatabaseGenerated(DatabaseGeneratedOption.Computed)]
    public DateTime LastUpdated { get; set; }
}

Fluent API

You can configure any property to have its value generated for inserted or updated entities using fluent API as shown below.

protected override void OnModelCreating(ModelBuilder modelBuilder)
{
    modelBuilder.Entity<Author>()
        .Property(a => a.LastUpdated)
        .ValueGeneratedOnAddOrUpdate();
}

Computed Columns

In some relational databases, you can configure a column in a way that its value is computed in the database.

protected override void OnModelCreating(ModelBuilder modelBuilder)
{
    modelBuilder.Entity<Author>()
        .Property(a => a.DisplayName)
        .HasComputedColumnSql("[LastName] + ', ' + [FirstName]");
}

In some cases, the column's value is computed every time it is fetched, and in others, it is computed on every update of the row and stored.