Generated Values

Value generation patterns

There are three value generation patterns that can be used for properties:

  • No value generation
  • Value generated on add
  • 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.

Value generated on add

Value generated on add means that a value is generated for new entities.

Depending on the database provider being used, values may be generated client side by EF or in the database. If the value is generated by the database, then EF may assign a temporary value when you add the entity to the context. This temporary value will then be replaced by the database generated value during SaveChanges().

If you add an entity to the context that has a value assigned to the property, then EF will attempt to insert that value rather than generating a new one. A property is considered to have a value assigned if it is not assigned the CLR default value (null for string, 0 for int, Guid.Empty for Guid, etc.). For more information, see Explicit values for generated properties.

Warning

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.

For example, when using SQL Server, values will be automatically generated for GUID properties (using the SQL Server sequential GUID algorithm). However, if you specify that a DateTime property is generated on add, then you must set up a way for the values to be generated. One way to do this, is to configure a default value of GETDATE(), see Default Values.

Value generated on add or update

Value generated on add or update means that a new value is generated every time the record is saved (insert or update).

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. For more information, see Explicit values for generated properties.

Warning

How the value is generated for added and updated entities will depend on the database provider being used. Database providers may automatically set up value generation for some property types, while others will require you to manually set up how the value is generated.

For example, when using SQL Server, byte[] properties that are set as generated on add or update and marked as concurrency tokens, will be set up with the rowversion data type - so that values will be generated in the database. However, if you specify that a DateTime property is generated on add or update, then you must set up a way for the values to be generated. One way to do this, is to configure a default value of GETDATE() (see Default Values) to generate values for new rows. You could then use a database trigger to generate values during updates (such as the following example trigger).

CREATE TRIGGER [dbo].[Blogs_UPDATE] ON [dbo].[Blogs]
    AFTER UPDATE
AS
BEGIN
    SET NOCOUNT ON;
                  
    IF ((SELECT TRIGGER_NESTLEVEL()) > 1) RETURN;
    
    DECLARE @Id INT
        
    SELECT @Id = INSERTED.BlogId
    FROM INSERTED
          
    UPDATE dbo.Blogs
    SET LastUpdated = GETDATE()
    WHERE BlogId = @Id
END

Value generated on add

By convention, non-composite primary keys of type short, int, long, or Guid are set up to have values generated for inserted entities, if a value isn't provided by the application. Your database provider typically takes care of the necessary configuration; for example, a numeric primary key in SQL Server is automatically set up to be an IDENTITY column.

You can configure any property to have its value generated for inserted entities as follows:

public class Blog
{
    public int BlogId { get; set; }
    public string Url { get; set; }
    [DatabaseGenerated(DatabaseGeneratedOption.Identity)]
    public DateTime Inserted { get; set; }
}

Warning

This just lets EF know that values are generated for added entities, it does not guarantee that EF will set up the actual mechanism to generate values. See Value generated on add section for more details.

Default values

On relational databases, a column can be configured with a default value; if a row is inserted without a value for that column, the default value will be used.

You can configure a default value on a property:

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

Specifying a default value will implicitly configure the property as value generated on add.

Value generated on add or update

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

Warning

This just lets EF know that values are generated for added or updated entities, it does not guarantee that EF will set up the actual mechanism to generate values. See Value generated on add or update section for more details.

Computed columns

On most relational databases, a column can be configured to have its value computed in the database, typically with an expression referring to other columns:

modelBuilder.Entity<Person>()
    .Property(p => p.DisplayName)
    .HasComputedColumnSql("[LastName] + ', ' + [FirstName]");

The above creates a virtual computed column, whose value is computed every time it is fetched from the database. You may also specify that a computed column be stored (sometimes called persisted), meaning that it is computed on every update of the row, and is stored on disk alongside regular columns:

modelBuilder.Entity<Person>()
    .Property(p => p.NameLength)
    .HasComputedColumnSql("LEN([LastName]) + LEN([FirstName])", stored: true);

Note

Support for creating stored computed columns was added in EF Core 5.0.

No value generation

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 value generated on add; you can disable this via the following:

public class Blog
{
    [DatabaseGenerated(DatabaseGeneratedOption.None)]
    public int BlogId { get; set; }
    public string Url { get; set; }
}