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DatabaseGenerated Attribute in Entity Framework Core
In this article, I am going to discuss DatabaseGenerated Data Annotation Attribute in Entity Framework Core (EF Core) with Examples. Please read our previous article, where we discussed MaxLength and MinLength Attribute in Entity Framework Core with Examples. In Entity Framework Core, the DatabaseGenerated attribute is used to indicate how a property’s value is generated or managed by the database when performing insert or update operations. This attribute can be applied to properties of our entity classes to control how their values are handled.
DatabaseGenerated Attribute in EF Core
As we already discussed, by default, Entity Framework Core creates an IDENTITY column in the database for the Key Property of the entity (It will not generate an identity column for the composite key or composite primary key). In the case of Primary Key with a single column, SQL Server creates an integer IDENTITY column with identity seed and increment values as 1. So, while inserting a record into the database table, the identity column value will be automatically generated by SQL Server and inserted into the identity column or primary key column.
EF Core provides the DatabaseGenerated Data Annotation Attribute, which configures how a property’s value will be generated. The DatabaseGenerated Attribute belongs to System.ComponentModel.DataAnnotations.Schema namespace. If you go to the definition of DatabaseGenerated Attribute class, then you will see the following signature. This class has one constructor and one property.
As you can see in the above image, the constructor takes one parameter of type DatabaseGeneratedOption. Now, if you go to the definition of DatabaseGeneratedOption, you will see that it is an enum with three values, as shown in the image below.
There are different values you can set for the DatabaseGenerated Attribute in EF Core:
- DatabaseGeneratedOption.None: This is the default option and indicates that the database does not generate the property value. It’s typically used for properties you want to set in your application manually. If you do not want the database to generate a value, then use this option.
- DatabaseGeneratedOption.Identity: This option indicates that the property is an identity column in the database, and the database automatically generates its value during an insert operation. This is often used for primary key columns. If you want the database to generate a value for a property when a row is inserted, then you need to use this option.
- DatabaseGeneratedOption.Computed: This option indicates that the database computes the property value during insert or update operations. This can be used for properties whose values are based on calculations or expressions involving other columns. If you want the database to generate a value for a property when a row is inserted or updated, then you need to use this option.
Let us understand each of the above options with examples.
DatabaseGeneratedOption.None in EF Core
This DatabaseGeneratedOption.None option is the default option, indicating that the database does not generate the property value. It’s typically used for properties you want to set in your application manually. If you do not want the database to generate a value, then use this option.
This option will be useful to override the default convention for the id properties. If you remember, for the Id Property or <Entity Name>+Id Property, Entity Framework Makes that column Identity Column and will automatically generate the value for that column.
For example, we want to provide our own values to Id Property or <Entity Name>+Id Property instead of autogenerated database values. Then, in this case, we need to use the DatabaseGeneratedOption.None option. For a better understanding, please modify the Student Entity class as follows. As you can see, we have decorated the StudentId property with the DatabaseGenerated Attribute with DatabaseGeneratedOption.None value.
using System.ComponentModel.DataAnnotations.Schema;
namespace EFCoreCodeFirstDemo.Entities
{
public class Student
{
[DatabaseGenerated(DatabaseGeneratedOption.None)]
public int StudentId { get; set; }
public string? FirstName { get; set; }
public string? LastName { get; set; }
}
}
In this case, the Entity Framework Core will mark the StudentId column in the database as the Primary Key Column but will not mark this column as an IDENTITY column. So, we need to specify values for this column explicitly while inserting a new record. Next, modify the context class as shown below. As you can see here, we have registered the Student model within the context class using DbSet.
using Microsoft.EntityFrameworkCore;
namespace EFCoreCodeFirstDemo.Entities
{
public class EFCoreDbContext : DbContext
{
protected override void OnConfiguring(DbContextOptionsBuilder optionsBuilder)
{
optionsBuilder.UseSqlServer(@"Server=LAPTOP-6P5NK25R\SQLSERVER2022DEV;Database=EFCoreDB;Trusted_Connection=True;TrustServerCertificate=True;");
}
protected override void OnModelCreating(ModelBuilder modelBuilder)
{
}
public DbSet<Student> Students { get; set; }
}
}
Note: Before Proceeding further, let us delete the EFCoreDB database using SSMS and Migration folder from our project.
With the above changes in place, open Package Manager Console and Execute the following add-migration and update-database commands. You can give any name to your migration. Here, I am giving EFCoreDBMigration1. The name that you are giving it should not be given earlier.
Now, if you check the Students database table SQL Script, you will see it created the primary key column without Identity, as shown in the image below.
Modifying the Main Method:
To prove this, modify the Program class’s Main method as follows. Here, you can see we are explicitly providing values for the StudentId. If we provide duplicate value or if we don’t provide any value for the StudentId, then it will throw Runtime Exception.
using EFCoreCodeFirstDemo.Entities;
namespace EFCoreCodeFirstDemo
{
internal class Program
{
static void Main(string[] args)
{
try
{
using EFCoreDbContext context = new EFCoreDbContext();
//We need to provide the unique StudentId value
var student1 = new Student() { StudentId = 101, FirstName = "Pranaya", LastName = "Kumar" };
context.Students.Add(student1);
//We need to provide the unique StudentId value
var student2 = new Student() { StudentId = 102, FirstName = "Hina", LastName = "Sharma" };
context.Students.Add(student2);
//The following Entity will throw an exception as StudentId has duplicate value
//var student3 = new Student() { StudentId = 102, FirstName = "Preety", LastName = "Tiwari" };
//context.Students.Add(student3);
//The following Entity will throw an exception as we have not supplied value for StudentId
//var student4 = new Student() { FirstName = "Preety", LastName = "Tiwari" };
//context.Students.Add(student4);
context.SaveChanges();
Console.WriteLine("Students Added");
Console.ReadKey();
}
catch (Exception ex)
{
Console.WriteLine($"Error: {ex.Message}"); ;
}
}
}
}
Once you execute the above code, please check the database table, and you should see the above two students as shown in the below image.
DatabaseGeneratedOption.Identity in EF Core
It is also possible to mark the non-key properties as DB-Generated properties in Entity Framework Core by using the DatabaseGeneratedOption.Identity option. This option specifies that the value of the property will be generated automatically by the database on the INSERT SQL statement. This Identity property cannot be updated.
Please note that the way the database will generate the value of the Identity property depends on the database provider. It can be identity, rowversion, or GUID. SQL Server makes an identity column for an integer property.
Let us modify the Student Entity as follows. As you can see here, we have applied the DatabaseGeneratedOption.Identity option with DatabaseGenerated Attribute on SequenceNumber property. This will mark the SequenceNumber as the Identity Column in the database. One more point that you need to remember is in a database table, only one column can be marked as Identity. So, in this case, StudentId will be the Primary Key column without Identity, and SequenceNumber will be the Identity column.
using System.ComponentModel.DataAnnotations.Schema;
namespace EFCoreCodeFirstDemo.Entities
{
public class Student
{
[DatabaseGenerated(DatabaseGeneratedOption.None)]
public int StudentId { get; set; }
public string? FirstName { get; set; }
public string? LastName { get; set; }
[DatabaseGenerated(DatabaseGeneratedOption.Identity)]
public int SequenceNumber { get; set; }
}
}
With the above changes in place, open Package Manager Console and Execute the following add-migration and update-database commands. You can give any name to your migration. Here, I am giving EFCoreDBMigration2. The name that you are giving it should not be given earlier.
Now, if you check the Students database table Create SQL Script, then you will see it created the primary key column without Identity and SequenceNumber with Identity column as shown in the below image.
Before proceeding further, let us first truncate the Students database table by executing the following Truncate table statement.
Truncate table Students;
Modifying the Main Method:
Next, we need to modify the Main method of the Program class as follows. Here, you can see we are explicitly providing values for the StudentId, but not for SequenceNumber. If we provide duplicate value or if we don’t provide any value for the StudentId, then it will throw Runtime Exception.
using EFCoreCodeFirstDemo.Entities;
namespace EFCoreCodeFirstDemo
{
internal class Program
{
static void Main(string[] args)
{
try
{
using EFCoreDbContext context = new EFCoreDbContext();
//We need to provide unique StudentId value as StudentId is Primary Key
//SequenceNumber is auto generated, so need to provide explicit value
var student1 = new Student() { StudentId = 101, FirstName = "Pranaya", LastName = "Kumar" };
context.Students.Add(student1);
//We need to provide the unique StudentId value as StudentId is Primary Key
//SequenceNumber is auto generated, so need to provide explicit value
var student2 = new Student() { StudentId = 102, FirstName = "Hina", LastName = "Sharma" };
context.Students.Add(student2);
context.SaveChanges();
Console.WriteLine("Students Added");
Console.ReadKey();
}
catch (Exception ex)
{
Console.WriteLine($"Error: {ex.Message}"); ;
}
}
}
}
Once you execute the above code, please check the database table, and you should see the above two students as shown in the below image.
DatabaseGeneratedOption.Compute in EF Core
The DatabaseGeneratedOption.Compute option specifies that the property’s value will be generated by the database when the value is first saved and subsequently regenerated every time the value is updated. The practical effect of this is that Entity Framework will not include the property in INSERT or UPDATE statements but will obtain the computed value from the database on retrieval.
Similar to Identity, the way the database generates the value depends on the database provider. You may configure a default value or use a trigger for this computed column.
Database providers differ in the way that values are automatically generated. Some will generate values for selected data types such as Identity, rowversion, and GUID. Others may require manual configuration, such as setting default values or triggers or configuring the column as Computed.
Before proceeding further, let us first truncate the Students database table by executing the following Truncate table statement.
Truncate table Students;
Let us modify the Student Entity as follows. As you can see here, we have applied the DatabaseGeneratedOption.Computed option with DatabaseGenerated Attribute on CreatedDate and FullName properties. This will mark the CreatedDate and FullName columns as the Computed Column in the database, and for the Computed column, we don’t need to pass any values.
using System.ComponentModel.DataAnnotations.Schema;
namespace EFCoreCodeFirstDemo.Entities
{
public class Student
{
[DatabaseGenerated(DatabaseGeneratedOption.None)]
public int StudentId { get; set; }
public string? FirstName { get; set; }
public string? LastName { get; set; }
[DatabaseGenerated(DatabaseGeneratedOption.Identity)]
public int SequenceNumber { get; set; }
[DatabaseGenerated(DatabaseGeneratedOption.Computed)]
public DateTime CreatedDate { get; set; }
[DatabaseGenerated(DatabaseGeneratedOption.Computed)]
public string? FullName { get; set; }
}
}
Modifying the Context Class:
We also need to provide some logic so that the values for this column is going to be calculated and stored in the database automatically. For this, in EF Core, we need to use Fluent API. So, modify the context class as follows. As you can see inside the OnModelCreating Method, we use Fluent API to set the computed column values. Here, we have set the GetUtcDate as the value for the CreatedDate property and First Name + Last Name as the value for the Full Name Property.
using Microsoft.EntityFrameworkCore;
using Microsoft.Extensions.Logging;
namespace EFCoreCodeFirstDemo.Entities
{
public class EFCoreDbContext : DbContext
{
protected override void OnConfiguring(DbContextOptionsBuilder optionsBuilder)
{
//Configuring the Connection String
optionsBuilder.UseSqlServer(@"Server=LAPTOP-6P5NK25R\SQLSERVER2022DEV;Database=EFCoreDB;Trusted_Connection=True;TrustServerCertificate=True;");
}
protected override void OnModelCreating(ModelBuilder modelBuilder)
{
//We will discuss OnModelCreating Method and Fluent API In Details in our upcoming articles
base.OnModelCreating(modelBuilder);
modelBuilder.Entity<Student>()
.Property(u => u.FullName)
.HasComputedColumnSql("[FirstName] + ' ' + [LastName]");
modelBuilder.Entity<Student>()
.Property(p => p.CreatedDate)
.HasComputedColumnSql("GetUtcDate()");
}
public DbSet<Student> Students { get; set; }
}
}
Note: In our upcoming articles, we will discuss OnModelCreating Method and Fluent API In Detail.
With the above changes in place, open Package Manager Console and Execute the following add-migration and update-database commands. You can give any name to your migration. Here, I am giving EFCoreDBMigration3. The name that you are giving it should not be given earlier.
Now, if you check the Students database table Create SQL Script, then you will see it created the primary key column without Identity and SequenceNumber with Identity column, Fullname, and CreatedDate as Computed Columns, as shown in the below image.
Next, modify the Main method of the Program class as follows:
using EFCoreCodeFirstDemo.Entities;
namespace EFCoreCodeFirstDemo
{
internal class Program
{
static void Main(string[] args)
{
try
{
using EFCoreDbContext context = new EFCoreDbContext();
//We need to provide unique StudentId value as StudentId is Primary Key
//SequenceNumber is auto generated, so need to provide explicit value
//CreatedDate is Computed, so need to provide the value
var student1 = new Student() { StudentId = 101, FirstName = "Pranaya", LastName = "Kumar" };
context.Students.Add(student1);
context.SaveChanges();
Console.WriteLine("Student Details:");
var student = context.Students.Find(101);
Console.WriteLine($"Id: {student?.StudentId}, FirsName: {student?.FirstName}, FirsName: {student?.LastName}");
Console.WriteLine($"FullName: {student?.FullName}, SequenceNumber: {student?.SequenceNumber}, CreatedDate: {student?.CreatedDate}");
Console.ReadKey();
}
catch (Exception ex)
{
Console.WriteLine($"Error: {ex.Message}"); ;
}
}
}
}
Output:
Once you execute the above code, please check the database table, and you should see the data as expected, as shown in the below image.
In the next article, I am going to discuss TimeStamp Attribute in Entity Framework Core with Examples. Here, in this article, I try to explain DatabaseGenerated Data Annotation Attribute in Entity Framework Core with Examples. I hope you enjoyed this DatabaseGenerated Attribute in EF Core with Examples article.
