Entity Framework 5: Using DatabaseGeneratedOption.Computed option

Question

I have an EF5 code first project that uses the [DatabaseGenerated(DatabaseGeneratedOption.Computed)] attribute.
This option is overriding my settings.

Consider this SQL table:

CREATE TABLE Vehicle (
  VehicleId  int identity(1,1) not null,
  Name varchar(100) not null default ('Not Set')
)

I am using the SQL default construct to set the [Name] is case it is not set.

In code behind, I have a class defined similar to:

public class Vehicle {
   ...

    [DatabaseGenerated(DatabaseGeneratedOption.Computed)]
    public string ShoulderYN { get; set; }
}

When I update the entity in code, the value set in the default overrides my new setting.

In code, I have (pseudo):

vehicle.Name = 'Update Name of Vehicle';
_dbContext.Update(vehicle);
_dbContext.SaveChanges();

The expected result is Vehicle.Name = 'Update Name of Vehicle'.
The actual result is Vehicle.Name = 'Not Set'.

Is there a way in EF5 to say:
if Vehicle.Name is null/empty, use the value defined in the database? Otherwise, if I set the value in code, I want to use this value.

Answer 1

Apparently, no there isn't. It's not that smart :)

As you may already read, Computed option just tells the EF not to update your column, because you will compute a value on the DB-side yourself. EF will then just return newly computed value from your database (which in your case is "Not Set").

Your basic three options are - as per EF Source code documentation:

• None - The database does not generate values.
• Identity - The database generates a value when a row is inserted.
• Computed - The database generates a value when a row is inserted or updated.

https://github.com/aspnet/EntityFramework6/blob/527ae18fe23f7649712e9461de0c90ed67c3dca9/src/EntityFramework/DataAnnotations/Schema/DatabaseGeneratedOption.cs

Since you expect a little more custom logic to be done, I'm afraid you would have to do it yourself. I would suggest you stop relying on database default constraint and do everything in code first approach. This way you would have a code like that:

public class Vehicle
{
  public Vehicle()
  {
    this.Name = "Not set";
  }
  
  // Without 'Generated' attribute
  public string Name { get; set; }
}

This way, when your Entity is created, it automatically starts with expected default value. And can be later changed by simply modifying the Name property.

Hope it helps!

Answer 2

Actually there is a simple solution for this:

• You need to leave default constraint with value in table creation script as it is now:

  CREATE TABLE Vehicle (
    VehicleId  int identity(1,1) not null,
    Name varchar(100) not null default ('Not Set')
  )
  

• Just remove DatabaseGenerated attribute from property in class definition:

  public class Vehicle {
     ...
  
     [DatabaseGenerated(DatabaseGeneratedOption.Computed)]
     public string ShoulderYN { get; set; }
  }
  

And that's it: now database will use default value only if you do not specify some value in code. Hope this helps.