What does the data annotation [DatabaseGenerated (DatabaseGenerationOption.Computed)] do?

Question

I'm trying to figure out what

[DatabaseGenerated(DatabaseGenerationOption.Computed)] 

DataAnnotation actually does. However, I cannot find any information using Google search or MSDN search. Somebody knows?

Answer 1

A computed column is a table column that is not updatable, but instead is based on other data in a row.

This is similar to a view, but it is lighter and can be PERSISTED without having to create an indexed view.

For example, you might have a computed column to combine two numbers like this (in T-SQL):

 CREATE TABLE [Foo] ( [FooId] int NOT NULL IDENTITY, CONSTRAINT [Foo_PK] PRIMARY KEY ([FooId]), [ItemA] int, [ItemB] int, [Sum] AS ([ItemA] + [ItemB]) ) 

Entity Framework needs to be aware of these columns so that it does not try to issue an Update statement that will attempt to change the value of this column.

Answer 2

This attribute is used in the first entity of the Entity Framework, in which it determines how the value is generated. DatabaseGenerationOptions is an enumeration of Identity , Computed , None , where Identity mapped to an identity column, Computed is a computed column.