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

Question

I am trying to figure out what the

[DatabaseGenerated(DatabaseGenerationOption.Computed)]

DataAnnotation actually does. However, I cannot find any information via google searches or MSDN searches. Does anyone have any idea?

Answer 1

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

It is a similar concept to a View, but is more light-weight, and can be PERSISTED without having to build an indexed view.

For example, you could have a computed column to add together 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 know about these columns so that it doesn't try to issue an Update statement that would try to change the value of that column.