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

I am trying to figure out w开发者_运维问答hat the

[DatabaseGenerated(DatabaseGenerationOption.Computed)]

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

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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.

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That attribute is used to in code-first Entity Framework whereby it specifies how the value is generated. DatabaseGenerationOptions is an enum of Identity, Computed, None, where Identity maps to an identity column, Computed is a computed column.