Changing the type of a property with EF Code First

weixin_30493401

于 2015-10-14 17:42:00 发布

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原文链接：http://www.cnblogs.com/dupeng0811/p/4878157.html

本文详细介绍了如何在数据库迁移中更改数据类型并更新数据的方法，包括添加新列、使用SQL语句迁移数据、删除旧列及重命名新列等步骤。通过遵循此流程，可以确保数据迁移过程的高效性和准确性。

The smartest way is probably to not alter types. If you need to do this, I'd suggest you to do the following steps:

Add a new column with your new type
Use Sql() to take over the data from the original column using an update statement
Remove the old column
Rename the new column

This can all be done in the same migration, the correct SQL script will be created. You can skip step 2 if you want your data to be discarded. If you want to take it over, add the appropriate statement (can also contain a switch statement).

Unfortunately Code First Migrations do not provide easier ways to accomplish this.

Here is the example code:

  AddColumn("dbo.People", "LocationTmp", c => c.Int(nullable: false)); 
Sql(@"
    UPDATE dbp.People
    SET LocationTmp =
        CASE Location
            WHEN 'London' THEN 1
            WHEN 'Edinburgh' THEN 2
            WHEN 'Cardiff' THEN 3
            ELSE 0
        END
    ");
DropColumn("dbo.People", "Location");
RenameColumn("dbo.People", "LocationTmp", "Location");