1.采用code first 做项目时，数据库已经生成，后期修改数据库表结构。再次运行时出现一下问题：
Entity Framework : The model backing the '' context has changed since the database was create
解决方法：
1.打开当前项目中的：程序包管理器控制台
2.输入：enable-migrations -ProjectName 'ProductModel' -Force
解释：’ProductModel‘，ef框架model层所在程序集名称
看到执行结果：
Checking if the context targets an existing database...
Code First Migrations enabled for project ProductModel.
输入：update-DataBase -ProjectName 'ProductModel' -Force
看到执行结果：
Specify the '-Verbose' flag to view the SQL statements being applied to the target database.
No pending explicit migrations.
Unable to update database to match the current model because there are pending changes and automatic migration is disabled. Either write the pending model changes to a code-based migration or enable automatic migration. Set DbMigrationsConfiguration.AutomaticMigrationsEnabled to true to enable automatic migration.
You can use the Add-Migration command to write the pending model changes to a code-based migration.
3.打开 ’ProductModel‘，找到Migrations-Configuration
public Configuration()
{
AutomaticMigrationsEnabled = true; （原来为false，改为true）
AutomaticMigrationDataLossAllowed = false; （是否丢数据）
}
下面时操作时的照片：
