Model compatibility cannot be checked because the database does not contain model metadata. Ensure that IncludeMetadataConvention has been added to the DbModelBuilder conventions

Model compatibility cannot be checked because the database does not contain model metadata. Ensure that IncludeMetadataConvention has been added to the DbModelBuilder conventions

调试MVC3 web项目出的错。

原因：App_data 中已经存在数据库了，并且数据库正在被各种连接着。