在我的 MVC 应用程序中,我使用了 Entity Framework 6 并使用代码优先方法创建了数据库。一段时间后,我通过添加新列和删除一些列来更新其中一个实体类。为了将这些更改反射(reflect)到数据库中,我按照以下步骤操作:
- Deleted the migrations folder in the project.
- Deleted the __MigrationHistory table in the database.
Then run the following command in the Package Manager Console:
Enable-Migrations -EnableAutomaticMigrations -ForceAdd the following lines in configuration file:
AutomaticMigrationsEnabled = true;
AutomaticMigrationDataLossAllowed = true;Run:
Add-Migration Initial- And finally, run:
Update-Database -Verbose
但是,我遇到错误“数据库中已存在名为 'xxx' 的对象。”
为了解决这个问题,我在第 5 步之后创建的初始文件中的 Up 方法中注释了代码。这可以防止错误,但数据库中没有任何更改(更新的实体表仍与以前一样)。 错误在哪里?预先感谢您的帮助。
这是我在migration.cs文件中注释的Up方法:
public override void Up()
{
CreateTable(
"dbo.City",
c => new
{
ID = c.Int(nullable: false, identity: true),
Name = c.String(nullable: false),
RegionID = c.Int(nullable: false),
})
.PrimaryKey(t => t.ID)
.ForeignKey("dbo.Region", t => t.RegionID)
.Index(t => t.RegionID);
CreateTable(
"dbo.Multiplier",
c => new
{
ID = c.Int(nullable: false, identity: true),
Status = c.Int(nullable: false),
Term = c.Int(nullable: false),
CityID = c.Int(nullable: false),
WhoIsOnline = c.String(nullable: false),
UserId = c.String(nullable: false),
InstituteName = c.String(nullable: false),
InstituteStatusID = c.Int(nullable: false),
InstituteAccreditationDate = c.DateTime(nullable: false),
Address = c.String(nullable: false),
Phone = c.String(nullable: false),
Fax = c.String(),
Email = c.String(nullable: false),
EurodeskEmail = c.String(nullable: false),
WebSite = c.String(),
ContactName = c.String(nullable: false),
ContactSurname = c.String(nullable: false),
ContactJobTitle = c.String(),
ContactAssignmentDate = c.DateTime(),
ContactWorkingStart = c.String(),
ContactWorkingkEnd = c.String(),
ContactPhone = c.String(),
ContactMobile = c.String(nullable: false),
ContactEmail = c.String(nullable: false),
ContactCityID = c.Int(nullable: false),
LegalRepresentativeName = c.String(nullable: false),
LegalRepresentativeSurname = c.String(nullable: false),
})
.PrimaryKey(t => t.ID)
.ForeignKey("dbo.City", t => t.CityID)
.ForeignKey("dbo.InstituteStatus", t => t.InstituteStatusID)
.Index(t => t.CityID)
.Index(t => t.InstituteStatusID);
CreateTable(
"dbo.InstituteStatus",
c => new
{
ID = c.Int(nullable: false, identity: true),
Name = c.String(nullable: false),
})
.PrimaryKey(t => t.ID);
CreateTable(
"dbo.TrainingParticipant",
c => new
{
ID = c.Int(nullable: false, identity: true),
TrainingID = c.Int(nullable: false),
ParticipantID = c.Int(nullable: false),
Multiplier_ID = c.Int(),
})
.PrimaryKey(t => t.ID)
.ForeignKey("dbo.Participant", t => t.ParticipantID)
.ForeignKey("dbo.Training", t => t.TrainingID)
.ForeignKey("dbo.Multiplier", t => t.Multiplier_ID)
.Index(t => t.TrainingID)
.Index(t => t.ParticipantID)
.Index(t => t.Multiplier_ID);
CreateTable(
"dbo.Participant",
c => new
{
ID = c.Int(nullable: false, identity: true),
Name = c.String(nullable: false),
Surname = c.String(nullable: false),
MultiplierID = c.Int(nullable: false),
})
.PrimaryKey(t => t.ID)
.ForeignKey("dbo.Multiplier", t => t.MultiplierID)
.Index(t => t.MultiplierID);
CreateTable(
"dbo.Training",
c => new
{
ID = c.Int(nullable: false, identity: true),
Name = c.String(nullable: false),
Date = c.DateTime(nullable: false),
CityID = c.Int(nullable: false),
})
.PrimaryKey(t => t.ID)
.ForeignKey("dbo.City", t => t.CityID)
.Index(t => t.CityID);
CreateTable(
"dbo.Region",
c => new
{
ID = c.Int(nullable: false, identity: true),
Name = c.String(nullable: false),
})
.PrimaryKey(t => t.ID);
}
这是migration.cs文件中的Down方法:
public override void Down()
{
DropForeignKey("dbo.City", "RegionID", "dbo.Region");
DropForeignKey("dbo.TrainingParticipant", "Multiplier_ID", "dbo.Multiplier");
DropForeignKey("dbo.TrainingParticipant", "TrainingID", "dbo.Training");
DropForeignKey("dbo.Training", "CityID", "dbo.City");
DropForeignKey("dbo.TrainingParticipant", "ParticipantID", "dbo.Participant");
DropForeignKey("dbo.Participant", "MultiplierID", "dbo.Multiplier");
DropForeignKey("dbo.Multiplier", "InstituteStatusID", "dbo.InstituteStatus");
DropForeignKey("dbo.Multiplier", "CityID", "dbo.City");
DropIndex("dbo.Training", new[] { "CityID" });
DropIndex("dbo.Participant", new[] { "MultiplierID" });
DropIndex("dbo.TrainingParticipant", new[] { "Multiplier_ID" });
DropIndex("dbo.TrainingParticipant", new[] { "ParticipantID" });
DropIndex("dbo.TrainingParticipant", new[] { "TrainingID" });
DropIndex("dbo.Multiplier", new[] { "InstituteStatusID" });
DropIndex("dbo.Multiplier", new[] { "CityID" });
DropIndex("dbo.City", new[] { "RegionID" });
DropTable("dbo.Region");
DropTable("dbo.Training");
DropTable("dbo.Participant");
DropTable("dbo.TrainingParticipant");
DropTable("dbo.InstituteStatus");
DropTable("dbo.Multiplier");
DropTable("dbo.City");
}
最佳答案
为什么要执行步骤 1-4?那就是你出错的地方。如果您有一个先前生成的数据库并且您只是对架构进行更改,那么只需生成迁移并应用它即可。通过执行步骤 1-4,您将有效地撤销 Entity Framework 对此数据库的了解,并最终以代码优先的方式使用现有数据库。此时,您要么必须手动更改架构,要么让 Entity Framework 将其破坏并重新开始。
就回到可以再次应用迁移的状态而言,您在生成迁移并清空 Up
方法方面走在正确的轨道上。但是,您需要根据应用程序的先前状态(即与当前数据库匹配的状态)执行此操作。否则, Entity Framework 将生成包含您的代码更改的创建表。因此要遵循的步骤是:
- 将代码恢复到开始修改 POCO 之前的状态。
- 生成迁移。
- 删除
Up
方法中的所有内容 - 使用
update-database
应用迁移 - 重新应用您对 POCO 所做的更改。
- 生成另一个迁移(这个迁移现在应该只有添加/更改列语句,而不是创建表)
- 应用迁移。
之后,您应该可以再次出发了。然后,下次更改代码时,只需按照步骤 5 和 6 进行操作即可。
关于sql-server - 在 MVC 中使用 Entity Framework Code First 更新现有数据库,我们在Stack Overflow上找到一个类似的问题: https://stackoverflow.com/questions/28547951/