在 EF Core 中添加外键列

Question

我有一个现有的 table Projects，我想向其中添加一个 UserId 列，其中 UserId 是一个外键。 Projects 现在有一个名称列表，但我希望每个用户都能管理自己的项目。我可以接受 "orphans" 一开始，因为列表足够小，我可以手动清理它们。

我更新了我的模型以包含 UserId 和导航属性 User（可能不相关，但 Entity 这里是基础 class Id 和 DateModified)

public class Project : Entity
{
    public string Name { get; set; }
    public Guid? UserId { get; set; } //tried this as nullable and non nullable
    public User User { get; set; }
}

而我的相关映射文件是

public class ProjectMap : IEntityTypeConfiguration<Project>
{
    public void Configure(EntityTypeBuilder<Project> builder)
    {
        builder.Property(x => x.Name).IsRequired();
        builder.Property(x => x.UserId).IsRequired();

        builder.HasOne(x => x.User)
            .WithMany(x => x.Projects)
            .HasForeignKey(x => x.UserId)
            .OnDelete(DeleteBehavior.SetNull);  //I have tried numerous combinations of things here....
    }
}

我还向项目的 User 实体添加了导航属性，但未对映射 class.

进行任何更改

public class User : Entity
{
    public string EmailAddress { get; set; }
    public string FirstName { get; set; }
    public string LastName { get; set; }
    public List<Task> Tasks { get; set; }
    public List<Project> Projects { get; set; }
}

由此生成的迁移：

    protected override void Up(MigrationBuilder migrationBuilder)
    {
        migrationBuilder.AddColumn<Guid>(
            name: "UserId",
            table: "Projects",
            nullable: false,
            defaultValue: new Guid("00000000-0000-0000-0000-000000000000"));

        migrationBuilder.CreateIndex(
            name: "IX_Projects_UserId",
            table: "Projects",
            column: "UserId");

        migrationBuilder.AddForeignKey(
            name: "FK_Projects_Users_UserId",
            table: "Projects",
            column: "UserId",
            principalTable: "Users",
            principalColumn: "Id",
            onDelete: ReferentialAction.SetNull);
    }

当运行 update-database

时转换为 SQL

ALTER TABLE [Projects] ADD CONSTRAINT [FK_Projects_Users_UserId] FOREIGN KEY ([UserId]) REFERENCES [Users] ([Id]) ON DELETE SET NULL;

并因此错误而失败

Cannot create the foreign key "FK_Projects_Users_UserId" with the SET NULL referential action, because one or more referencing columns are not nullable.

我做错了什么？

Answer 1

以下行导致了问题，因为您想要 Project

上的可为空 FK

builder.Property(x => x.UserId).IsRequired();

另外，您生成的迁移已经有提示：

 nullable: false,

只需删除您的 Configure 方法中的那一行，它就会起作用。您可能还需要通过运行 Remove-Migration 删除迁移，然后再次运行添加迁移。这次你应该有这个：

protected override void Up(MigrationBuilder migrationBuilder)
{
    migrationBuilder.AlterColumn<string>(
        name: "Name",
        table: "Projects",
        nullable: false,
        oldClrType: typeof(string),
        oldNullable: true);

    migrationBuilder.AddColumn<Guid>(
        name: "UserId",
        table: "Projects",
        nullable: true);

    migrationBuilder.CreateIndex(
        name: "IX_Projects_UserId",
        table: "Projects",
        column: "UserId");

    migrationBuilder.AddForeignKey(
        name: "FK_Projects_Users_UserId",
        table: "Projects",
        column: "UserId",
        principalTable: "Users",
        principalColumn: "Id",
        onDelete: ReferentialAction.SetNull);
}

如果需要，您可以在配置关系时通过显式设置 IsRequired 来更具体：

private static void ConfigureProject(EntityTypeBuilder<Project> b)
{
    b.Property(x => x.Name).IsRequired();

    b.HasOne(x => x.User)
        .WithMany(x => x.Projects)
        .HasForeignKey(x => x.UserId)
        .IsRequired(false)
        .OnDelete(DeleteBehavior.SetNull);
}

我喜欢这个，尽管属性已经可以为 null，并且 EF 将使用它的约定来正确创建它，但对于阅读配置并了解它而无需转到实际项目的人来说仍然很好class.

Answer 2

在函数配置中指定：

builder.Property(x => x.UserId).IsRequired();

这不是说UserId不能为null吗？但您希望它为空？

在 EF Core 中添加外键列

Add Foreign Key Column In EF Core

entity-framework

entity-framework-migrations

ef-core-2.0