EF Core 变更外键关联字段的名称或数据类型沐见南

主外键关联着的字段，无法变更其中任意一个的名称、类型、大小。要想变更，可以使用Sql解除这些状态。  
而EF为我生成的代码，并没有先解除字段关联再更改字段类型。  
三种方案，一是迁移前备份库结构，解除外键关联，顺利迁移之后再加回来；二是迁移前备份库（表），删除所有迁移记录，删除相关表格，生成新的迁移语句（也就是创建语句）之后直接重新建库（表），最后按先主表后关联表的顺序导入原数据；三是在EF生成的代码的基础上进行修改，在必要的位置加上解除关联的语句，操作完成后的位置加上还原关联的语句。  

下面是我使用方案三的案例：  
1. EF Core为我生成的代码

        using Microsoft.EntityFrameworkCore.Migrations;

        namespace MuJianNanBlog.Migrations
        {
            public partial class ChangeDataType : Migration
            {
                protected override void Up(MigrationBuilder migrationBuilder)
                {
                    migrationBuilder.AlterColumn<string>(
                        name: "Name",
                        table: "AspNetUserTokens",
                        nullable: false,
                        oldClrType: typeof(string),
                        oldMaxLength: 128);
                    migrationBuilder.AlterColumn<string>(
                        name: "LoginProvider",
                        table: "AspNetUserTokens",
                        nullable: false,
                        oldClrType: typeof(string),
                        oldMaxLength: 128);

                    migrationBuilder.AlterColumn<string>(
                        name: "UserId",
                        table: "AspNetUserTokens",
                        nullable: false,
                        oldClrType: typeof(string));

                    migrationBuilder.AlterColumn<string>(
                        name: "UserName",
                        table: "AspNetUsers",
                        maxLength: 256,
                        nullable: true,
                        oldClrType: typeof(string),
                        oldMaxLength: 128,
                        oldNullable: true);

                    migrationBuilder.AlterColumn<string>(
                        name: "Id",
                        table: "AspNetUsers",
                        nullable: false,
                        oldClrType: typeof(string),
                        oldMaxLength: 128);

                    migrationBuilder.AlterColumn<string>(
                        name: "RoleId",
                        table: "AspNetUserRoles",
                        nullable: false,
                        oldClrType: typeof(string));

                    migrationBuilder.AlterColumn<string>(
                        name: "UserId",
                        table: "AspNetUserRoles",
                        nullable: false,
                        oldClrType: typeof(string));

                    migrationBuilder.AlterColumn<string>(
                        name: "UserId",
                        table: "AspNetUserLogins",
                        nullable: false,
                        oldClrType: typeof(string));

                    migrationBuilder.AlterColumn<string>(
                        name: "ProviderKey",
                        table: "AspNetUserLogins",
                        nullable: false,
                        oldClrType: typeof(string),
                        oldMaxLength: 128);

                    migrationBuilder.AlterColumn<string>(
                        name: "LoginProvider",
                        table: "AspNetUserLogins",
                        nullable: false,
                        oldClrType: typeof(string),
                        oldMaxLength: 128);

                    migrationBuilder.AlterColumn<string>(
                        name: "UserId",
                        table: "AspNetUserClaims",
                        nullable: false,
                        oldClrType: typeof(string));

                    migrationBuilder.AlterColumn<string>(
                        name: "Name",
                        table: "AspNetRoles",
                        maxLength: 256,
                        nullable: true,
                        oldClrType: typeof(string),
                        oldMaxLength: 128,
                        oldNullable: true);

                    migrationBuilder.AlterColumn<string>(
                        name: "Id",
                        table: "AspNetRoles",
                        nullable: false,
                        oldClrType: typeof(string),
                        oldMaxLength: 128);

                    migrationBuilder.AlterColumn<string>(
                        name: "RoleId",
                        table: "AspNetRoleClaims",
                        nullable: false,
                        oldClrType: typeof(string));

                }

                protected override void Down(MigrationBuilder migrationBuilder)
                {
                    migrationBuilder.AlterColumn<string>(
                        name: "Name",
                        table: "AspNetUserTokens",
                        maxLength: 128,
                        nullable: false,
                        oldClrType: typeof(string));

                    migrationBuilder.AlterColumn<string>(
                        name: "LoginProvider",
                        table: "AspNetUserTokens",
                        maxLength: 128,
                        nullable: false,
                        oldClrType: typeof(string));

                    migrationBuilder.AlterColumn<string>(
                        name: "UserId",
                        table: "AspNetUserTokens",
                        nullable: false,
                        oldClrType: typeof(string));

                    migrationBuilder.AlterColumn<string>(
                        name: "UserName",
                        table: "AspNetUsers",
                        maxLength: 128,
                        nullable: true,
                        oldClrType: typeof(string),
                        oldMaxLength: 256,
                        oldNullable: true);

                    migrationBuilder.AlterColumn<string>(
                        name: "Id",
                        table: "AspNetUsers",
                        maxLength: 128,
                        nullable: false,
                        oldClrType: typeof(string));

                    migrationBuilder.AlterColumn<string>(
                        name: "RoleId",
                        table: "AspNetUserRoles",
                        nullable: false,
                        oldClrType: typeof(string));

                    migrationBuilder.AlterColumn<string>(
                        name: "UserId",
                        table: "AspNetUserRoles",
                        nullable: false,
                        oldClrType: typeof(string));

                    migrationBuilder.AlterColumn<string>(
                        name: "UserId",
                        table: "AspNetUserLogins",
                        nullable: false,
                        oldClrType: typeof(string));

                    migrationBuilder.AlterColumn<string>(
                        name: "ProviderKey",
                        table: "AspNetUserLogins",
                        maxLength: 128,
                        nullable: false,
                        oldClrType: typeof(string));

                    migrationBuilder.AlterColumn<string>(
                        name: "LoginProvider",
                        table: "AspNetUserLogins",
                        maxLength: 128,
                        nullable: false,
                        oldClrType: typeof(string));

                    migrationBuilder.AlterColumn<string>(
                        name: "UserId",
                        table: "AspNetUserClaims",
                        nullable: false,
                        oldClrType: typeof(string));

                    migrationBuilder.AlterColumn<string>(
                        name: "Name",
                        table: "AspNetRoles",
                        maxLength: 128,
                        nullable: true,
                        oldClrType: typeof(string),
                        oldMaxLength: 256,
                        oldNullable: true);

                    migrationBuilder.AlterColumn<string>(
                        name: "Id",
                        table: "AspNetRoles",
                        maxLength: 128,
                        nullable: false,
                        oldClrType: typeof(string));

                    migrationBuilder.AlterColumn<string>(
                        name: "RoleId",
                        table: "AspNetRoleClaims",
                        nullable: false,
                        oldClrType: typeof(string));
                }
            }
        }

2. 我修改后的代码  
注意两处注释
        using Microsoft.EntityFrameworkCore.Migrations;

        namespace MuJianNanBlog.Migrations
        {
            public partial class ChangeDataType : Migration
            {
                protected override void Up(MigrationBuilder migrationBuilder)
                {
                    //Drop FKs
                    migrationBuilder.DropForeignKey(name: "FK_AspNetRoleClaims_AspNetRoles_RoleId", table: "AspNetRoleClaims");
                    migrationBuilder.DropForeignKey(name: "FK_AspNetUserClaims_AspNetUsers_UserId", table: "AspNetUserClaims");
                    migrationBuilder.DropForeignKey(name: "FK_AspNetUserLogins_AspNetUsers_UserId", table: "AspNetUserLogins");
                    migrationBuilder.DropForeignKey(name: "FK_AspNetUserRoles_AspNetRoles_RoleId", table: "AspNetUserRoles");
                    migrationBuilder.DropForeignKey(name: "FK_AspNetUserRoles_AspNetUsers_UserId", table: "AspNetUserRoles");
                    migrationBuilder.DropForeignKey(name: "FK_AspNetUserTokens_AspNetUsers_UserId", table: "AspNetUserTokens");

                    migrationBuilder.AlterColumn<string>(
                        name: "Name",
                        table: "AspNetUserTokens",
                        nullable: false,
                        oldClrType: typeof(string),
                        oldMaxLength: 128);

                    migrationBuilder.AlterColumn<string>(
                        name: "LoginProvider",
                        table: "AspNetUserTokens",
                        nullable: false,
                        oldClrType: typeof(string),
                        oldMaxLength: 128);

                    migrationBuilder.AlterColumn<string>(
                        name: "UserId",
                        table: "AspNetUserTokens",
                        nullable: false,
                        oldClrType: typeof(string));

                    migrationBuilder.AlterColumn<string>(
                        name: "UserName",
                        table: "AspNetUsers",
                        maxLength: 256,
                        nullable: true,
                        oldClrType: typeof(string),
                        oldMaxLength: 128,
                        oldNullable: true);

                    migrationBuilder.AlterColumn<string>(
                        name: "Id",
                        table: "AspNetUsers",
                        nullable: false,
                        oldClrType: typeof(string),
                        oldMaxLength: 128);

                    migrationBuilder.AlterColumn<string>(
                        name: "RoleId",
                        table: "AspNetUserRoles",
                        nullable: false,
                        oldClrType: typeof(string));

                    migrationBuilder.AlterColumn<string>(
                        name: "UserId",
                        table: "AspNetUserRoles",
                        nullable: false,
                        oldClrType: typeof(string));

                    migrationBuilder.AlterColumn<string>(
                        name: "UserId",
                        table: "AspNetUserLogins",
                        nullable: false,
                        oldClrType: typeof(string));

                    migrationBuilder.AlterColumn<string>(
                        name: "ProviderKey",
                        table: "AspNetUserLogins",
                        nullable: false,
                        oldClrType: typeof(string),
                        oldMaxLength: 128);

                    migrationBuilder.AlterColumn<string>(
                        name: "LoginProvider",
                        table: "AspNetUserLogins",
                        nullable: false,
                        oldClrType: typeof(string),
                        oldMaxLength: 128);

                    migrationBuilder.AlterColumn<string>(
                        name: "UserId",
                        table: "AspNetUserClaims",
                        nullable: false,
                        oldClrType: typeof(string));

                    migrationBuilder.AlterColumn<string>(
                        name: "Name",
                        table: "AspNetRoles",
                        maxLength: 256,
                        nullable: true,
                        oldClrType: typeof(string),
                        oldMaxLength: 128,
                        oldNullable: true);

                    migrationBuilder.AlterColumn<string>(
                        name: "Id",
                        table: "AspNetRoles",
                        nullable: false,
                        oldClrType: typeof(string),
                        oldMaxLength: 128);

                    migrationBuilder.AlterColumn<string>(
                        name: "RoleId",
                        table: "AspNetRoleClaims",
                        nullable: false,
                        oldClrType: typeof(string));

                    //Restore FKs
                    migrationBuilder.AddForeignKey(
                        name: "FK_AspNetRoleClaims_AspNetRoles_RoleId",
                        table: "AspNetRoleClaims",
                        column: "RoleId",
                        principalTable: "AspNetRoles",
                        principalColumn: "Id",
                        onDelete: ReferentialAction.Cascade);

                    migrationBuilder.AddForeignKey(
                        name: "FK_AspNetUserClaims_AspNetUsers_UserId",
                        table: "AspNetUserClaims",
                        column: "UserId",
                        principalTable: "AspNetUsers",
                        principalColumn: "Id",
                        onDelete: ReferentialAction.Cascade);

                    migrationBuilder.AddForeignKey(
                        name: "FK_AspNetUserLogins_AspNetUsers_UserId",
                        table: "AspNetUserLogins",
                        column: "UserId",
                        principalTable: "AspNetUsers",
                        principalColumn: "Id",
                        onDelete: ReferentialAction.Cascade);

                    migrationBuilder.AddForeignKey(
                        name: "FK_AspNetUserRoles_AspNetRoles_RoleId",
                        table: "AspNetUserRoles",
                        column: "RoleId",
                        principalTable: "AspNetRoles",
                        principalColumn: "Id",
                        onDelete: ReferentialAction.Cascade);

                    migrationBuilder.AddForeignKey(
                        name: "FK_AspNetUserRoles_AspNetUsers_UserId",
                        table: "AspNetUserRoles",
                        column: "UserId",
                        principalTable: "AspNetUsers",
                        principalColumn: "Id",
                        onDelete: ReferentialAction.Cascade);

                    migrationBuilder.AddForeignKey(
                        name: "FK_AspNetUserTokens_AspNetUsers_UserId",
                        table: "AspNetUserTokens",
                        column: "UserId",
                        principalTable: "AspNetUsers",
                        principalColumn: "Id",
                        onDelete: ReferentialAction.Cascade);
                }

                protected override void Down(MigrationBuilder migrationBuilder)
                {
                    migrationBuilder.AlterColumn<string>(
                        name: "Name",
                        table: "AspNetUserTokens",
                        maxLength: 128,
                        nullable: false,
                        oldClrType: typeof(string));

                    migrationBuilder.AlterColumn<string>(
                        name: "LoginProvider",
                        table: "AspNetUserTokens",
                        maxLength: 128,
                        nullable: false,
                        oldClrType: typeof(string));

                    migrationBuilder.AlterColumn<string>(
                        name: "UserId",
                        table: "AspNetUserTokens",
                        nullable: false,
                        oldClrType: typeof(string));

                    migrationBuilder.AlterColumn<string>(
                        name: "UserName",
                        table: "AspNetUsers",
                        maxLength: 128,
                        nullable: true,
                        oldClrType: typeof(string),
                        oldMaxLength: 256,
                        oldNullable: true);

                    migrationBuilder.AlterColumn<string>(
                        name: "Id",
                        table: "AspNetUsers",
                        maxLength: 128,
                        nullable: false,
                        oldClrType: typeof(string));

                    migrationBuilder.AlterColumn<string>(
                        name: "RoleId",
                        table: "AspNetUserRoles",
                        nullable: false,
                        oldClrType: typeof(string));

                    migrationBuilder.AlterColumn<string>(
                        name: "UserId",
                        table: "AspNetUserRoles",
                        nullable: false,
                        oldClrType: typeof(string));

                    migrationBuilder.AlterColumn<string>(
                        name: "UserId",
                        table: "AspNetUserLogins",
                        nullable: false,
                        oldClrType: typeof(string));

                    migrationBuilder.AlterColumn<string>(
                        name: "ProviderKey",
                        table: "AspNetUserLogins",
                        maxLength: 128,
                        nullable: false,
                        oldClrType: typeof(string));

                    migrationBuilder.AlterColumn<string>(
                        name: "LoginProvider",
                        table: "AspNetUserLogins",
                        maxLength: 128,
                        nullable: false,
                        oldClrType: typeof(string));

                    migrationBuilder.AlterColumn<string>(
                        name: "UserId",
                        table: "AspNetUserClaims",
                        nullable: false,
                        oldClrType: typeof(string));

                    migrationBuilder.AlterColumn<string>(
                        name: "Name",
                        table: "AspNetRoles",
                        maxLength: 128,
                        nullable: true,
                        oldClrType: typeof(string),
                        oldMaxLength: 256,
                        oldNullable: true);

                    migrationBuilder.AlterColumn<string>(
                        name: "Id",
                        table: "AspNetRoles",
                        maxLength: 128,
                        nullable: false,
                        oldClrType: typeof(string));

                    migrationBuilder.AlterColumn<string>(
                        name: "RoleId",
                        table: "AspNetRoleClaims",
                        nullable: false,
                        oldClrType: typeof(string));
                }
            }
        }

可以看到，我只是简单地在Up方法的首尾加了外键的移除和恢复代码。  
如果需要还原迁移，可以在Down方法里也加上一些代码（不过一般没必要，直接进数据库还原库、Git还原代码会更方便）。  

**参考：**  
1. [自定义迁移操作](https://docs.microsoft.com/zh-cn/ef/core/managing-schemas/migrations/operations)

EF Core 变更外键关联字段的名称或数据类型沐见南

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