Laravel - 迁移，表结构修改 - 正确方法

Question

我现在的桌子是

Schema::create('students', function (Blueprint $table) {
    $table->bigIncrements('id');
    $table->string('first_name', 255);
    $table->string('last_name', 255);
    $table->enum('gender', ['m', 'f']);
    $table->date('date_of_birth');
    $table->integer('roll_number');
    $table->char('section', 1);
    $table->integer('class');
    $table->unsignedBigInteger('school_id');            
    $table->string('photo')->nullable;
    $table->timestamps();

    $table->foreign('school_id')
        ->references('id')->on('schools')
        ->onUpdate('cascade')->onDelete('cascade');

    $table->unique(['roll_number', 'section', 'class', 'school_id']);
});

标准

Schema::create('standards', function (Blueprint $table) {
       $table->bigIncrements('id');
       $table->string('name');
       $table->unsignedBigInteger('school_id');
       $table->timestamps();

       $table->foreign('school_id')
       ->references('id')->on('schools')
       ->onUpdate('cascade')->onDelete('cascade');
    });

部分

Schema::create('sections', function (Blueprint $table) {
    $table->bigIncrements('id');
    $table->char('name', 1);
    $table->unsignedBigInteger('standard_id');
    $table->timestamps();

    $table->foreign('standard_id')
    ->references('id')->on('standards')
    ->onUpdate('cascade')->onDelete('cascade');
});

现在我有标准和部分表，这些表中的外键将替换现有结构中的类和部分列，并保持 roll_number、section_id、standard_id 和 school_id 的组合是唯一的。

我试过了

 public function up()
    {
        Schema::table('students', function (Blueprint $table) {
            $table->dropUnique(['roll_number', 'section', 'class', 'school_id']);

            $table->dropColumn('section');
            $table->dropColumn('class');
            $table->unsignedBigInteger('standard_id')->after('roll_number');
            $table->unsignedBigInteger('section_id')->after('standard_id');

            $table->foreign('standard_id')->references('id')->on('standards')
            ->onUpdate('cascade')->onDelete('cascade');
            $table->foreign('section_id')->references('id')->on('sections')
            ->onUpdate('cascade')->onDelete('cascade');

            $table->unique(['roll_number', 'standard_id', 'section_id', 'school_id']); // unique combination
        });
    }

但它似乎不起作用。

错误

Illuminate\Database\QueryException : SQLSTATE[23000]: 完整性 违反约束：1452 无法添加或更新子行：外国 键约束失败（myapp_extra.#sql-2f78_29d, CONSTRAINT students_standard_id_foreign 外键 (standard_id) 参考 NCES standards (id) ON DELETE CASCADE ON UPDATE CASCADE) (SQL: alter table students add constraint students_standard_id_foreign 删除时外键 (standard_id) 引用 standards (id) 级联更新级联）

  at \myapp\vendor\laravel\framework\src\Illuminate\Database\Connection.php:664

注意：标准和部分表是在进行此迁移之前创建的，因此这两个列都可用。

Answer 1

为什么会这样？

发生此错误是因为您正在向已有行的表中插入一个没有默认值且没有NULLABLE 的新列。

当你这样做时，MySQL 会将它的值设置为0，所以你所有的表都将standard_id 和section_id 设置为0，所以当你尝试添加外部索引时，它会失败，因为0 不是您的 standards/sections 表上的有效 ID。

那么如何解决呢？

你有一些方法可以解决这个问题：

1st：设置默认值

如果对您的应用有意义，您可以为您的列设置一个默认（有效）值，这样外键就不会失败：

        $table->unsignedBigInteger('standard_id')->default(1)->after('roll_number');
        $table->unsignedBigInteger('section_id')->default(1)->after('standard_id');

大多数时候它并不是那么简单，所以你需要动态地定义值

2nd：动态设置值

如果您有一些逻辑可以为这些新列设置默认值，则可以将迁移分为两个步骤：

// Add the fields first
Schema::table('students', function (Blueprint $table) {
        $table->dropUnique(['roll_number', 'section', 'class', 'school_id']);

        $table->dropColumn('section');
        $table->dropColumn('class');
        $table->unsignedBigInteger('standard_id')->after('roll_number');
        $table->unsignedBigInteger('section_id')->after('standard_id');
}

App\Students::get()->each(function($student) {
    // Apply your logic here
    $student->standard_id = 3;
    $student->section_id = 3;
    $student->save();
});

// Now you can add your foreign keys.
Schema::table('students', function (Blueprint $table) {
        $table->foreign('standard_id')->references('id')->on('standards')
        ->onUpdate('cascade')->onDelete('cascade');
        $table->foreign('section_id')->references('id')->on('sections')
        ->onUpdate('cascade')->onDelete('cascade');

        $table->unique(['roll_number', 'standard_id', 'section_id', 'school_id']); // unique combination
});

3rd：使字段可以为空

如果您只是不知道或没有此字段的默认值，那么您的字段应该可以为空：

    $table->unsignedBigInteger('standard_id')->nullable()->after('roll_number');
    $table->unsignedBigInteger('section_id')->nullable()->after('standard_id');