Laravel：如何解决“SQLSTATE[HY000]：一般错误：1215 无法添加外键约束”

Question

我正在进行迁移，其中存在使用外键将一个表与其他各种表映射的特定迁移。并且在运行迁移时出现以下错误。

SQLSTATE[HY000]：一般错误：1215 无法添加外键约束（SQL：更改表bookmaps 添加约束bookmaps_subject_id_foreign 外键（subject_id）在删除级联时引用subject（id））

这是导致错误的迁移：

    public function up()
    {
        Schema::create('bookmaps', function (Blueprint $table) {
            $table->unsignedBigInteger('book_id')->unique();
            $table->unsignedBigInteger('subject_id')->nullable();
            $table->unsignedBigInteger('grade_id')->nullable();
            $table->unsignedBigInteger('author_id')->nullable();
            $table->unsignedBigInteger('catagory_id')->nullable();
            $table->unsignedBigInteger('language_id')->nullable();
            $table->timestamps();
        });

        Schema::table('bookmaps', function($table) {
            $table->foreign('book_id')->references('id')->on('book')->onDelete('cascade');
            $table->foreign('subject_id')->references('id')->on('subject')->onDelete('cascade');
            $table->foreign('grade_id')->references('id')->on('grade')->onDelete('cascade');
            $table->foreign('author_id')->references('id')->on('author')->onDelete('cascade');
            $table->foreign('catagory_id')->references('id')->on('catagories')->onDelete('cascade');
            $table->foreign('language_id')->references('id')->on('language')->onDelete('cascade');
        });
    }

其他相关表的迁移是：

    public function up()
    {
        Schema::create('book', function (Blueprint $table) {
            $table->id();
            $table->string('name');
            $table->string('discription')->nullable();
            $table->string('book_file')->nullable();
            $table->string('card_image')->nullable();
            $table->unsignedBigInteger('user_id')->nullable();
            $table->timestamps();
        });

        Schema::table('book', function($table) {
            $table->foreign('user_id')->references('id')->on('users')->onDelete('cascade');
        });
     
    }

上述迁移创建的表仅通过外键映射，其余迁移未映射。

    public function up()
    {
        Schema::create('subject', function (Blueprint $table) {
            $table->id();
            $table->string('subject_name');
            $table->timestamps();
        });
    }

public function up()
    {
        Schema::create('grade', function (Blueprint $table) {
            $table->id();
            $table->string('grade_name');
            $table->timestamps();
        });
    }

public function up()
    {
        Schema::create('author', function (Blueprint $table) {
            $table->id();
            $table->string('author_name');
            $table->string('author_discription')->nullable();
            $table->timestamps();
        });
    }

    public function up()
    {
        Schema::create('grade', function (Blueprint $table) {
            $table->id();
            $table->string('grade_name');
            $table->timestamps();
        });
    }

    public function up()
    {
        Schema::create('author', function (Blueprint $table) {
            $table->id();
            $table->string('author_name');
            $table->string('author_discription')->nullable();
            $table->timestamps();
        });
    }

    public function up()
    {
        Schema::create('catagories', function (Blueprint $table) {
            $table->id();
            $table->string('catagories_name');
            $table->timestamps();
        });
    }

public function up()
    {
        Schema::create('language', function (Blueprint $table) {
            $table->id();
            $table->string('language_name');
            $table->timestamps();
        });
    }

可能是什么问题？

Answer 1

您必须在书签之前创建主题迁移。当 Laravel 尝试迁移书签表时，它没有找到链接到主题表的外键。