Laravel: How to solve "SQLSTATE[HY000]: General error: 1215 Cannot add foreign key constraint"

Question

I am making migrations where there is a particular migration that maps one table with various other tables using a foreign key. and I am getting the following error while running the migration.

SQLSTATE[HY000]: General error: 1215 Cannot add foreign key constraint (SQL: alter table bookmaps add constraint bookmaps_subject_id_foreign foreign key (subject_id) references subject (id) on delete cascade)

Here is the migration that is creating the error:

    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');
        });
    }

Migrations for other related tables are:

    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');
        });
     
    }

The table created by above migration is only mapped by foreign key rest of the below migrations are not mapped.

    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();
        });
    }

What could be the problem?

Answer 1

You have to create the subject migration before the bookmarks. When Laravel tries to migrate the bookmarks table, there is a foreign key linked to the subject table that it does not find.