Laravel分类迁移出现errno:150外键约束错误求助

The error you're hitting happens because the data type of your parent_id field doesn't match the type of the id field it references. Let's break this down and fix it step by step:

Why This Error Occurs #

• $table->bigIncrements('id') creates an unsigned big integer (unsigned BIGINT) for your primary key.
• But your parent_id is defined as $table->integer('parent_id')->unsigned() which makes it an unsigned integer (unsigned INT)—a smaller, incompatible data type. MySQL rejects foreign key constraints when column types don't match exactly.

Solution 1: Align Column Types in the Original Migration #

Update your parent_id to use unsignedBigInteger so it matches the id column's type:

public function up() {
    Schema::create('categories', function (Blueprint $table) {
        $table->bigIncrements('id');
        $table->string('title');
        $table->string('slug');
        // Match the id column's type with unsignedBigInteger
        $table->unsignedBigInteger('parent_id')->default(0);
        $table->timestamps();
    });
    Schema::table('categories', function (Blueprint $table) {
        $table->foreign('parent_id')
              ->references('id')
              ->on('categories')
              ->onUpdate('cascade')
              ->onDelete('cascade');
    });
}

Solution 2: Use Laravel's Shorthand (Laravel 8+) #

If you're on Laravel 8 or newer, you can simplify this with the foreignId method—it automatically creates an unsignedBigInteger field and sets up the foreign key in one line:

public function up() {
    Schema::create('categories', function (Blueprint $table) {
        $table->id(); // Shorthand for bigIncrements('id')
        $table->string('title');
        $table->string('slug');
        // foreignId handles type and constraint in one step
        $table->foreignId('parent_id')
              ->default(0)
              ->constrained('categories')
              ->onUpdate('cascade')
              ->onDelete('cascade');
        $table->timestamps();
    });
}

If You've Already Run the Old Migration #

If you already executed the original migration (and the table exists), don't edit the old migration file directly. Instead:

1. Roll back the migration first:

   php artisan migrate:rollback
   

2. Run the updated migration:

   php artisan migrate
   

Alternatively, create a new migration to modify the parent_id column type:

// Example migration file: 2024_05_20_123456_fix_category_parent_id_type.php
public function up() {
    Schema::table('categories', function (Blueprint $table) {
        // Drop existing foreign key if it was partially created
        try {
            $table->dropForeign('categories_parent_id_foreign');
        } catch (\Exception $e) {
            // Ignore if the foreign key doesn't exist
        }
        // Change column type to match id
        $table->unsignedBigInteger('parent_id')->default(0)->change();
        // Re-add the foreign key constraint
        $table->foreign('parent_id')
              ->references('id')
              ->on('categories')
              ->onUpdate('cascade')
              ->onDelete('cascade');
    });
}

public function down() {
    Schema::table('categories', function (Blueprint $table) {
        $table->dropForeign('categories_parent_id_foreign');
        $table->integer('parent_id')->unsigned()->default(0)->change();
    });
}

Run this new migration with:

php artisan migrate

内容的提问来源于stack exchange，提问作者Mostafa Norzade