laravel - 向现有表添加新的自动增量列

Question

我的旧迁移是:

Schema::create('item_tag', function (Blueprint $table) {

    $table->integer('item_id')->unsigned()->index();
    $table->foreign('item_id')->references('id')->on('items')->onDelete('cascade');

    $table->integer('tag_id')->unsigned()->index();
    $table->foreign('tag_id')->references('id')->on('tags')->onDelete('cascade');

    $table->primary(['item_id', 'tag_id']);

});

现在我想为此添加新的自动增量列并删除旧的主键

我试试这个:

Schema::table('item_tag', function (Blueprint $table) {

    $table->unsignedInteger('id', true)->first();

    $table->dropPrimary();
    $table->primary('id');

});

但是迁移后报错:

SQLSTATE[42000]: Syntax error or access violation: 1068 Multiple primary key defined

Answer 1

在单独的迁移中删除主键并删除 $table->primary('id') .添加 AUTO_INCREMENT column 自动创建主键:

Schema::table('item_tag', function (Blueprint $table) {   
    $table->dropPrimary();
});

Schema::table('item_tag', function (Blueprint $table) {
    $table->unsignedInteger('id', true)->first();
});

您还可以简化第二次迁移:

Schema::table('item_tag', function (Blueprint $table) {
    $table->increments('id')->first();
});