title: whats causing the migration to fail with error " there can only be on auto increment column"
body: <p>My table posts does not have any limits, i am letting <code>Laravel</code> choose the auto increment by default. It still fails with “there can only be on auto increment column”.</p>
  <h2>migration</h2>
  <pre><code>public function up()
  {
      Schema::create('posts', function (Blueprint $table) {
          $table-&gt;id();
          $table-&gt;id('user_id')-&gt;nullable();
          $table-&gt;id('admin_id')-&gt;nullable();
          $table-&gt;string('about')-&gt;nullable();
          $table-&gt;decimal('price')-&gt;nullable();
          $table-&gt;string('new_used')-&gt;nullable();
          $table-&gt;string('negotiable')-&gt;nullable();
          $table-&gt;dateTime('expire_date')-&gt;nullabe();
  
        
          $table-&gt;timestamps();
      });
  }
  </code></pre>