Laravel - how to use belongsToMany relationship using second index and not primary key

App overview

I'm learning Laravel and my dummy app allows users to log issues against projects. A project can have many issues and an issue can have many tags assigned to it.

I'm currently working on projects/show.blade.php which outputs all issues associated with a project in an HTML table with columns for name and tags.

Issue

I've setup 'belongs to many' relationships so that my issues, tags and issues_tags (intermediary) tables can talk to each other but I've run into a problem whereby tags are being output against the wrong issues.

The problem stems from the fact that my issues table (by design) includes an item_id column, which is an index but not the primary key. I use item_id for routing/display on the frontend because I wanted the issue ID that users see to start from 1 for every new project added instead of being sequential and everyone knowing how many there are in the database.

For example, tags assigned to issue #3 are called using issues.id and not issues.item_id.

Sample data

Code

Issues migration

    Schema::create('issues', function (Blueprint $table) {
        $table->increments('id');
        $table->integer('item_id')->index()->unsigned();
        $table->integer('project_id')->unsigned();
        $table->string('name');
        $table->longText('description');
        $table->tinyInteger('status')->default('1');
        $table->integer('created_by');
        $table->timestamps();
    });

Tags migration

Schema::create('tags', function (Blueprint $table) {
    $table->increments('id');
    $table->string('name');
    $table->timestamps();
});

Issues tags migration

Schema::create('issues_tags', function (Blueprint $table) {
    $table->increments('id');
    $table->integer('tag_id')->unsigned()->index();
    $table->foreign('tag_id')->references('id')->on('tags');
    $table->integer('issue_id')->unsigned()->index();
    $table->foreign('issue_id')->references('item_id')->on('issues');
    $table->timestamps();
});

Issue model

public function tags()
{
    return $this->belongsToMany('App\Tag', 'issues_tags');
}

Tag model

public function issues()
{
    return $this->belongsToMany('App\Issue', 'issues_tags', 'issue_id')->withPivot('item_id');
}

View (projects/show.blade.php)

  <table>
    <tr>
      <th>ID</th>
      <th>Item ID</th>
      <th>Issue</th>
      <th>Tags</th>
    </tr>
    @foreach($issues as $issue)
    <tr>
      <td></td>
      <td></td>
      <td></td>
      <td>
        @foreach($issue->tags as $tag)
          <span class="badge badge-primary"></span>
        @endforeach
      </td>
    </tr>
    @endforeach
  </table>

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Question

How can I use issues.item_id instead of issues.id when using a belongsToMany relationship in Laravel?

via Chebli Mohamed