Course.php

<?php

class Course extends Eloquent implements Collectable {
  use SoftDeletingTrait;
  use EventGenerator;
  use TaggableTrait;
  use ImagableTrait;

  /**
   * @return \Illuminate\Database\Eloquent\Relations\BelongsToMany
   */
  public function completedBy()
  {
    return $this->belongsToMany('User', 'course_progress')->withTimestamps()->withPivot(['scope_type', 'scope_id']);
  }

  /**
   * @param Model  $parent
   * @param array  $attributes
   * @param string $table
   * @param bool   $exists
   *
   * @return CourseProgress|Pivot
   */
  public function newPivot(Model $parent, array $attributes, $table, $exists)
  {
    if($table == 'course_progress')
      return new CourseProgress($parent, $attributes, $table, $exists);

    return new Pivot($parent, $attributes, $table, $exists);
  }
}

CourseProgress.php

<?php

class CourseProgress extends Pivot {
  use EventGenerator;

  /**
   * @var string
   */
  protected $table = 'course_progress'; // The pivot table between users and courses

  /**
   * @return \Illuminate\Database\Eloquent\Relations\BelongsTo
   */
  public function user()
  {
    return $this->belongsTo('User');
  }

  /**
   * @return \Illuminate\Database\Eloquent\Relations\BelongsTo
   */
  public function course()
  {
    return $this->belongsTo('Course');
  }

  /**
   * @return \Illuminate\Database\Eloquent\Relations\MorphTo
   */
  public function scope()
  {
    return $this->morphTo();
  }
}

User.php

<?php

class User extends Eloquent implements UserInterface, RemindableInterface, CourseProgressScope {
  use UserTrait, RemindableTrait, EventGenerator;

  /**
   * @return \Illuminate\Database\Eloquent\Relations\BelongsToMany
   */
  public function courseProgress()
  {
    return $this->belongsToMany('Course', 'course_progress')->withTimestamps()->withPivot(['scope_type', 'scope_id']);
  }

  /**
   * @param Model  $parent
   * @param array  $attributes
   * @param string $table
   * @param bool   $exists
   *
   * @return CourseProgress|Pivot
   */
  public function newPivot(Model $parent, array $attributes, $table, $exists)
  {
    if($table == 'course_progress')
      return new CourseProgress($parent, $attributes, $table, $exists);

    return new Pivot($parent, $attributes, $table, $exists);
  }

}