Category1
    Goal1
        Activity1
    Goal2
        Activity2
        Activity3

@tasks.groupBy(_.goal).map { case (goal, tasks) =>
    <ul>
        <li>@goal.name</li>
        <ul>
            @tasks.map { task =>
                <li>@task.name</li>
            }
        </ul>
    </ul>
}

Goal1
    Activity1
Goal2
    Activity2
    Activity3

tasks.groupBy(_.goal).groupBy { case (goal, _) => goal.category }

tasks.groupBy(_.goal).groupBy(_._1.category)

<ul>
  @tasks.groupBy(_.goal).groupBy(_._1.category).map { case (category, goals) =>
    <li>@category.name
      <ul>
        @goals.map { case (goal, tasks) =>
          <li>@goal.name
            <ul>
              @tasks.map { task => <li>@task.name</li> }
            </ul>
          </li>
        }
      </ul>
    </li>
  }
</ul>

<ul>
    @defining(tasks.map(_.goal.category).distinct) {categories =>
        @categories.map{category =>
            <li>
                @category.name
            </li>
            <ul>
                @tasks.filter(_.goal.category == category).groupBy(_.goal).map{ case (goal, tasks) =>
                    <ul>
                        <li>@goal.name</li>
                        <ul>
                            @tasks.map {task =>
                                <li>@task.name</li>
                            }
                        </ul>
                    </ul>
                }
            </ul>
        }
    }
</ul>