場景

. 多對多的關係中經常需要用到的一種方式
. 例項   Question Model and Follower Model(問題關注者)
  假設點選一個按鈕，則如果使用者已經是關注者，則取消關注 否則成為關注者

分析

1.  belongsToMany 在哪裡實現了toggle?  內部trait實現

class BelongsToMany extends Relation
{
    use Concerns\InteractsWithPivotTable;
}

2.  toggle實現的過程 :
   . 將傳入的引數全部轉成Key為id的陣列 （傳入的引數可能是Model Collection BaseCollection 或者本身就是id 陣列或者字串）
   . 和已經存在的關係(id array)找交集 $detch，交集執行detach
   . 和已經存在的關係(id array)找相對補集  執行attach 
     這裡的實現有點意思  採用的方式是在 解析出來的id array中找$deatch補集
         $attach = array_diff_key($records, array_flip($detach));
 

  /**
     * Toggles a model (or models) from the parent.
     *
     * Each existing model is detached, and non existing ones are attached.
     *
     * @param  mixed  $ids
     * @param  bool   $touch
     * @return array
     */
    public function toggle($ids, $touch = true)
    {
        $changes
 
 = [
            'attached' => [], 'detached' => [],
        ];

        $records = $this->formatRecordsList($this->parseIds($ids));

        // Next, we will determine which IDs should get removed from the join table by
        // checking which of the given ID/records is in the list of current records
 

        // and removing all of those rows from this "intermediate" joining table.
        $detach = array_values(array_intersect(
            $this->newPivotQuery()->pluck($this->relatedPivotKey)->all(),
            array_keys($records)
        ));

        if (count($detach) > 0) {
            $this->detach($detach, false);

            $changes['detached'] = $this->castKeys($detach);
        }

        // Finally, for all of the records which were not "detached", we'll attach the
        // records into the intermediate table. Then, we will add those attaches to
        // this change list and get ready to return these results to the callers.
        $attach = array_diff_key($records, array_flip($detach));

        if (count($attach) > 0) {
            $this->attach($attach, [], false);

            $changes['attached'] = array_keys($attach);
        }

        // Once we have finished attaching or detaching the records, we will see if we
        // have done any attaching or detaching, and if we have we will touch these
        // relationships if they are configured to touch on any database updates.
        if ($touch && (count($changes['attached']) ||
                       count($changes['detached']))) {
            $this->touchIfTouching();
        }

        return $changes;
    }