Laravel 查詢生成器 - 連接三個表（A 一對多 B，B 一對多 C）

Question

我有三張桌子。

國家：id，country_name
State：id，country_id，state_name
城市：id、state_id、city_name

我想獲得一個特定國家（$id = 1），其中包含每個 state 中的所有城市。
我試試這段代碼。

DB::table('country as a')
   ->join('state as b', 'b.country_id', '=', 'a.id')
   ->join('city as c', 'c.state_id', '=', 'b.id')
   ->select('a.*', 'b.*', 'c.*') 
   ->where('a.id', '=', $id)
   ->get();

並得到這樣的結果。

result: [
  {id: 1, country_id: 1, country_name: ABC, state_name: aaa, state_id: 0, city_name: city a1},
  {id: 2, country_id: 1, country_name: ABC, state_name: aaa, state_id: 0, city_name: city a2},
  {id: 3, country_id: 1, country_name: ABC, state_name: aaa, state_id: 0, city_name: city a3},
  {id: 4, country_id: 1, country_name: ABC, state_name: aaa, state_id: 0, city_name: city a4},
  {id: 5, country_id: 1, country_name: ABC, state_name: bbb, state_id: 1, city_name: city b1},
  {id: 6, country_id: 1, country_name: ABC, state_name: bbb, state_id: 1, city_name: city b2},
  {id: 7, country_id: 1, country_name: ABC, state_name: bbb, state_id: 1, city_name: city b3},
  {id: 8, country_id: 1, country_name: ABC, state_name: bbb, state_id: 1, city_name: city b4},
  .....
]

我希望能得到這樣的結果。

result: [
  {
    id: 1, 
    country_id: 1,
    country_name: ABC, 
    state_name: aaa, 
    state_id: 0, 
    city: [
      {city_name: city a1},
      {city_name: city a2},
      {city_name: city a3},
      {city_name: city a4}
    ]
  },
  {
    id: 2, 
    country_id: 1,
    country_name: ABC, 
    state_name: bbb, 
    state_id: 1, 
    city: [
      {city_name: city b1},
      {city_name: city b2},
      {city_name: city b3},
      {city_name: city b4}
    ]
  },
  .....
  and like that for other states.
]

也許有人可以幫助我提供新的見解。
非常感謝。

Answer 1

利用 Eloquent 關系

模型與關系

class Country extends Model
{
    public function states()
    {
        return $this->hasMany(State::class);
    }
}

class State extends Model
{
    public function country()
    {
        return $this->belongsTo(Country::class);
    }

    public function cities()
    {
        return $this->hasMany(City::class);
    }
}

class City extends Model
{
    public function state()
    {
        return $this->belongTo(State::class);
    }
}

詢問

$countries = Country::query()
    ->with('states.cities')
    ->get();