How to create query with twice a connection to a t

问题:

I need get two city names with one query:

For example:

City table:

+---------+----------+
|  Pana   |   Name   |
+---------+----------+
|   THR   |  Tehran  |
|   LON   |  London  |
+---------+----------+

In Model: from_city is THR and to_city is LON

public function scopePrintQuery($query, $id)
{
    $join = $query
        -> join('cities', 'cities.pana', 'flights.from_city')
        -> join('cities', 'cities.pana', 'flights.to_city')
        -> where('flights.id', $id)
        ->get([
            'flights.*',
            'cities.name as from_city'
            ??? for to_city?
        ]);
    return $join;
}

Now, I need get from_city name and to_city name in this query.

The query does not work with two joins from one table!

How to create this query?

回答1:

With straight SQL you could give each joined table an alias - e.g.

SELECT flights.*
FROM flights as f
 JOIN cities as fromCity on fromCity.pana = f.from_city
 JOIN cities as toCity on toCity.pana = f.to_city
WHERE f.id = 3 --

With Eloquent, use select() to specify select fields. Also use DB::raw() to use raw SQL (e.g. giving an alias to table like DB::raw('cities as toCity').

public function scopePrintQuery($query, $id)
{
  $join = $query
    -> join(DB::raw('cities as fromCity'), 'fromCity.pana', 'flights.from_city')
    -> join(DB::raw('cities as toCity'), 'toCity.pana', 'flights.to_city')
    -> where('flights.id', $id)
    ->select([
        'flights.*',
        DB::raw('fromCity.name as from_city')
        DB::raw('toCity.name as to_city')
    ]);
    return $join->get();
}

回答2:

you can also use the eloquent model with defining the relationship.

Also for more detail visit https://laravel.com/docs/5.3/eloquent-relationships

crate two model -- 1st is "Flights"

<?php


class Flights extends Model
{
    protected $table = 'flights';

    /**
     * Get the From City detail.
     */
    public function fromCity()
    {
        return $this->hasOne('App\Models\City', 'Pana', 'from_city');
    }

    /**
     * Get the To city state.
     */
   public function toCity()
   {
        return $this->hasOne('App\Models\City', 'Pana', 'to_city');
   }

}

2nd Model is "City"

<?php
class City extends Model
{
    protected $table = 'city';
}

Now for fetching

Flights::where(id, $id)->with('toCity', 'fromCity')->get();