[PYTHON] Django model: ManyToManyField

Model example and definition

Consider a soccer player (Player) and its position (Position) as an example.

1. Players can have multiple positions
2. There are multiple players in the same position

In this case, ManyToMany comes into play.

Model example

class Position(models.Model):
    id = models.IntegerField(primary_key=True)
    name = models.CharField(max_length=20, unique=True)


class Player(models.Model):
... (Omitted) ...
    position = models.ManyToManyField(Position)

If you migrate Django using the above model definition, the following three tables will be created.

The table called player_position is the PK of the player table and the PK of the position table, and the information is managed as follows.

Search example

Extract all positions of the player "Pirlo"

positions = Player.objects.get(name='Pirlo').position.all()
[print(p.name) for p in positions]

Execution result:
DM
CM

Extract all players in the position of "CB" (Method 1)

players = Player.objects.filter(position=Position.objects.get(name='CB'))
[print(p.name) for p in players]

Execution result:
Maldini
Nesta

Extract all players in the position of "CB" (Method 2)

Model example (method 2)

class Player(models.Model):
... (Omitted) ...
    position = models.ManyToManyField(Position, related_name='player')

• If you add the ** reverse reference ** of "** related_name ='player' **" to the position definition of the player model as described above, the player will be acquired directly from the position execution result as shown below. It is also possible *** (no migration required) ***

players = Position.objects.get(name='CB').player.all()
[print(p.name) for p in players]

Execution result:
Maldini
Nesta

Postscript

Method 1 and Method 2 extracted all the players in the position of "CB", but when I checked the created query, both generated the following SQL.

SELECT "soccer_player"."id", "soccer_player"."name", "soccer_player"."team_id"
FROM "soccer_player"
INNER JOIN "soccer_player_position" ON ("soccer_player"."id" = "soccer_player_position"."player_id")
WHERE "soccer_player_position"."position_id" = 1