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Django Model Meta, attribute, methods (해시태그 기능을 위한 공부)

09 Nov 2019 | Django

Django model - Meta, attribute, methods

1. Meta

Models | Django documentation | Django

from django.db import models

class Ox(models.Model):
    horn_length = models.IntegerField()

    class Meta:
        ordering = ["horn_length"]
        verbose_name_plural = "oxen"

Model metadata is “anything that’s not a field”, such as ordering options ( ordering ), database table name ( db_table ), or human-readable singular and plural names ( verbose_name and verbose_name_plural ). None are required, and adding class Meta to a model is completely optional. A complete list of all possible Meta options can be found in the model option reference .

모델 메타데이터는 필드가 아닌 모든것을 뜻합니다. 예를들어 정렬옵션, db table이름, 사람이 읽기 편한 이름 등이 있습니다. 필수는 아니고 옵션입니다. 관련 옵션들은 여기에 있습니다.

2. Model attributes (Manager)

Models | Django documentation | Django

objects

The most important attribute of a model is the Manager . It’s the interface through which database query operations are provided to Django models and is used to retrieve the instances from the database. If no custom Manager is defined, the default name is objects . Managers are only accessible via model classes, not the model instances.

모델의 가장 중요한 속성중 하나는 Manager 입니다. 이 인터페이스를 통해 Django models로 쿼리작업이 이루어지고 데이터베이스로부터 인스턴스를 받아올 수 있습니다. Manager를 따로 정해주지 않으면 디폴트 이름값은 objects입니다. models instances가 아닌 model class를 통해서만 접근할 수 있습니다.

Custom managers

Managers | Django documentation | Django

There are two reasons you might want to customize a Manager: to add extra Manager methods, and/or to modify the initial QuerySet the Manager returns.

커스텀하는 이유

  1. manager methods 추가
  2. initail queryset 수정

Adding extra manager methods

Managers | Django documentation | Django

Adding extra Manager methods is the preferred way to add “table-level” functionality to your models. (For “row-level” functionality – i.e., functions that act on a single instance of a model object – use Model methods , not custom Manager methods.) A custom Manager method can return anything you want. It doesn’t have to return a QuerySet. For example, this custom Manager offers a method with_counts(), which returns a list of all OpinionPoll objects, each with an extra num_responses attribute that is the result of an aggregate query:

from django.db import models

class PollManager(models.Manager):
    def with_counts(self):
        from django.db import connection
        with connection.cursor() as cursor:
            cursor.execute("""
                SELECT p.id, p.question, p.poll_date, COUNT(*)
                FROM polls_opinionpoll p, polls_response r
                WHERE p.id = r.poll_id
                GROUP BY p.id, p.question, p.poll_date
                ORDER BY p.poll_date DESC""")
            result_list = []
            for row in cursor.fetchall():
                p = self.model(id=row[0], question=row[1], poll_date=row[2])
                p.num_responses = row[3]
                result_list.append(p)
        return result_list

class OpinionPoll(models.Model):
    question = models.CharField(max_length=200)
    poll_date = models.DateField()
    objects = PollManager()

class Response(models.Model):
    poll = models.ForeignKey(OpinionPoll, on_delete=models.CASCADE)
    person_name = models.CharField(max_length=50)
    response = models.TextField()

3. Model methods

Models | Django documentation | Django

Define custom methods on a model to add custom “row-level” functionality to your objects. Whereas Manager methods are intended to do “table-wide” things, model methods should act on a particular model instance. This is a valuable technique for keeping business logic in one place – the model. For example, this model has a few custom methods:

모델의 객체에서 row level 수준으로 동작할 custom methods를 정의할 수 있다. Manager methods는 table wide인 반면 model methodssms 특정 모델 인스턴스에서 작동한다. 이는 비즈니스 로직에서 중요한 기술이다.

from django.db import models

class Person(models.Model):
    first_name = models.CharField(max_length=50)
    last_name = models.CharField(max_length=50)
    birth_date = models.DateField()

    def baby_boomer_status(self):
        "Returns the person's baby-boomer status."
        import datetime
        if self.birth_date < datetime.date(1945, 8, 1):
            return "Pre-boomer"
        elif self.birth_date < datetime.date(1965, 1, 1):
            return "Baby boomer"
        else:
            return "Post-boomer"

    @property
    def full_name(self):
        "Returns the person's full name."
        return '%s %s' % (self.first_name, self.last_name)

결론

요약

  1. model attribute (manager) : 하나의 모델, 하나의 테이블, 모델의 인스턴스에 국한된 속성 
    Book.dahl_objects.all()
Book.dahl_objects.filter(title='Matilda')
Book.dahl_objects.count()
  2. model method : low level부터 접근가능, 다른 모델과 연계해서 사용할 수 있다. 
    class Person(models.Model):
 first_name = models.CharField(max_length=50)
 last_name = models.CharField(max_length=50)
 birth_date = models.DateField()

 def baby_boomer_status(self):
     "Returns the person's baby-boomer status."
     import datetime
     if self.birth_date < datetime.date(1945, 8, 1):
         return "Pre-boomer"
     elif self.birth_date < datetime.date(1965, 1, 1):
         return "Baby boomer"
     else:
         return "Post-boomer"

내 프로젝트에 적용

내가 작업하고 있는 인스타그램 클론프로젝트의 Django : models.py의 일부분이다.

class Post(models.Model):
		...
    content = models.TextField(blank=True)
    tag_set = models.ManyToManyField("Tag")

class Tag(models.Model):
    name = models.CharField(max_length=100, unique=True)

내가 구현하려 했던 기능은

  1. Post를 작성하고 저장하면서 내용에서 해시태그를 추출
  2. 추출한 태그를 자동으로 Post table의 tag_set 필드에 저장.

model methods만 활용하면 충분하다고 판단했고 간략하게 다음과 같이 코딩할 예정이다.

class Post(models.Model):
		...
    content = models.TextField(blank=True)
    tag_set = models.ManyToManyField("Tag")

	  def set_tags(self):
		  tag_set = self.content에서 tag 추출
		  t = Tag.objects.get_or_create(name=tag_set)
		  self.tag_set.add(t)

#study/programming/django 9 Nov 2019 3:51 PM

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