Creating Model Managers in Django

Model Managers:

  • object is the default manager of every model that retrieves all objects in the database. 
  • We can also define custom managers for your models. 
  • We will create a custom manager to retrieve all posts with the published status.
  • There are 2 ways to add or customize managers for your models: you can add extra manager methods to an existing manager, or create a new manager by modifying the initial QuerySet that the manager returns. 
    • The first method provides you with a QuerySet API such as Posts.objects.my_manager()
    • The 2nd method provides you with Posts.my_manager.all(). 
      • The manager will allow you to retrieve posts using Posts.published.all().
  • Edit the models.py file of your <<xx>> application to add the custom manager:

class xxManager(models.Manager):

    def get_queryset(self):

        return super(xxManager,

                            self).get_queryset().filter(status='published')

class Posts(models.Model):

    objects = models.Manager() # The Default Manager.

    published = xxManager() # The Custom Manager.

  • The 1st manager declared in a model becomes the Default Manager. You can use the Meta attribute default_manager_name to specify a different default manager. 
  • If no manager is defined in the model, Django automatically creates the objects default manager for it. 
  • If you declare any managers for your model but you want to keep the objects manager as well, you have to add it explicitly to your model.
  • In the above code, you add the Default Objects Manager and the Published Custom Manager(xxManager()) to the Post model.
  • The get_queryset() method of a manager returns the QuerySet that will be executed. 
  • We can  override get_queryset() method to include your custom filter in the final QuerySet.
  • We have now defined the Custom Manager and added it to the Posts model; 
    • from blog.models import Posts
    • Posts.published.filter(title__startswith='abc')
    • To obtain results for this QuerySet, make sure that you set the published field to True in the Posts object whose title starts with abc.


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