Question

I have read the definition in the official Django documentation, and I am still confused by what a Manager does.

The documentation says that they allow you to operate on database tables/models, but I still don’t understand this.

Can someone explain managers and their role to me? An answer with an example would be preferable.

Answer 1

A manager is usually something hidden away from django programmers that django uses to interface between model code and the database backend.

When you query the django ORM, you do so through calls to

from my_app.models import MyModel

mms = MyModel.objects.all()

In this case, the objects part of the function is what is returned by the manager. If you wanted MyModel to only ever get blue MyModel instances (the database might contain red models too) then you could create a manager and hack your model thus

class BlueManager(models.Manager):
    def get_query_set(self):
        return super(BlueManager, self).get_query_set().filter(colour='Blue')

class MyModel(models.Model):
     colour = models.CharField(max_length=64)
     blue_objects = BlueManager()

and calling

MyModel.blue_objects.all()

would only return objects with colour as blue. Note, this is a very poor way to filter models!

One would usually need to modify a Manager interface if they were going to modify the QuerySets that a manager would usually return or if you needed to add “table” level queries (rather than regular django “row” level). The documentation for managers is quite complete and contains several examples.