Django Model vs. Manager

问题

Not really sure what the difference is. Seems like all the Manager does is have a bunch of functions related to the Model. But these functions could also be placed in the Model too....

Django documentation describes the Manager as follows,

A Manager is the interface through which database query operations are provided to Django models.

So is there anything else fundamentally different about the Manager than this simple abstraction?

Or a better question: what methods should be defined in the Model vs. the Manager? Is there an ACTUAL difference or just stylistic one?

回答1:

In Django, a models' manager is the object through which models perform database queries. Each Django model has at least one manager, which is objects, and you can create your own to change the default behavior.

So, your statement

But these functions could also be placed in the Model too

Well, not really because the model is still depending on the default manager to retrieve the queryset.

Let me try to explain in terms of an example. Lets say your application requires a model object to show only objects with a status of published. Now, MyModel.objects.all() retrieves everything, and you would have to specify the filter MyModel.objects.filter(published=True) every single time.

Now, you can override this default behavior.

class MyModelAdmin(admin.ModelAdmin):

    def queryset(self, request):
        return MyModel.objects.filter(published=True)

What we just did was override the default behaviour of the default manager.

Now, lets say you want everything, You can do something like

class MyModelAdmin(admin.ModelAdmin):    
    def queryset(self, request):
        return MyModel.objects.filter(published=True)
    def all_objects(self, request):
        return MyModel.objects.all()

and while accessing all objects, just do

MyModel.objects.all_objects()

It is also possible to have multiple managers to a single model

In short, managers give a lot of flexibility in terms of accessing querysets to the model.

来源：https://stackoverflow.com/questions/17320333/django-model-vs-manager

标签

django

django-models

django-managers