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In working with the Django user model I've noticed model managers include a using=self._db parameter when acting on a database. If I'm only using a single database, is this necessary? What does using=self._db do other than specify the database. Is this specified as a fail-safe in the event another database is added?
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This is used in case you have multiple databases by which you define which database you need to use for operation. An example user.save(using=self._db) usually defined as "default" from your database configuration in settings.py .
A model's manager is an object through which Django models perform database queries. Each Django model has at least one manager, and you can create custom managers in order to customise database access.
A Manager is the interface through which database query operations are provided to Django models. At least one Manager exists for every model in a Django application. The way Manager classes work is documented in Making queries; this document specifically touches on model options that customize Manager behavior.
You can use a custom Manager in a particular model by extending the base Manager class and instantiating your custom Manager in your model. 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.
Django Model Manager is the interface through which database query operations are provided to Django models. objects is the default model manager and each model needs to have at least one model manager. Let's see a few use-cases when we can use model managers and when to use custom model managers.
We all know that model data operation in Django is so easy, you do not need to write any sql command at all, what you need to do is just use model class’s attribute objects ‘s methods to create, query, update and delete mode data records in backend database table like below. Suppose our model class name is Employee.
By default, Django adds a Manager with the name objects to every Django model class. However, if you want to use objects as a field name, or if you want to use a name other than objects for the Manager, you can rename it on a per-model basis. To rename the Manager for a given class, define a class attribute of type models.Manager () on that model.
# the first defined model manager is the default model manager. # create two custom model data table level manager. ......
Managers use that parameter to define on which database the underlying queryset the manager uses should operate on. This is simply there to optionally override it in case you have e.g. multiple databases and you want your manger/queryset to operate on a specific one.
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