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Sorry for not being this as programming question, but this caught my eye when I was trying to introspect my class objects.
I found this
{'user_id': 1, '_state': <django.db.models.base.ModelState object at 0x10ac2a750>, 'id': 2, 'playlist_id': 8} What is the role of _state and what ModelState does?
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str function in a django model returns a string that is exactly rendered as the display name of instances for that model. # Create your models here. This will display the objects as something always in the admin interface.
To save changes to an object that's already in the database, use save() . This performs an UPDATE SQL statement behind the scenes. Django doesn't hit the database until you explicitly call save() .
Django includes a “signal dispatcher” which helps decoupled applications get notified when actions occur elsewhere in the framework. In a nutshell, signals allow certain senders to notify a set of receivers that some action has taken place.
Use update_fields in save() If you would like to explicitly mention only those columns that you want to be updated, you can do so using the update_fields parameter while calling the save() method. You can also choose to update multiple columns by passing more field names in the update_fields list.
From the Django source code, _state is an instance variable defined in each Model instance that is an instance of ModelState that is defined as:
class ModelState(object): """ A class for storing instance state """ def __init__(self, db=None): self.db = db # If true, uniqueness validation checks will consider this a new, as-yet-unsaved object. # Necessary for correct validation of new instances of objects with explicit (non-auto) PKs. # This impacts validation only; it has no effect on the actual save. self.adding = True So basically this instance variable is used to know if the Model instance was already written to a db (knowing that Django support multiple db backends) and to hold the db used, this instance variable attribute adding is set to false after saving the model instance, and it's used mostly (as the comment in the code above) for validating if the primary keys is unique.
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