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Django Using models

what a model is, how it is declared, and some of the main field types. It also briefly shows a few of the main ways you can access model data??!! Django web applications access and manage data through Python objects referred to as models. Models define the structure of stored data, including the field types and possibly also their maximum size, default values, selection list options, help text for documentation, label text for forms, etc. The definition of the model is independent of the underlying database — you can choose one of several as part of your project settings. Once you’ve chosen what database you want to use, you don’t need to talk to it directly at all — you just write your model structure and other code, and Django handles all the dirty work of communicating with the database for you.

Model definition Models are usually defined in an application’s models.py file. They are implemented as subclasses of django.db.models.Model, and can include fields, methods and metadata.

Fields A model can have an arbitrary number of fields, of any type — each one represents a column of data that we want to store in one of our database tables. Each database record (row) will consist of one of each field value. Let’s look at the example seen below:

 my_field_name = models.CharField(max_length=20, help_text='Enter field documentation')

Common field types: The following list describes some of the more commonly used types of fields.

CharField is used to define short-to-mid sized fixed-length strings. You must specify the max_length of the data to be stored.
TextField is used for large arbitrary-length strings. You may specify a max_length for the field, but this is used only when the field is displayed in forms (it is not enforced at the database level).
IntegerField is a field for storing integer (whole number) values, and for validating entered values as integers in forms.
DateField and DateTimeField are used for storing/representing dates and date/time information (as Python datetime.date in and datetime.datetime objects, respectively). These fields can additionally declare the (mutually exclusive) parameters auto_now=True (to set the field to the current date every time the model is saved), auto_now_add (to only set the date when the model is first created) , and default (to set a default date that can be overridden by the user).
EmailField is used to store and validate email addresses.
FileField and ImageField are used to upload files and images respectively (the ImageField adds additional validation that the uploaded file is an image). These have parameters to define how and where the uploaded files are stored.
AutoField is a special type of IntegerField that automatically increments. A primary key of this type is automatically added to your model if you don’t explicitly specify one.
ForeignKey is used to specify a one-to-many relationship to another database model (e.g. a car has one manufacturer, but a manufacturer can make many cars). The “one” side of the relationship is the model that contains the “key” (models containing a “foreign key” referring to that “key”, are on the “many” side of such a relationship).
ManyToManyField is used to specify a many-to-many relationship (e.g. a book can have several genres, and each genre can contain several books). In our library app we will use these very similarly to ForeignKeys, but they can be used in more complicated ways to describe the relationships between groups. These have the parameter on_delete to define what happens when the associated record is deleted (e.g. a value of models.SET_NULL would set the value to NULL).

Django admin site: we’ll use the Django Admin site to add some “real” book data. First we’ll show you how to register the models with the admin site, then we’ll show you how to login and create some data. At the end of the article we will show some of the ways you can further improve the presentation of the Admin site.

Creating a superuser In order to log into the admin site, we need a user account with Staff status enabled. In order to view and create records we also need this user to have permissions to manage all our objects. You can create a “superuser” account that has full access to the site and all needed permissions using manage.py.

Call the following command, in the same directory as manage.py, to create the superuser. You will be prompted to enter a username, email address, and strong password. python3 manage.py createsuperuser

Logging in and using the site To login to the site, open the /admin URL (e.g. http://127.0.0.1:8000/admin) and enter your new superuser userid and password credentials (you’ll be redirected to the login page, and then back to the /admin URL after you’ve entered your details).
Advanced configuration Django does a pretty good job of creating a basic admin site using the information from the registered models:

Each model has a list of individual records, identified by the string created with the model’s str() method, and linked to detail views/forms for editing. By default, this view has an action menu at the top that you can use to perform bulk delete operations on records. The model detail record forms for editing and adding records contain all the fields in the model, laid out vertically in their declaration order.