Django: Using models

From: MDN Web Docs

Models

• Python objects
• Define the data structure
• Independent from the database schema
• Facilitate communication between Django and the database via Object-Relational Mapper

Designing Models

Create separate models for every object. Models can also be used to represent selection-list options.

Consider the relationships between objects. Relationships include:

• one to one (OneToOneField)
• one to many (ForeignKey)
• many to many (ManyToManyField)

Multiplicities define the maximum and minimum number of each model that may be present in the relationship.

Model definition

Models:

• Defined in models.py
• Extend django.db.models.Model class
• Can include fields, methods and metadata

Fields

• Fields in a model represent columns in a database table.
• Each record (row) in the table contains values for these fields.
• Field types are designated using specific classes that define the type of data stored.
• Field types can also be used for HTML form validation.

Field Arguments

• https://docs.djangoproject.com/en/5.0/ref/models/fields/#field-options
• help_text
• verbose_name
• default
• null
• blank
• unique
• primary_key

COMMON FIELD TYPES

• https://docs.djangoproject.com/en/5.0/ref/models/fields/#field-types
• CharField
• TextField
• IntegerField
• other fields for different types of numbers
• DateField
• EmailField
• FileField
• ImageField
• AutoField
• ForeignKey specifies one-to-many relationship to another database model
• ManyToManyField

Metadata

• https://docs.djangoproject.com/en/5.0/ref/models/options/
• Declare model-level metadata with class Meta
• Useful for controlling the ordering of records returned by queries
• Other metadata options control the database used for the model and how the data is stored

Methods

__str__()

• Python class method
• Provides a readable string representation of the object
• The string represents individual records

get_absolute_url()

• Generates a URL to view individual records of the model

Model management

Use model classes to create, update, or delete records, and to run queries.

Creating and modifying records

• Create a record by instantiating the model and using the model's constructor.
• Then call save() to save the object to the database.
• Access fields in the record using dot notation.
• Search for records using the model's objects attribute

The full list of lookups:

https://docs.djangoproject.com/en/5.0/ref/models/querysets/#field-lookups

Making queries:

https://docs.djangoproject.com/en/5.0/topics/db/queries/

Constraints

https://docs.djangoproject.com/en/5.0/ref/models/constraints/

Defining the LocalLibrary Models

At the top of /django-locallibrary-tutorial/catalog/models.py, the boilerplate imports the models module, which provides the models.Model base class that our models inherit from.

Genre model

• Stores information about the book genre
• A single CharField field is used to describe the genre
• Limited to 200 characters
• Has some help_text
• Set tounique=True
• One record for each genre
• a __str__() method returns the name of the genre
• get_absolute_url() method used to access a detail record

Book model

• Represents all the general information about an available book
• CharField is used for the book's title and isbn.
• unique as true ensures all books have a unique ISBN
• title is not set to be unique, because it is possible for different books to have the same name.
• TextField for longer summary
• ManyToManyField a book can have multiple genres and a genre can have many books

In both field types:

• The first unnamed parameter should specify the related model class
  • either directly by the model class
  • or as a string with the name of the related model
• If the associated class is not yet defined, use the model's name as a string in this file
• Setting null=True permits the database to store Null if no author is selected
• Using on_delete=models.RESTRICT prevents the deletion of the book's author if it is referenced by any book

Warning:

• The default behavior is on_delete=models.CASCADE
• This means that if the author is deleted, the book would also be deleted
• Use RESTRICT or PROTECT to avoid the author being deleted while it is referenced by any book
• Alternatively, use SET_NULL to set the book's author to Null if the author record is deleted
• The __str__() method represents a Book record by its title field
• The get_absolute_url() method provides a URL to access a detailed record

BookInstance model

• Represents a specific copy of a book
• Includes information about whether the copy is available
• The date expected back
• version details
• unique id for the book in the library
• ForeignKey identifies the associated Book
  • each book can have many copies
  • a copy can only have one Book
• on_delete=models.RESTRICT ensures the Book cannot be deleted while referenced by a BookInstance
• CharField represents the specific release
• UUIDField for the id field to set it as the primary_key
• This allocates a globally unique value for each instance
• DateField for the due_back date
• status CharField defines a choice/selection list
• str() uses a combination of its unique id and title

Re-run the database migrations

After models have now been created, re-run database migrations to add them to the database.

python3 manage.py makemigrations
python3 manage.py migrate