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: the maximum and minimum number of each model that may be present in the relationship.
Model definition
Defined in models.py
Implemented as subclasses of django.db.models.Model
Can include fields, methods and metadata
Fields
A model has fields that represent columns in a database table.
Each record (row) in the table contains values for these fields.
Field types are assigned with classes, which determine the type of record that is used to store the data in the database.
Field types can 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
Returns a human-readable string representation of the object
The string represents individual records
get_absolute_url()
Returns a URL for displaying individual model records
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 to represent 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 related model class is declared as the first unnamed parameter
using either the model class
or a string containing the name of the related model
Use the name of the model as a string if the associated class has not yet been defined in this file before it is referenced
null=True allows the database to store a Null value if no author is selected
on_delete=models.RESTRICT prevents the book’s associated author being deleted if it is referenced by any book
Warning:
By default on_delete=models.CASCADE
If the author was deleted, this book would be deleted too! – – Use RESTRICT or PROTECT to prevent the author being deleted while any book uses it
or SET_NULL to set the book’s author to Null if the record is deleted
__str__() uses the book’s title field to represent a Book record
get_absolute_url() returns a URL used to access a detail 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 migrate
