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Samuel Lubliner
Samuel Lubliner

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Django: Using models

From: MDN Web Docs


  • 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


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


  • 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





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


  • 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:

Making queries:


Defining the LocalLibrary Models

At the top of /django-locallibrary-tutorial/catalog/, 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


  • 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 makemigrations
python3 migrate
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