Django Model Best Practices

# Django Model Best Practices
=====================================

## Core Principles
-------------------

1. **Separation of Concerns**: Keep models focused on data representation and avoid mixing business logic.
2. **Readability**: Use clear and concise naming conventions, and follow PEP 8 guidelines.
3. **Reusability**: Design models to be reusable across the application.

## Code Style Guidelines
------------------------

### Naming Conventions

* **Model Names**: Use singular nouns (e.g., `Book`, not `Books`).
* **Field Names**: Use lowercase with words separated by underscores (e.g., `first_name`).
* **Related Field Names**: Use the related model name followed by `_id` (e.g., `author_id`).

### Typing

* **Use Type Hints**: Include type hints for model fields and methods.
* **Field Types**: Use Django's built-in field types (e.g., `models.CharField`, `models.IntegerField`).

### Model Structure

* **Use `Meta` Class**: Define a `Meta` class to specify model metadata (e.g., `verbose_name`, `ordering`).
* **Field Ordering**: Define fields in the following order:
	1. Primary key field (`id`)
	2. Foreign key fields
	3. Other fields

## Best Practices
------------------

### Model Definition

* **Use `models.Model` as Base Class**: Inherit from `models.Model` for all models.
* **Define `__str__` Method**: Implement a `__str__` method to provide a human-readable representation of the model instance.
* **Use `verbose_name` and `verbose_name_plural`**: Specify verbose names for models and their plural forms.

### Field Definitions

* **Use `null` and `blank` Correctly**: Set `null=True` for fields that can be null in the database, and `blank=True` for fields that can be blank in forms.
* **Specify Field Lengths**: Define lengths for character fields (e.g., `models.CharField(max_length=255)`).

### Relationships

* **Use `ForeignKey` for Many-to-One Relationships**: Establish many-to-one relationships using `ForeignKey`.
* **Use `ManyToManyField` for Many-to-Many Relationships**: Establish many-to-many relationships using `ManyToManyField`.

## Common Pitfalls to Avoid
---------------------------

* **Avoid Using `null=True` for Foreign Keys**: Instead, use `blank=True` and set a default value or use a `ForeignKey` with `on_delete=models.SET_NULL`.
* **Don't Override `save` Method**: Avoid overriding the `save` method; instead, use signals or custom methods to perform additional logic.
* **Don't Use `select_related` and `prefetch_related` Excessively**: Use these methods judiciously to avoid unnecessary database queries.

Example of a well-structured Django model:
```python
from django.db import models

class Book(models.Model):
    title = models.CharField(max_length=255)
    author = models.ForeignKey('Author', on_delete=models.CASCADE)
    publication_date = models.DateField()

    class Meta:
        verbose_name = 'book'
        verbose_name_plural = 'books'
        ordering = ['title']

    def __str__(self):
        return self.title
```