In which order a Django model is created?
Tags: django
Published: 2024-06-25 09:34:28

# Introduction to Order of Items in Django Models

In Django models, maintaining a consistent and logical order of items is essential for enhancing code readability, maintainability, and functionality. Establishing a standardized sequence within your model classes can help developers and collaborators rapidly comprehend the structure and purpose of the model. The following sequence is commonly adopted to organize the contents of a Django model:

1. **Class Attributes:** 

   These are constants or class-level attributes often used for choices or predefined options. For example, `PRODUCT_CHOICES` defines the permissible values for a product type field.


2. **Field Definitions:**

   The core of the model, field definitions specify the attributes of the model and their types. These include `CharField`, `IntegerField`, `DateField`, etc.


3. **Meta Class:**

   The `Meta` class provides metadata about the model such as ordering, verbose name, indexes, and database table name.

4. **Dunder Methods:**

   Special methods like `__str__()` and `__repr__()` customize the string representation of the model instances. These methods define how objects are displayed and can aid in debugging and logging.

5. **Save Method:**

   The `save` method allows for custom save logic to be executed whenever an object is saved. This can include data validation, modification, or additional side effects such as sending notifications.

6. **Property Methods:**

   Methods decorated with `@property` provide computed attributes that are derived from the model’s fields. These attributes behave like fields but do not require storage in the database.

7. **Other Custom Methods:**

   These are additional methods that implement specific functionalities related to the model. They encapsulate business logic or provide utility functions for the model's data.


### Example with `PRODUCT_CHOICES`

Here's a complete example of a Django model with `PRODUCT_CHOICES` included:

```python
from django.db import models

class Product(models.Model):
    # Class attribute for choices
    PRODUCT_CHOICES = [
        ('EL', 'Electronics'),
        ('CL', 'Clothing'),
        ('FD', 'Food'),
        ('FR', 'Furniture'),
    ]

    # Field definitions
    name = models.CharField(max_length=100)
    category = models.CharField(max_length=2, choices=PRODUCT_CHOICES)
    price = models.DecimalField(max_digits=10, decimal_places=2)
    discount = models.DecimalField(max_digits=5, decimal_places=2, default=0)
    created_at = models.DateTimeField(auto_now_add=True)
    updated_at = models.DateTimeField(auto_now=True)

    # Meta class for model options
    class Meta:
        ordering = ['-created_at']
        verbose_name = 'Product'
        verbose_name_plural = 'Products'

    # __str__ method for readable string representation
    def __str__(self):
        return f"{self.name} ({self.get_category_display()})"

    # save method to customize the saving process
    def save(self, *args, **kwargs):
        # Custom save logic here
        if not self.name:
            self.name = 'Unnamed Product'
        super(Product, self).save(*args, **kwargs)

    # Property methods
    @property
    def discounted_price(self):
        return self.price - self.discount

    @property
    def is_discounted(self):
        return self.discount > 0

    # Other custom methods
    def apply_discount(self, amount):
        self.discount = amount
        self.save()

    def get_price_details(self):
        return f"Price: {self.price}, Discount: {self.discount}, Discounted Price: {self.discounted_price}"
```

### Detailed Explanation

1. **Class Attribute for Choices**: 
   - `PRODUCT_CHOICES` is defined at the top of the class. It’s a list of tuples, where each tuple represents a choice with a key and a human-readable label. This is used by the `category` field to restrict its possible values.
   - Placing `PRODUCT_CHOICES` at the top makes it easy to find and understand the options available for the `category` field.

2. **Field Definitions**: 
   - Fields like `name`, `category`, `price`, `discount`, `created_at`, and `updated_at` are defined next.
   - The `category` field uses the `choices` attribute to restrict its values to those defined in `PRODUCT_CHOICES`.

3. **Meta Class**: 
   - The `Meta` class defines options like default ordering and verbose names for the model.

4. **Dunder Methods**: 
   - The `__str__()` method provides a readable string representation of the model instance, including the category display name using `get_category_display()`, which is a built-in method for fields with choices.

5. **Save Method**: 
   - The `save()` method includes custom logic to ensure that the `name` field is not empty before saving.

6. **Property Methods**: 
   - The `discounted_price` property computes the price after applying the discount.
   - The `is_discounted` property checks if the product has a discount.

7. **Other Custom Methods**: 
   - Methods like `apply_discount` and `get_price_details` provide additional functionality for the model, such as applying discounts and getting detailed pricing information.

### Why This Order?

- **Clarity**: Defining `PRODUCT_CHOICES` at the top ensures that it’s visible and understandable before getting into the specifics of field definitions and other methods.
- **Maintainability**: By following this structured order, your code remains organized, making it easier to manage and modify as your application evolves.
- **Readability**: This order makes the model easy to read, with each section logically flowing into the next.

Following this approach helps maintain a clean and understandable codebase, which is crucial for collaborative development and long-term project maintenance.