Django Migrations Example Explanation

This walkthrough demonstrates creating models, applying migrations, adding fields, creating new models, and basic data operations using Django's ORM.

Overview

The example demonstrates how to:

1. Set up a Django project and app
2. Define models using Django's ORM
3. Create and apply database migrations
4. Add new fields to existing models
5. Create new models and corresponding migrations

Project Structure

django_migrations_example/
├── django_migrations_example/
│   ├── __init__.py
│   ├── settings.py
│   ├── urls.py
│   └── wsgi.py
├── data_app/
│   ├── migrations/
│   │   ├── __init__.py
│   │   ├── 0001_initial.py
│   │   ├── 0002_rawdata_data_quality.py
│   │   └── 0003_processeddata.py
│   ├── __init__.py
│   ├── admin.py
│   ├── apps.py
│   ├── models.py
│   ├── tests.py
│   └── views.py
└── manage.py

Code Explanation

Model Definition (data_app/models.py)

from django.db import models

class DataSource(models.Model):
    name = models.CharField(max_length=100)
    url = models.URLField(max_length=500)
    type = models.CharField(max_length=50)

    def __str__(self):
        return self.name

class RawData(models.Model):
    source = models.ForeignKey(DataSource, on_delete=models.CASCADE)
    timestamp = models.DateTimeField()
    content = models.TextField()
    data_quality = models.FloatField(null=True)

    def __str__(self):
        return f"Data from {self.source.name} at {self.timestamp}"

class ProcessedData(models.Model):
    raw_data = models.ForeignKey(RawData, on_delete=models.CASCADE)
    processed_content = models.TextField()
    processing_timestamp = models.DateTimeField(auto_now_add=True)

    def __str__(self):
        return f"Processed data for {self.raw_data}"

This file defines three models: DataSource, RawData, and ProcessedData, representing a basic data processing system.

Running the Example

Here's a step-by-step guide to set up and run the Django Migrations example:

1. Set up a new Django project:

   pip install django
   django-admin startproject django_migrations_example
   cd django_migrations_example
   

2. Create a new app:

   python manage.py startapp data_app
   

3. Add 'data_app' to INSTALLED_APPS in django_migrations_example/settings.py:

   INSTALLED_APPS = [
       # ...
       'data_app',
   ]

4. Create data_app/models.py with the content:

   from django.db import models
   
   class DataSource(models.Model):
       name = models.CharField(max_length=100)
       url = models.URLField(max_length=500)
       type = models.CharField(max_length=50)
   
       def __str__(self):
           return self.name
   
   class RawData(models.Model):
       source = models.ForeignKey(DataSource, on_delete=models.CASCADE)
       timestamp = models.DateTimeField()
       content = models.TextField()
   
       def __str__(self):
           return f"Data from {self.source.name} at {self.timestamp}"

5. Create and apply the initial migrations:

   python manage.py makemigrations
   python manage.py migrate
   

6. Add a new field to RawData in data_app/models.py:

   class RawData(models.Model):
       # ... existing fields ...
       data_quality = models.FloatField(null=True)

7. Create and apply the new migration:

   python manage.py makemigrations
   python manage.py migrate
   

8. Add a new model to data_app/models.py:

   class ProcessedData(models.Model):
       raw_data = models.ForeignKey(RawData, on_delete=models.CASCADE)
       processed_content = models.TextField()
       processing_timestamp = models.DateTimeField(auto_now_add=True)
   
       def __str__(self):
           return f"Processed data for {self.raw_data}"

9. Create and apply the migration for the new model:

   python manage.py makemigrations
   python manage.py migrate
   

10. To see the SQL that Django would run for these migrations:

    python manage.py sqlmigrate data_app 0001_initial
    python manage.py sqlmigrate data_app 0002_rawdata_data_quality
    python manage.py sqlmigrate data_app 0003_processeddata
    

11. To view migration history:

    python manage.py showmigrations
    

12. To use the models in Django shell:

    python manage.py shell
    
    # In the shell:
    from data_app.models import DataSource, RawData, ProcessedData
    from django.utils import timezone
    
    # Create a DataSource
    source = DataSource.objects.create(name="Example Source", url="https://example.com", type="API")
    
    # Create a RawData instance
    raw_data = RawData.objects.create(source=source, timestamp=timezone.now(), content="Sample raw data", data_quality=0.95)
    
    # Create a ProcessedData instance
    processed_data = ProcessedData.objects.create(raw_data=raw_data, processed_content="Processed: Sample raw data")
    
    # Query the data
    print(DataSource.objects.all())
    print(RawData.objects.all())
    print(ProcessedData.objects.all())
    

This example showcases how Django's migration system can manage database schema changes over time, allowing for version control of your database structure and smooth collaboration in team environments. It integrates seamlessly with Django's ORM, making it easy to define and evolve your data models as your project grows.