Django models relationships

 from django.db import models

# One-to-One Relationship

class Profile(models.Model):

    user = models.OneToOneField(User, on_delete=models.CASCADE, related_name='profile')

    bio = models.TextField()

# Many-to-One Relationship

class Post(models.Model):

    author = models.ForeignKey(User, on_delete=models.CASCADE, related_name='posts')

    title = models.CharField(max_length=200)

# Many-to-Many Relationship

class Course(models.Model):

    students = models.ManyToManyField(Student, related_name='courses')

    title = models.CharField(max_length=200)

# Summary of Relationship Types

# | Relationship Type   | Model Field Type         | Description                          |

# |----------------------|--------------------------|--------------------------------------|

# | One-to-One           | OneToOneField           | Each instance relates to one instance. |

# | Many-to-One          | ForeignKey              | Many instances relate to one instance. |

# | Many-to-Many         | ManyToManyField         | Many instances relate to many instances. |

Here’s a summary of the most common Django model queries you can use in a single window for quick reference:

```python

# 1. Import your model

from myapp.models import Author

# 2. Create a new object

author = Author(first_name="John", last_name="Doe")

author.save()

# 3. Retrieve all objects

authors = Author.objects.all()

# 4. Retrieve a single object by ID

author = Author.objects.get(id=1)

# 5. Filter objects (e.g., by last name)

authors = Author.objects.filter(last_name="Doe")

# 6. Exclude objects

authors = Author.objects.exclude(last_name="Doe")

# 7. Retrieve the first and last object

first_author = Author.objects.first()

last_author = Author.objects.last()

# 8. Order results (ascending or descending)

authors = Author.objects.order_by('last_name')

authors = Author.objects.order_by('-last_name')

# 9. Limit results (get the first 10)

limited_authors = Author.objects.all()[:10]

# 10. Field lookups (e.g., contains, exact match)

authors = Author.objects.filter(first_name__icontains="oh")

author = Author.objects.get(first_name__exact="John")

# 11. Count objects

author_count = Author.objects.count()

# 12. Check if a QuerySet contains data

exists = Author.objects.filter(last_name="Doe").exists()

# 13. Update an object

author = Author.objects.get(id=1)

author.first_name = "Jane"

author.save()

# 14. Delete an object

author = Author.objects.get(id=1)

author.delete()

# 15. Aggregation (e.g., count)

from django.db.models import Count

author_count = Author.objects.aggregate(Count('id'))

# 16. Access related objects (ForeignKey or ManyToManyField)

books = author.book_set.all()  # Reverse ForeignKey relation

books = author.books.all()  # Many-to-Many relation

# 17. Chaining queries (e.g., filter, order, limit)

authors = Author.objects.filter(last_name__icontains="Doe").order_by('first_name')[:5]

```

authors = Author.objects.raw('SELECT * FROM myapp_author WHERE last_name = %s', ['Doe']) 

This is a compact and practical summary of common Django model queries for use in the shell.