Code-Memo

Complex Lookups

Using Q Objects

Q objects provide a way to build complex queries by combining multiple conditions with logical operators (& for AND, | for OR, and ~ for NOT). Q objects can be used in methods like filter(), exclude(), and get().

Suppose you have an Author model and you want to retrieve authors named ‘John’ or born after January 1, 2000:

from django.db.models import Q

# Retrieve authors named 'John' or born after January 1, 2000
authors = Author.objects.filter(Q(name='John') | Q(birth_date__gt='2000-01-01'))

• Q(name='John') | Q(birth_date__gt='2000-01-01') constructs a query to match either condition.

Combining Q Objects

You can combine multiple Q objects using the & and | operators to create more complex queries.

Retrieve authors named ‘John’ and born after January 1, 2000:

# Retrieve authors named 'John' and born after January 1, 2000
authors = Author.objects.filter(Q(name='John') & Q(birth_date__gt='2000-01-01'))

• Q(name='John') & Q(birth_date__gt='2000-01-01') constructs a query to match both conditions.

Negating Q Objects

You can negate Q objects using the ~ operator to exclude specific conditions.

Retrieve authors not named ‘John’:

# Retrieve authors not named 'John'
authors = Author.objects.filter(~Q(name='John'))

• ~Q(name='John') constructs a query to exclude authors named ‘John’.

Complex Lookups with Relationships

Django’s ORM allows you to perform complex lookups involving related models using double underscores (__) to traverse relationships.

Suppose you have Book and Author models where a book has a foreign key to an author. Retrieve books written by authors named ‘John’:

# Retrieve books written by authors named 'John'
books_by_john = Book.objects.filter(author__name='John')

• author__name='John' traverses the author relationship to filter books based on the author’s name.

Using Aggregation with Complex Lookups

Aggregation functions can be combined with complex lookups to perform calculations on subsets of data.

Suppose you have an Order model with a foreign key to a Customer model and you want to calculate the total value of orders for customers from ‘New York’:

from django.db.models import Sum

# Calculate total order value for customers from New York
total_value = Order.objects.filter(customer__city='New York').aggregate(Sum('total_amount'))

• customer__city='New York' filters orders based on the customer’s city.
• aggregate(Sum('total_amount')) calculates the total value of the filtered orders.

Combining Multiple Complex Lookups

You can combine multiple complex lookups to handle intricate querying scenarios.

Suppose you have an Article model with a foreign key to an Author model, and you want to retrieve articles published in 2021 by authors named ‘John’ or who have more than 5 articles:

# Retrieve articles published in 2021 by authors named 'John' or who have more than 5 articles
articles = Article.objects.filter(
    Q(author__name='John') |
    Q(author__articles__count__gt=5),
    published_date__year=2021
)

• Q(author__name='John') | Q(author__articles__count__gt=5) constructs a query to match either author condition.
• published_date__year=2021 filters articles based on the publication year.