Field lookups (exact, contains, gt, lt) in Django

Introduction

Field lookups help you find specific data in your database by matching conditions easily.

You want to find users with a specific exact name.

You need to search for products containing a certain word in their description.

You want to get all orders with amounts greater than a certain value.

You want to list events that happened before a certain date.

Syntax

Django

Model.objects.filter(field__lookup=value)

Use double underscores __ to separate the field name and the lookup type.

Common lookups include exact, contains, gt (greater than), and lt (less than).

Examples

Find users whose name is exactly 'Alice'.

Django

User.objects.filter(name__exact='Alice')

Find products with 'organic' anywhere in their description.

Django

Product.objects.filter(description__contains='organic')

Find orders where the amount is greater than 100.

Django

Order.objects.filter(amount__gt=100)

Find events that happened before January 1, 2024.

Django

Event.objects.filter(date__lt='2024-01-01')

Sample Program

This example defines a Product model and shows how to use field lookups to filter products by exact name, description containing a word, price greater than, and price less than.

Printing the query shows the SQL Django will run.

Django

from django.db import models

class Product(models.Model):
    name = models.CharField(max_length=100)
    description = models.TextField()
    price = models.DecimalField(max_digits=6, decimal_places=2)

# Example queries
products_exact = Product.objects.filter(name__exact='Apple')
products_contains = Product.objects.filter(description__contains='fresh')
products_gt = Product.objects.filter(price__gt=10.00)
products_lt = Product.objects.filter(price__lt=5.00)

# For demonstration, print the querysets' query strings
print(products_exact.query)
print(products_contains.query)
print(products_gt.query)
print(products_lt.query)

Important Notes

Field lookups are case-sensitive by default. Use icontains for case-insensitive contains.

Lookups can be combined with other filters for more complex queries.

Summary

Field lookups let you filter database records by conditions on fields.

Use exact for exact matches, contains for substring matches, gt and lt for greater or less than comparisons.

They make querying your data simple and readable.

Practice

1. Which Django field lookup would you use to find records where a field exactly matches a given value?

easy

A. exact

B. contains

C. gt

D. lt

Solution

1. Step 1: Understand the purpose of each lookup

   The exact lookup matches fields that are exactly equal to the given value. contains checks for substring presence, gt means greater than, and lt means less than.

2. Step 2: Match the requirement to the lookup

   Since the question asks for exact matches, exact is the correct lookup.

3. Final Answer:

   exact -> Option A

4. Quick Check:

   Exact match = exact [OK]

Hint: Exact match uses 'exact' lookup in Django queries [OK]

Common Mistakes:

• Confusing 'contains' with exact match
• Using 'gt' or 'lt' for equality checks
• Assuming 'exact' is default without specifying

2. Which of the following is the correct syntax to filter a Django model Book for titles containing the word 'django'?

easy

A. Book.objects.filter(title__exact='django')

B. Book.objects.filter(title__gt='django')

C. Book.objects.filter(title__contains='django')

D. Book.objects.filter(title__lt='django')

Solution

1. Step 1: Identify the lookup for substring matching

   The contains lookup is used to find records where the field contains the given substring anywhere inside it.

2. Step 2: Check the syntax for filtering

   The correct syntax uses double underscores to specify the lookup: title__contains='django'.

3. Final Answer:

   Book.objects.filter(title__contains='django') -> Option C

4. Quick Check:

   Substring search = contains [OK]

Hint: Use __contains for substring filters in Django queries [OK]

Common Mistakes:

• Using __exact for substring search
• Using __gt or __lt for string matching
• Missing double underscores in lookup

3. Given the model Product with a field price, what will this query return?

Product.objects.filter(price__gt=100)

medium

A. All products with price greater than 100

B. All products with price exactly 100

C. All products with price less than 100

D. All products with price containing '100'

Solution

1. Step 1: Understand the lookup used

   The lookup price__gt=100 means filter products where the price is greater than 100.

2. Step 2: Interpret the query result

   The query returns all products with price values strictly greater than 100, excluding 100 itself.

3. Final Answer:

   All products with price greater than 100 -> Option A

4. Quick Check:

   gt means greater than [OK]

Hint: gt means greater than in Django filters [OK]

Common Mistakes:

• Confusing gt with exact or contains
• Thinking gt includes the value 100
• Assuming it filters less than 100

4. You wrote this Django query but it raises an error:

Entry.objects.filter(date__gt='2023-01-01')

What is the most likely cause?

medium

A. You must use 'exact' lookup for date filtering

B. The field 'date' is not a DateField or DateTimeField

C. The date string format is incorrect for filtering

D. The lookup 'gt' is not valid for date fields

Solution

1. Step 1: Check field type compatibility

   The gt lookup works with fields that support ordering like DateField or DateTimeField. If date is not one of these, the query will error.

2. Step 2: Validate other options

   The lookup gt is valid for date fields, and the string format is acceptable for Django's date parsing. Using exact is not required.

3. Final Answer:

   The field 'date' is not a DateField or DateTimeField -> Option B

4. Quick Check:

   gt requires comparable field type [OK]

Hint: Ensure field type supports lookup before filtering [OK]

Common Mistakes:

• Assuming all fields support gt lookup
• Incorrect date string format causing error
• Using exact lookup unnecessarily

5. You want to find all Order records where the status field contains 'pending' (case insensitive) and the total is less than 500. Which Django query correctly applies these filters?

hard

A. Order.objects.filter(status__contains='pending', total__gt=500)

B. Order.objects.filter(status__contains='pending', total__lt=500)

C. Order.objects.filter(status__exact='pending', total__lt=500)

D. Order.objects.filter(status__icontains='pending', total__lt=500)

Solution

1. Step 1: Choose case-insensitive substring lookup

   To find 'pending' regardless of case, use icontains instead of contains.

2. Step 2: Apply less than filter on total

   The total__lt=500 filters orders with total less than 500.

3. Step 3: Combine filters correctly

   Both filters are passed as keyword arguments to filter() to apply AND logic.

4. Final Answer:

   Order.objects.filter(status__icontains='pending', total__lt=500) -> Option D

5. Quick Check:

   Case-insensitive contains + less than = icontains + lt [OK]

Hint: Use icontains for case-insensitive substring filters [OK]

Common Mistakes:

• Using contains instead of icontains for case insensitivity
• Mixing lt and gt incorrectly
• Using exact instead of contains for substring search