What is Duplication in Django ORM? The Need and Usage of the distinct() Method

1. What is Duplication in Django ORM?

Duplication in Django ORM refers to the case where specific fields or combinations of fields repeat with the same values. This can occur when querying data based on a specific field, even if the primary key (PK) in the database is unique.

Example: Article Table

id	title	author	category
1	Python Basics	Alice	Python
2	Django Intro	Bob	Django
3	Python Basics	Alice	Python
4	Advanced Django	Bob	Django
5	Python Basics	Alice	Python

Looking at the table above, the id values of all records are unique. However, when considering the title field, "Python Basics" appears three times, resulting in duplication.

2. Why is `distinct()` Necessary?

There are frequent situations where you need to eliminate duplicate data based on specific fields when querying data. distinct() operates the same as SQL's SELECT DISTINCT, returning only unique data from the queryset.

Example: Situations Requiring Duplicate Removal

When you want to get a list of unique titles only

Article.objects.values('title').distinct()

Result:

[
    {'title': 'Python Basics'},
    {'title': 'Django Intro'},
    {'title': 'Advanced Django'}
]

When you want to query only unique categories

Article.objects.values('category').distinct()

Result:

[
    {'category': 'Python'},
    {'category': 'Django'}
]

3. When is `distinct()` Useful?

When you need to retrieve unique values for a specific field
When duplicate records are returned due to joins
For data analysis and statistics

4. How to Use `distinct()`

Removing duplicates for all fields

unique_articles = Article.objects.distinct()

Removing duplicates for specific fields

unique_titles = Article.objects.values('title').distinct()

Removing duplicates based on multiple fields

unique_combinations = Article.objects.values('author', 'category').distinct()

Removing duplicates based on specific fields in PostgreSQL
```
unique_authors = Article.objects.distinct('author')
```

5. Precautions When Using `distinct()`

Combination with order_by()
distinct() can conflict when used with order_by().
```
Article.objects.order_by('title').distinct()
```
Database Support
PostgreSQL supports distinct() based on specific fields, but MySQL and SQLite do not.
Performance
distinct() performs duplicate removal at the database level, which can lead to performance issues when dealing with large datasets.

6. Conclusion

Duplication in Django ORM refers to instances where specific fields or combinations of fields appear the same, which can be an issue depending on the query purpose.

The distinct() method helps remove duplicate data and return only unique data. However, it should be used appropriately, keeping in mind compatibility with databases and performance issues.

distinct() is an important tool for data cleaning, analysis, and performance optimization, and can be very useful in Django projects. 😊