Best practices for ORM in Django framework

Django is a popular Python web framework, and its ORM (Object Relational Mapping) layer is one of its core features. ORM provides developers with a more convenient way to handle interaction with databases in web applications. Due to its powerful functions, ORM is becoming more and more popular among developers. However, ORM complexity can also lead to poor design choices and performance issues. To avoid these problems, this article will introduce the best practices of Django ORM.

1. Use query set (QuerySet) to simplify data query

The query set in Django ORM provides an extremely powerful query method. Querysets should always be used when you need to query data in a database. It is one of the main concepts in ORM. This concept represents a series of objects retrieved from the database. Querysets can have chained calls applied, making it easy to combine multiple filters in code. Using query sets, you don't have to worry about manually writing SQL query statements, and it's very convenient.

The following is an example of a query set:

books = Book.objects.filter(author__name='Jane Doe').exclude(published_at__year=2020).order_by('title ')

The above code is used to query books whose author is named 'Jane Doe' and was not published in 2020, sorted alphabetically by title.

2. Minimize the number of query sets

Every request to the database will cause overhead. Therefore, the number of requests to the database should be minimized.

When you need to query multiple query sets, you can use prefetch_related and select_related to optimize the query set.

3. Use properties or methods in the model to perform calculations

One of the advantages of the ORM framework is that all database operations are converted into object operations. Using this approach can simplify the code and reduce the developer's workload. We can execute properties or methods on the model object to get the results we need.

For example, if you need to add a method to calculate the number of pages in a Book model:

class Book(models.Model):
    ...
    def get_num_pages(self):
        return self.word_count / self.words_per_page

In this way, we can call the get_num_pages() function and do not need to manually calculate the number of pages. . This is much faster than querying the value from the database.

4. Use indexes to improve performance

Indexes are a powerful tool for optimizing query performance. Adding indexes to the model can speed up queries. The index can be a single column index or a compound index. It can prioritize the frequently accessed data columns that are often used in OrderBy, Where, and Join to achieve optimization purposes. The establishment of indexes will increase the maintenance cost of the table, and the long-term maintenance cost should be considered.

For example, if you need to query a certain column of a model (such as book title), you can add an order_with_respect_to statement in the Meta class of the model, for example:

class Book(models.Model):
    title = models.CharField(max_length=255)
    author = models.ForeignKey('Author', on_delete=models.CASCADE)

    class Meta:
        ordering = ('title',)
        indexes = [
            models.Index(fields=['title', 'author'], name='book_title_author_idx')
        ]

The above code implements by title Sort by the author attribute and add an index to this combination.

5. Pay attention to the performance of database operations

When using ORM, we should try to reduce the number of accesses to the database to improve application performance. Here are some ways to reduce access to the database:

a. 通过对数据的预加载和缓存来减少查询

b. 避免在循环中进行多次查询

c. 如果使用的是 PostgreSQL 数据库，在 signals 中加载计算后的数据，可以避免查询重复

Summary:

This article lists some best practice techniques in Django ORM. By carefully following these tips, developers can avoid common mistakes and anti-patterns when using ORMs. When optimizing your ORM for performance and speed with these recommendations, trade-offs and testing are required to avoid the wrong optimizations. With proper ORM design, application performance and reliability can be greatly improved.

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