{"id":1341,"date":"2022-03-07T13:48:07","date_gmt":"2022-03-07T13:48:07","guid":{"rendered":"https:\/\/zerotobyte.com\/?p=1341"},"modified":"2022-03-07T15:33:10","modified_gmt":"2022-03-07T15:33:10","slug":"how-to-use-django-select-related-and-prefetch-related","status":"publish","type":"post","link":"https:\/\/zerotobyte.com\/how-to-use-django-select-related-and-prefetch-related\/","title":{"rendered":"How to use Django select_related and prefetch_related?"},"content":{"rendered":"\n<p><strong>Django ORM (object-relational mapping) is a tool that helps us make queries on relational databases using Python code that is later translated into SQL. <\/strong>ORM provides methods that are used to optimize queries on the database.\u00a0 Django <code>select_related<\/code> and <code>prefetch_related<\/code> are methods used to join tables using Python or SQL. Their optimization purpose is to reduce the number of queries.<\/p>\n\n\n\n<ul><li><code>select_related<\/code> &#8211; creates <strong>INNER JOIN<\/strong> between tables.<\/li><li><code>prefetch_related<\/code> &#8211; connects tables using Python (but queries both tables).<\/li><\/ul>\n\n\n\n<p>In all defined sections, we\u2019ll use models:<\/p>\n\n\n\n<pre class=\"EnlighterJSRAW\" data-enlighter-language=\"python\" data-enlighter-theme=\"\" data-enlighter-highlight=\"\" data-enlighter-linenumbers=\"\" data-enlighter-lineoffset=\"\" data-enlighter-title=\"\" data-enlighter-group=\"\">from django.db import models\n\nclass Author(models.Model):\n   first_name = models.CharField(max_length=512)\n   last_name = models.CharField(max_length=512)\n\nclass Book(models.Model):\n   title = models.CharField(max_length=512)\n   author = models.ForeignKey(Author, on_delete=models.CASCADE)<\/pre>\n\n\n\n<p>And our database is pre-populated with data.<\/p>\n\n\n\n<p><strong>Author table<\/strong><\/p>\n\n\n\n<figure class=\"wp-block-table\"><table><tbody><tr><td><strong>id<\/strong><\/td><td><strong>first_name<\/strong><\/td><td><strong>last_name<\/strong><\/td><\/tr><tr><td>1<\/td><td>William<\/td><td>Shakespeare<\/td><\/tr><tr><td>2<\/td><td>Miguel<\/td><td>de Cervantes<\/td><\/tr><\/tbody><\/table><\/figure>\n\n\n\n<p><strong>Book table<\/strong><\/p>\n\n\n\n<figure class=\"wp-block-table\"><table><tbody><tr><td><strong>id<\/strong><\/td><td><strong>title<\/strong><\/td><td><strong>author_id<\/strong><\/td><\/tr><tr><td>1<\/td><td>Hamlet<\/td><td>1<\/td><\/tr><tr><td>2<\/td><td>Romeo and Juliet<\/td><td>1<\/td><\/tr><tr><td>3<\/td><td>Macbeth<\/td><td>1<\/td><\/tr><tr><td>4<\/td><td>Don Quixote<\/td><td>2<\/td><\/tr><\/tbody><\/table><\/figure>\n\n\n\n<h2 class=\"wp-block-heading\">Django select_related<\/h2>\n\n\n\n<p><strong>The <code>select_related<\/code> in Django is a method used to <code>INNER JOIN<\/code> connected tables. This implies retrieving all columns from the specified model related to the model on which we are executing this method.<\/strong><\/p>\n\n\n\n<p>It might sound a bit abstract, but it will become clearer with examples.<\/p>\n\n\n\n<p>Let&#8217;s take a look at an example without using <code>select_related<\/code> first. In examples, we&#8217;ll show how Django ORM code translates into SQL and how many queries are needed to get the result.<\/p>\n\n\n\n<p>Plain lookup method:<\/p>\n\n\n\n<pre class=\"EnlighterJSRAW\" data-enlighter-language=\"python\" data-enlighter-theme=\"\" data-enlighter-highlight=\"\" data-enlighter-linenumbers=\"\" data-enlighter-lineoffset=\"\" data-enlighter-title=\"\" data-enlighter-group=\"\">>>> Book.objects.get(id=1).author.first_name\n\n\u2018William\u2019<\/pre>\n\n\n\n<p>This translates into two SQL queries:<\/p>\n\n\n\n<pre class=\"EnlighterJSRAW\" data-enlighter-language=\"generic\" data-enlighter-theme=\"\" data-enlighter-highlight=\"\" data-enlighter-linenumbers=\"\" data-enlighter-lineoffset=\"\" data-enlighter-title=\"\" data-enlighter-group=\"\">SELECT \"book\".\"id\", \"book\".\"title\", \"book\".\"author_id\" FROM \"book\" WHERE \"book\".\"id\" = 1\n\nSELECT \"author\".\"id\", \"author\".\"first_name\", \"author\".\"last_name\" FROM \"author\" WHERE \"author\".\"id\" = 1<\/pre>\n\n\n\n<p>First SQL query finds the author_id connected to book with id=1.<br>The second SQL query finds column values to the author with id=1 (found from the first query).<\/p>\n\n\n\n<p>Let\u2019s now take a look at the Django <code>select_related<\/code> method example.<\/p>\n\n\n\n<pre class=\"EnlighterJSRAW\" data-enlighter-language=\"python\" data-enlighter-theme=\"\" data-enlighter-highlight=\"\" data-enlighter-linenumbers=\"\" data-enlighter-lineoffset=\"\" data-enlighter-title=\"\" data-enlighter-group=\"\">>>> Book.objects.select_related('author').get(id=1).author.first_name\n\n\u2018William\u2019<\/pre>\n\n\n\n<pre class=\"EnlighterJSRAW\" data-enlighter-language=\"generic\" data-enlighter-theme=\"\" data-enlighter-highlight=\"\" data-enlighter-linenumbers=\"\" data-enlighter-lineoffset=\"\" data-enlighter-title=\"\" data-enlighter-group=\"\">SELECT \"book\".\"id\", \"book\".\"title\", \"book\".\"author_id\", \"author\".\"id\", \"author\".\"first_name\", \"author\".\"last_name\" FROM \"book\" INNER JOIN \"author\" ON (\"book\".\"author_id\" = \"author\".\"id\") WHERE \"book\".\"id\" = 1<\/pre>\n\n\n\n<p>We get the same result as the plain method, but we spared one database query. We can see that the Django <code>select_related<\/code> method uses <code>INNER JOIN<\/code> to connect tables by matching ids from both tables.<\/p>\n\n\n\n<figure class=\"wp-block-image size-full\"><img loading=\"lazy\" decoding=\"async\" width=\"721\" height=\"491\" src=\"https:\/\/zerotobyte.com\/wp-content\/uploads\/2022\/03\/django-select-related.webp\" alt=\"Django select_related INNER JOIN example.\" class=\"wp-image-1342\" srcset=\"https:\/\/zerotobyte.com\/wp-content\/uploads\/2022\/03\/django-select-related.webp 721w, https:\/\/zerotobyte.com\/wp-content\/uploads\/2022\/03\/django-select-related-300x204.webp 300w\" sizes=\"(max-width: 721px) 100vw, 721px\" \/><\/figure>\n\n\n\n<p>Now, we can understand the whole logic behind <code>INNER JOIN<\/code>. First, <code>INNER JOIN<\/code> connects two tables by matching their foreign key ids and connected table id. Then, it appends corresponding table columns to create a new table that can be accessed without having to \u201chit\u201d the database again.<\/p>\n\n\n\n<p>Also, <code>select_related<\/code> can be used in loops. Again, we&#8217;ll check both plain and <code>select_related<\/code> methods. In these examples, the goal is to fetch the first_name of all the books in our database defined in the first section.<\/p>\n\n\n\n<p>Plain lookup method:<\/p>\n\n\n\n<pre class=\"EnlighterJSRAW\" data-enlighter-language=\"python\" data-enlighter-theme=\"\" data-enlighter-highlight=\"\" data-enlighter-linenumbers=\"\" data-enlighter-lineoffset=\"\" data-enlighter-title=\"\" data-enlighter-group=\"\">books = Book.objects.all() \n# &lt;QuerySet [&lt;Book: Book object (1)>, &lt;Book: Book object (2)>, &lt;Book: Book object (3)>, &lt;Book: Book object (4)>]>\n\nfor book in books: \n    print(book.author.first_name)<\/pre>\n\n\n\n<p>Since we have four books in our database, this queries the database five times, the first time fetching all books, and four times to get authors first_name every time we iterate over the book.<\/p>\n\n\n\n<pre class=\"EnlighterJSRAW\" data-enlighter-language=\"generic\" data-enlighter-theme=\"\" data-enlighter-highlight=\"\" data-enlighter-linenumbers=\"\" data-enlighter-lineoffset=\"\" data-enlighter-title=\"\" data-enlighter-group=\"\">SELECT \"book\".\"id\", \"book\".\"title\", \"book\".\"author_id\" FROM \"book\";\n\nSELECT \"author\".\"id\", \"author\".\"first_name\", \"author\".\"last_name\" FROM \"author\" WHERE \"author\".\"id\" = 1;\nSELECT \"author\".\"id\", \"author\".\"first_name\", \"author\".\"last_name\" FROM \"author\" WHERE \"author\".\"id\" = 1;\nSELECT \"author\".\"id\", \"author\".\"first_name\", \"author\".\"last_name\" FROM \"author\" WHERE \"author\".\"id\" = 1;\nSELECT \"author\".\"id\", \"author\".\"first_name\", \"author\".\"last_name\" FROM \"author\" WHERE \"author\".\"id\" = 2;<\/pre>\n\n\n\n<p>Let\u2019s take a look at <code>select_related<\/code> method.<\/p>\n\n\n\n<pre class=\"EnlighterJSRAW\" data-enlighter-language=\"python\" data-enlighter-theme=\"\" data-enlighter-highlight=\"\" data-enlighter-linenumbers=\"\" data-enlighter-lineoffset=\"\" data-enlighter-title=\"\" data-enlighter-group=\"\">books = Book.objects.select_related('author')\n# &lt;QuerySet [&lt;Book: Book object (1)>, &lt;Book: Book object (2)>, &lt;Book: Book object (3)>, &lt;Book: Book object (4)>]>\n\nfor book in books: \n    print(book.author.first_name)\n\n# William\n# William\n# William\n# Miguel<\/pre>\n\n\n\n<p>This creates only one query to the database, since we join books table with authors table, we already have first_name and no additional queries are required:<\/p>\n\n\n\n<pre class=\"EnlighterJSRAW\" data-enlighter-language=\"generic\" data-enlighter-theme=\"\" data-enlighter-highlight=\"\" data-enlighter-linenumbers=\"\" data-enlighter-lineoffset=\"\" data-enlighter-title=\"\" data-enlighter-group=\"\">SELECT \"book\".\"id\", \"book\".\"title\", \"book\".\"author_id\", \"author\".\"id\", \"author\".\"first_name\", \"author\".\"last_name\" FROM \"book\" INNER JOIN \"author\" ON (\"book\".\"author_id\" = \"author\".\"id\") <\/pre>\n\n\n\n<p>Already here, you can see how many database calls we can save using Django <code>select_related<\/code> method. Only by changing one line of code, we save four database calls since we have four books in our database. Imagine if we had thousands or even millions of records in our database!<\/p>\n\n\n\n<h2 class=\"wp-block-heading\">Django prefetch_related<\/h2>\n\n\n\n<p>The Django <code>prefetch_related<\/code> method as <code>select_related<\/code> has an optimization purpose, and both achieve the same purpose but in different ways<strong>. The <code>prefetch_related<\/code> method queries both tables and connects them using Python (not SQL), so basically, its efficiency is somewhat lower than the <code>select_related<\/code> method.<\/strong> <strong>But <code>prefetch_related<\/code> has some other qualities.<\/strong><\/p>\n\n\n\n<p>Let&#8217;s first check the differences between plain lookups and the <code>prefetch_related<\/code> methods in Django.<\/p>\n\n\n\n<p>As we&#8217;ve already seen, the plain lookup method translates into two SQL queries. Can we get a better result using the <code>prefetch_related<\/code> method?<\/p>\n\n\n\n<pre class=\"EnlighterJSRAW\" data-enlighter-language=\"python\" data-enlighter-theme=\"\" data-enlighter-highlight=\"\" data-enlighter-linenumbers=\"\" data-enlighter-lineoffset=\"\" data-enlighter-title=\"\" data-enlighter-group=\"\">>>> Book.objects.prefetch_related('author').get(id=1).author.first_name\n\n'William'<\/pre>\n\n\n\n<p>This is also translated into two SQL queries, as seen below:<\/p>\n\n\n\n<pre class=\"EnlighterJSRAW\" data-enlighter-language=\"generic\" data-enlighter-theme=\"\" data-enlighter-highlight=\"\" data-enlighter-linenumbers=\"\" data-enlighter-lineoffset=\"\" data-enlighter-title=\"\" data-enlighter-group=\"\">SELECT \"book\".\"id\", \"book\".\"title\", \"book\".\"author_id\" FROM \"book\" WHERE \"book\".\"id\" = 1;\n\nSELECT \"author\".\"id\", \"author\".\"first_name\", \"author\".\"last_name\" FROM \"author\" WHERE \"author\".\"id\" IN (1);<\/pre>\n\n\n\n<p>As for now, we can\u2019t see optimization potential, but let\u2019s try other examples.<\/p>\n\n\n\n<p>Now, let\u2019s check optimizing in loops.<\/p>\n\n\n\n<pre class=\"EnlighterJSRAW\" data-enlighter-language=\"python\" data-enlighter-theme=\"\" data-enlighter-highlight=\"\" data-enlighter-linenumbers=\"\" data-enlighter-lineoffset=\"\" data-enlighter-title=\"\" data-enlighter-group=\"\">books = Book.objects.prefetch_related('author')\n# &lt;QuerySet [&lt;Book: Book object (1)>, &lt;Book: Book object (2)>, &lt;Book: Book object (3)>, &lt;Book: Book object (4)>]>\n\nfor book in books: \n    print(book.author.first_name)\n\n# William\n# William\n# William\n# Miguel<\/pre>\n\n\n\n<p>Our output of authors remains the same.<br>Let\u2019s take a quick look at SQL queries:<\/p>\n\n\n\n<pre class=\"EnlighterJSRAW\" data-enlighter-language=\"generic\" data-enlighter-theme=\"\" data-enlighter-highlight=\"\" data-enlighter-linenumbers=\"\" data-enlighter-lineoffset=\"\" data-enlighter-title=\"\" data-enlighter-group=\"\">SELECT \"book\".\"id\", \"book\".\"title\", \"book\".\"author_id\" FROM \"book\";\n\nSELECT \"author\".\"id\", \"author\".\"first_name\", \"author\".\"last_name\" FROM \"author\" WHERE \"author\".\"id\" IN (1, 2);<\/pre>\n\n\n\n<p>As you can see, no additional queries are made. Instead, we have two queries that fetch both tables. And Python connects these tables making &#8220;join in Python&#8221; (<code>INNER JOIN <\/code>we can&#8217;t see).<\/p>\n\n\n\n<p>That, of course, has its pros and cons. Efficiency-wise, <code>select_related<\/code> is faster and more efficient, but <code>select_related<\/code> could not be used for <code>ManyToManyField<\/code>. And that&#8217;s the main <code>prefetch_related<\/code> advantage.<\/p>\n\n\n\n<pre class=\"wp-block-verse\"><strong>NOTE<\/strong> -  ManyToMany relationships should not be handled using SQL because many performance issues could appear when dealing with large tables. That's why the prefetch_related method joins tables inside Python avoiding making large SQL joins.<\/pre>\n\n\n\n<p>We will now change models to demonstrate how to use <code>prefetch_related<\/code> in <code>ManyToManyField<\/code> and why it is used that way.<\/p>\n\n\n\n<pre class=\"EnlighterJSRAW\" data-enlighter-language=\"python\" data-enlighter-theme=\"\" data-enlighter-highlight=\"\" data-enlighter-linenumbers=\"\" data-enlighter-lineoffset=\"\" data-enlighter-title=\"\" data-enlighter-group=\"\">from django.db import models\n\n\nclass Author(models.Model):\n   first_name = models.CharField(max_length=512)\n   last_name = models.CharField(max_length=512)\n\n\nclass Book(models.Model):\n   title = models.CharField(max_length=512)\n   authors = models.ManyToManyField(Author)\n\n   def __str__(self):\n       return \"%s (%s)\" % (\n           self.title,\n           \", \".join(author.last_name for author in self.authors.all()),\n       )<\/pre>\n\n\n\n<p>We now have an additional table that connects authors and books in the database. Many To Many relationships are explained in this <a href=\"https:\/\/zerotobyte.com\/django-many-to-many-relationship-explained\/\" target=\"_blank\" rel=\"noreferrer noopener\">article<\/a>.<\/p>\n\n\n\n<p><strong>Author table<\/strong><\/p>\n\n\n\n<figure class=\"wp-block-table\"><table><tbody><tr><td><strong>id<\/strong><\/td><td><strong>first_name<\/strong><\/td><td><strong>last_name<\/strong><\/td><\/tr><tr><td>1<\/td><td>Stephen<\/td><td>King<\/td><\/tr><tr><td>2<\/td><td>Peter<\/td><td>Straub<\/td><\/tr><\/tbody><\/table><\/figure>\n\n\n\n<p><strong>Book table<\/strong><\/p>\n\n\n\n<figure class=\"wp-block-table\"><table><tbody><tr><td><strong>id<\/strong><\/td><td><strong>title<\/strong><\/td><\/tr><tr><td>1<\/td><td>The Talisman<\/td><\/tr><tr><td>2<\/td><td>Black House<\/td><\/tr><\/tbody><\/table><\/figure>\n\n\n\n<p><strong>Book_author table<\/strong><\/p>\n\n\n\n<figure class=\"wp-block-table\"><table><tbody><tr><td><strong>id<\/strong><\/td><td><strong>book_id<\/strong><\/td><td><strong>author_id<\/strong><\/td><\/tr><tr><td>1<\/td><td>1<\/td><td>1<\/td><\/tr><tr><td>2<\/td><td>1<\/td><td>2<\/td><\/tr><tr><td>3<\/td><td>2<\/td><td>1<\/td><\/tr><tr><td>4<\/td><td>2<\/td><td>2<\/td><\/tr><\/tbody><\/table><\/figure>\n\n\n\n<p>We&#8217;ve deliberately defined <code>__str__<\/code> method connected to the ManyToMany table for examples.<\/p>\n\n\n\n<pre class=\"EnlighterJSRAW\" data-enlighter-language=\"python\" data-enlighter-theme=\"\" data-enlighter-highlight=\"\" data-enlighter-linenumbers=\"\" data-enlighter-lineoffset=\"\" data-enlighter-title=\"\" data-enlighter-group=\"\">>>> Book.objects.all()\n\n&lt;QuerySet [&lt;Book: The Talisman (King, Straub)>, &lt;Book: Black House (King, Straub)>]><\/pre>\n\n\n\n<p>For only two books with same authors, we get three queries:<\/p>\n\n\n\n<pre class=\"EnlighterJSRAW\" data-enlighter-language=\"generic\" data-enlighter-theme=\"\" data-enlighter-highlight=\"\" data-enlighter-linenumbers=\"\" data-enlighter-lineoffset=\"\" data-enlighter-title=\"\" data-enlighter-group=\"\">SELECT \"book\".\"id\", \"book\".\"title\" FROM \"book\u201d;\n\nSELECT \"author\".\"id\", \"author\".\"first_name\", \"author\".\"last_name\" FROM \"author\" INNER JOIN \"book_author\" ON (\"author\".\"id\" = \"book_author\".\"author_id\") WHERE \"book_author\".\"book_id\" = 5; \n\nSELECT \"author\".\"id\", \"author\".\"first_name\", \"author\".\"last_name\" FROM \"author\" INNER JOIN \"book_author\" ON (\"author\".\"id\" = \"book_author\".\"author_id\") WHERE \"book_author\".\"book_id\" = 6;<\/pre>\n\n\n\n<p>Because we are calling <code>authors.all()<\/code> every time for every book we get, this can become very unoptimized, very soon. That&#8217;s why <code>prefetch_related<\/code> exists! So we can simply do this:<\/p>\n\n\n\n<pre class=\"EnlighterJSRAW\" data-enlighter-language=\"python\" data-enlighter-theme=\"\" data-enlighter-highlight=\"\" data-enlighter-linenumbers=\"\" data-enlighter-lineoffset=\"\" data-enlighter-title=\"\" data-enlighter-group=\"\">>>> Book.objects.all().prefetch_related('author')\n\n&lt;QuerySet [&lt;Book: The Talisman (King, Straub)>, &lt;Book: Black House (King, Straub)>]><\/pre>\n\n\n\n<pre class=\"EnlighterJSRAW\" data-enlighter-language=\"generic\" data-enlighter-theme=\"\" data-enlighter-highlight=\"\" data-enlighter-linenumbers=\"\" data-enlighter-lineoffset=\"\" data-enlighter-title=\"\" data-enlighter-group=\"\">SELECT \"book\".\"id\", \"book\".\"title\" FROM \"book\u201d;\n\nSELECT (\"book_author\".\"book_id\") AS \"_prefetch_related_val_book_id\", \"author\".\"id\", \"author\".\"first_name\", \"author\".\"last_name\" FROM \"author\" INNER JOIN \"book_author\" ON (\"author\".\"id\" = \"book_author\".\"author_id\") WHERE \"book_author\".\"book_id\" IN (5, 6);<\/pre>\n\n\n\n<p>Now, we have only two database calls, and no matter the amount of connections in Many To Many relationship, we\u2019ll always have only two calls.<\/p>\n\n\n\n<p>Instead of having to call <code>authors.objects.all()<\/code>, it will find them in a QuerySet cache that was made using the Django <code>prefetch_related<\/code> method.<\/p>\n\n\n\n<h2 class=\"wp-block-heading\">Conclusion<\/h2>\n\n\n\n<p>Database calls can be costly operations not only in Django but in every web framework, so it&#8217;s important to optimize as much as possible with all the tools these frameworks provide. Sparing one database call doesn&#8217;t seem much. Still, when performing queries on complex database tables with multiple optimizing potentials, we can save seconds and even minutes using the methods described in the article.<\/p>\n\n\n\n<p>Use the <code>select_related<\/code> method when using One To One and One To Many (Many To One) relationships. This is because the <code>select_related<\/code> method creates <code>INNER JOIN<\/code> between tables, and it&#8217;s the most efficient way to fetch data from the referenced table. On the other hand, You should use the <code>prefetch_related<\/code> method with Many To Many relationships as it makes JOIN inside Python.<\/p>\n","protected":false},"excerpt":{"rendered":"<p>Django ORM (object-relational mapping) is a tool that helps us make queries on relational databases using Python code that is &#8230; <\/p>\n<p class=\"read-more-container\"><a title=\"How to use Django select_related and prefetch_related?\" class=\"read-more button\" href=\"https:\/\/zerotobyte.com\/how-to-use-django-select-related-and-prefetch-related\/#more-1341\" aria-label=\"More on How to use Django select_related and prefetch_related?\">Read more<\/a><\/p>\n","protected":false},"author":2,"featured_media":0,"comment_status":"closed","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[7],"tags":[8],"yoast_head":"<!-- This site is optimized with the Yoast SEO plugin v20.4 - https:\/\/yoast.com\/wordpress\/plugins\/seo\/ -->\n<title>How to use Django select_related and prefetch_related? - ZeroToByte<\/title>\n<meta name=\"description\" content=\"Are your queries becoming slow as the application grows? 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