DatabaseQueryInLoop.qhelp

<!DOCTYPE qhelp PUBLIC
  "-//Semmle//qhelp//EN"
  "qhelp.dtd">
<qhelp>

<overview>
<p>
When a database query operation, for example a call to a query method in the Rails `ActiveRecord::Relation` class, is executed in a loop, this can lead to a performance issue known as an "n+1 query problem".
The database query will be executed in each iteration of the loop.
Performance can usually be improved by performing a single database query outside of a loop, which retrieves all the required objects in a single operation.
</p>
</overview>
<recommendation>
<p>If possible, pull the database query out of the loop and rewrite it to retrieve all the required objects. This replaces multiple database operations with a single one.
</p>
</recommendation>
<example>
<p>The following (suboptimal) example code queries the <code>User</code> object in each iteration of the loop:</p>
<sample src="examples/straight_loop.rb" />
<p>To improve the performance, we instead query the <code>User</code> object once outside the loop, gathering all necessary information in a single query:</p>
<sample src="examples/preload.rb" />
</example>
</qhelp>