In this post, we are going to untangle a commonly misunderstood feature of database behavior and how Ruby on Rails handles it with the
implicit_order_column-attribute. This feature proves to be particularly handy when we want to specify a default column for ordering database records.
Before diving into the essentials of
implicit_order_column, let’s take a moment to demystify a common misconception about database behavior: the order of returned rows when no specific order is specified.
Contrary to popular belief, the order in which rows are returned by a database query is not guaranteed to be in the order of their insertion or based on their primary key (like
id). The actual order of returned rows depends on several factors including the database management system in use, the table structure, and the query’s execution plan. In practice, rows often appear to be returned in the order of their primary key, particularly when using auto-incrementing integer ids. However, this behavior is not something to rely on as it’s not guaranteed!
ActiveRecord, when no order is specified, Rails defaults to ordering by the primary key. The aim is to provide consistent results, though it’s always recommended to explicitly specify an order when it’s required.
Now, let’s switch gears to our main topic of the day: the
implicit_order_column attribute introduced in Rails 6.
class Book < ApplicationRecord self.implicit_order_column = "created_at" end
In this example, whenever you invoke
Book.all or any similar method that fetches multiple records without an explicit order, Rails will now order the Books by the
created_at column instead of the
id. This proves particularly useful for UUID primary keys where ordering by id does not yield meaningful results, as UUIDs are not sequentially created.
It’s important to note that
implicit_order_column only applies to queries without an explicit order clause. If you call
Book.order(:title), Rails will still order the books by the title.
To sum up,
implicit_order_column is a powerful Rails feature that contributes to the flexibility and customization offered by the framework. It simplifies your code by reducing redundancy and ensuring consistency across your application. However, it is critical to understand and remember the underlying database behaviors when dealing with ordered data sets.