Best Practices for Adding a Boolean Column in Rails: A Case Study

When adding a boolean column to a table in a Rails application, one common question is whether to set a default value for it. Let’s explore why this choice matters, the best practices around it, and how it impacts data handling. Using a real-world example, we’ll take a look at a migration adding a women_only column to a trips table.

Imagine you need to add a women_only attribute to the trips table in a Ruby on Rails application. This attribute is intended to indicate if a specific slot is restricted to women-only access. Here’s how the migration might look:

class AddWomenOnlyToTrips < ActiveRecord::Migration[6.1]
  def change
    add_column :trips, :women_only, :boolean, default: false
  end
end

This migration adds a new column, women_only, with a default value of false.

Why Set a Default Value?

Setting a default value in this migration brings several advantages:

Consistency in Data
By setting a default of false, we ensure that all existing records in the trips table will have a consistent value (false) once the migration runs. Without a default, all pre-existing records would have nil for women_only. This can lead to unexpected behavior if the application code relies on true or false values and does not handle nil.
Reducing Code Complexity
With a default, you avoid additional nil checks. For example, without a default value, code checking women_only might need to account for three states: true, false, and nil. Setting false as a default simplifies this to just true or false.
Performance Considerations
Default values in the database can improve performance in queries by making your records predictable. Querying women_only: true or women_only: false becomes more efficient if all rows have a defined value, compared to filtering records where women_only might be nil.

Considerations for Omitting the Default Value

While setting a default value is generally a good practice, there are scenarios where you might avoid it. If women_only is optional and you want the application to interpret missing values differently than false, it might make sense to leave the default value off and allow nil as an option.

For instance, in some applications, nil could mean “not specified,” while false explicitly means “not women-only.” In this case, the code might differentiate between nil, true, and false:

if slot.women_only.nil?
  # Handle cases where it’s not specified
elsif slot.women_only
  # Handle women-only cases
else
  # Handle cases where it’s explicitly not women-only
end

Adding a boolean column in Rails with a default value of false is a standard, best practice in many applications. This approach prevents null values, simplifies code, and ensures data consistency. However, if your application logic specifically needs to distinguish between nil, true, and false, omitting the default value may be the right choice. By understanding how defaults affect data handling and querying, you can make an informed decision on whether a default fits your use case.