Rails 8.0: `NoMethodError: undefined method 'right' for an instance of String` in `select_for_count`

[StephenVNelson]

Mobility Rails 8.0 Compatibility Issue - GitHub Issue Template

Environment

• Mobility version: 1.3.2
• Rails version: 8.0.3
• Ruby version: 3.3.3
• Database: PostgreSQL

Description

The Rails 8.0 compatibility fix introduced in Mobility 1.3.2 (referencing PR #654 and #655) contains an incomplete implementation that causes a NoMethodError when select_values contains plain strings instead of Arel nodes.

Error

NoMethodError: undefined method `right' for an instance of String

Root Cause

The select_for_count method in lib/mobility/plugins/active_record/query.rb assumes all values in select_values respond to .right and .left methods (i.e., are Arel::Nodes::As objects):

if select_values.any? { |value| value.right.start_with?(ATTRIBUTE_ALIAS_PREFIX) }
  filtered_select_values = select_values.map do |value|
    value.right.start_with?(ATTRIBUTE_ALIAS_PREFIX) ? value.left : value
  end
  # ...
end

However, as noted in PR #654's comments, Rails 8.0 changed behavior (commit: rails/rails@ba468db) such that select_values can contain plain strings like "id" instead of Arel nodes.

Steps to Reproduce

1. Upgrade to Rails 8.0.x
2. Use Mobility 1.3.2
3. Perform any query that triggers select_for_count with a simple select value
4. Observe the NoMethodError on .right

Proposed Fix

Check if values respond to .right before calling it:

[StephenVNelson]

Adding a note that a couple of devs have mentioned this issue in this thread

@bvogel & @KonstantinReido

[StephenVNelson]

I added a PR for a possible fix. Interested in your thoughts @jukra and @shioyama (this is my first contribution to open source so forgive me if I've made obvious blunders)