Generates a unified CHANGELOG.md for Rails releases.

rails_changes.rb

# This script generates unified CHANGELOG output in Markdown
# of all Rails changes between two versions of Rails.
#
# By default, it shows the latest changes in the specified version only.
#
# You can also pass the "since earlier" version you're interested in.
#
# Usage: ruby rails_changes.rb {latest_version} [since_version]
#
# Examples:
#
#   $ ruby rails_changes.rb 5.2.2.rc1
#
#   Generates a unified CHANGELOG to stdout of changes in 5.2.2.rc1 since
#   previous released version.
#
#
#   $ ruby rails_changes.rb 5.2.2.rc1 5.2.1
#
#   Generates a unified CHANGELOG to stdout of changes in Rails since
#   5.2.1 was released, up to and including the changesin 5.2.2.rc1
#

require 'open-uri'

def generate_unified_changelog(version, since_previous_version)
  changelog, new_version, since_version = read_all_change_logs(version, since_previous_version)

  puts
  puts "## New in Rails #{new_version} since #{since_version}:"
  puts

  changelog.each do |line|
    puts line
  end
end

# Returns a list of Markdown-formatted lines.
def read_all_change_logs(latest_version, since_version)
  unified_changelog = { }
  path_template = "https://raw.githubusercontent.com/rails/rails/v#{latest_version}/*/CHANGELOG.md"

  libs = %w|actioncable actionmailer actionpack actionview activejob activemodel activerecord
            activestorage activesupport railties|

  versions = []
  current_version = nil

  libs.each do |lib|
    open(path_template.sub('*', lib)).readlines.each do |line|
      if line =~ /\# Rails (\S+)/
        current_version = $1
        versions.push(current_version) unless versions.index(current_version)
        unified_changelog[current_version] ||= []
      elsif current_version && line =~ /^\*+\s+(.+)$/
        commit_title = $1
        unified_changelog[current_version].push([lib, commit_title]) unless commit_title =~ /No changes/i
      end
    end
  end
  new_version ||= versions.first
  since_version ||= (version.count > 1 ? versions[1] : current_version)
  new_version_index = versions.index(new_version)
  since_version_index = versions.index(since_version)
  versions = versions.slice(new_version_index, since_version_index-new_version_index)

  changelog = versions.map do |version|
    unified_changelog[version].map do |lib, commit|
      "* [#{version}] [#{lib}] #{commit}"
    end
  end.flatten
  return [changelog, new_version, since_version]
end

if ARGV[0]
  generate_unified_changelog ARGV[0], ARGV[1]
else
  puts "Usage: ruby rails_changes.rb {released_version} [since_version]"
  puts
  puts " Examples:"
  puts
  puts "   $ ruby rails_changes.rb 5.2.2.rc1"
  puts
  puts "   Generates a unified CHANGELOG to stdout of changes in 5.2.2.rc1 since"
  puts "   the previous released version."
  puts
  puts
  puts "   $ ruby rails_changes.rb 5.2.2.rc1 5.2.1"
  puts
  puts "   Generates a unified CHANGELOG to stdout of changes in Rails since"
  puts "   5.2.1 was released, up to and including the changes in 5.2.2.rc1."
  puts
end

Note

The script above works only within a single major Rails version (changelogs for different major Rails versions are separate files)

This script worked for me on Ruby 3.2 after making a couple adjustments:

• replace open to URI.open (seems like Kernel.open used to support URLs but it doesn't now?)
• replace version.count with versions.count (seems like a typo)

I also adjusted versions.slice to versions.slice(new_version_index, since_version_index-new_version_index + 1) to make it include the changes from the oldest version, but it may break other stuff

Nice script BTW, thanks for sharing :-)