Rails: array of IDs

example.rb

# If you want to go directly to the DB
ActiveRecord::Base.connection.select_values("select id from users")

# If you already have a model
User.all(:select => :id).collect(&:id)

# In Rails 3 you can do this
User.pluck(:id)

# Rails helper
User.comment_ids

# Without DB
User.all.collect { |obj| obj.id }