Backport pluck to Rails 3.1

active_record.rb

# config/initializers/extensions/active_record.rb
module ActiveRecord
  class Base
    class << self
      delegate :pluck, to: :scoped
    end
  end

  class CollectionProxy
    delegate :pluck, to: :scoped
  end

  # = Active Record Relation
  class Relation
    # Returns <tt>Array</tt> with values of the specified column name
    # The values has same data type as column.
    #
    # Examples:
    #
    #   Person.pluck(:id) # SELECT people.id FROM people
    #   Person.uniq.pluck(:role) # SELECT DISTINCT role FROM people
    #   Person.where(:confirmed => true).limit(5).pluck(:id)
    #
    def pluck(column_name)
      scope = self.select(column_name)
      self.connection.select_values(scope.to_sql).map! do |value|
        type_cast_using_column(value, column_for(column_name))
      end
    end
  end
end

That's awesome. There is only one detail, the self.select you're using, if its on a CollectionProxy, it will select

.* on tables involved on joins. That is why my example was using Arel::SelectManager#projections=