pluck backported to Rails 3.x

pluck.rb

module ActiveRecord
  class Base
    class << self
      delegate :pluck, :ids, :to => :scoped
    end
  end

  class CollectionProxy
    delegate :pluck, :ids, :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)
    #
    #     Person.pluck(:id, :name)
    #     # SELECT people.id, people.name FROM people
    #     # [[1, 'David'], [2, 'Jeremy'], [3, 'Jose']]
    #
    def pluck(*column_names)
      # Make sure we have an array of text representations of the column names
      column_names = Array(column_names).map(&:to_s)

      # Define a scope for selecting just those fields
      scope = self.select(column_names.join(", "))

      # Precache the column definitions of those fields
      column_definitions = Hash[column_names.map { |column| [column, column_for(column)] }]

      # Select the resultant SQL
      select_all = self.connection.select_all(scope.to_sql)

      # Map the row hashes returned from select_all into arrays such that the
      # values are in the same order as the originally supplied columns
      results = select_all.map! do |row_hash|
                  column_names.map.with_index do |column, index|
                    value      = row_hash.values[index]
                    column_def = column_definitions[column_names[index]]

                    type_cast_using_column(value, column_def)
                  end
                end

      # If only a single column was given then flatten the results
      column_names.size == 1 ? results.flatten : results
    end

    def ids
      pluck("#{table_name}.id")
    end
  end
end