pluck_from_activerecord.rb

def pluck(*column_names)
  if loaded? && (column_names.map(&:to_s) - @klass.attribute_names - @klass.attribute_aliases.keys).empty?
    return records.pluck(*column_names)
  end

  if has_include?(column_names.first)
    construct_relation_for_association_calculations.pluck(*column_names)
  else
    relation = spawn
    relation.select_values = column_names.map { |cn|
      @klass.has_attribute?(cn) || @klass.attribute_alias?(cn) ? arel_attribute(cn) : cn
    }
    result = klass.connection.select_all(relation.arel, nil, bound_attributes)
    result.cast_values(klass.attribute_types)
  end
end

activerecord/lib/active_record/relation/calculations.rb