def calculate(operation, column_name, options = {})
      if options.except(:distinct).present?
        apply_finder_options(options.except(:distinct)).calculate(operation, column_name, :distinct => options[:distinct])
      else
        relation = with_default_scope

        if relation.equal?(self)
          if eager_loading? || (includes_values.present? && references_eager_loaded_tables?)
            construct_relation_for_association_calculations.calculate(operation, column_name, options)
          else
            perform_calculation(operation, column_name, options)
          end
        else
          relation.calculate(operation, column_name, options)
        end
      end
    rescue ThrowResult
      0
    end
def calculate(operation, column_name, options = {})
      if options.except(:distinct).present?
        apply_finder_options(options.except(:distinct)).calculate(operation, column_name, :distinct => options[:distinct])
      else
        relation = with_default_scope

        if relation.equal?(self)
          if eager_loading? || (includes_values.present? && references_eager_loaded_tables?)
            construct_relation_for_association_calculations.calculate(operation, column_name, options)
          else
            perform_calculation(operation, column_name, options)
          end
        else
          relation.calculate(operation, column_name, options)
        end
      end
    rescue ThrowResult
      0
    end
This calculates aggregate values in the given column. Methods for count, sum, average,
 minimum, and maximum have been added as shortcuts. Options such as <tt>:conditions</tt>,
 <tt>:order</tt>, <tt>:group</tt>, <tt>:having</tt>, and <tt>:joins</tt> can be passed to customize the query.

 There are two basic forms of output:
   * Single aggregate value: The single value is type cast to Fixnum for COUNT, Float
     for AVG, and the given column's type for everything else.
   * Grouped values: This returns an ordered hash of the values and groups them by the
     <tt>:group</tt> option. It takes either a column name, or the name of a belongs_to association.

       values = Person.maximum(:age, :group => 'last_name')
       puts values["Drake"]
       => 43

       drake  = Family.find_by_last_name('Drake')
       values = Person.maximum(:age, :group => :family) # Person belongs_to :family
       puts values[drake]
       => 43

       values.each do |family, max_age|
       ...
       end

 Options:
 * <tt>:conditions</tt> - An SQL fragment like "administrator = 1" or [ "user_name = ?", username ].
   See conditions in the intro to ActiveRecord::Base.
 * <tt>:include</tt>: Eager loading, see Associations for details. Since calculations don't load anything,
   the purpose of this is to access fields on joined tables in your conditions, order, or group clauses.
 * <tt>:joins</tt> - An SQL fragment for additional joins like "LEFT JOIN comments ON comments.post_id = id".
   (Rarely needed).
   The records will be returned read-only since they will have attributes that do not correspond to the
   table's columns.
 * <tt>:order</tt> - An SQL fragment like "created_at DESC, name" (really only used with GROUP BY calculations).
 * <tt>:group</tt> - An attribute name by which the result should be grouped. Uses the GROUP BY SQL-clause.
 * <tt>:select</tt> - By default, this is * as in SELECT * FROM, but can be changed if you for example
   want to do a join, but not include the joined columns.
 * <tt>:distinct</tt> - Set this to true to make this a distinct calculation, such as
   SELECT COUNT(DISTINCT posts.id) ...

 Examples:
   Person.calculate(:count, :all) # The same as Person.count
   Person.average(:age) # SELECT AVG(age) FROM people...
   Person.minimum(:age, :conditions => ['last_name != ?', 'Drake']) # Selects the minimum age for
                                                                    # everyone with a last name other than 'Drake'

   # Selects the minimum age for any family without any minors
   Person.minimum(:age, :having => 'min(age) > 17', :group => :last_name)

   Person.sum("2 * age")