calculate

This calculates aggregate values in the given column. Methods for count, sum, average, minimum, and maximum have been added as shortcuts. Options such as :conditions, :order, :group, :having, and :joins 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:

• :conditions - An SQL fragment like “administrator = 1” or [ “user_name = ?”, username ]. See conditions in the intro to ActiveRecord::Base.

• :include: 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.

• :joins - 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.

• :order - An SQL fragment like “created_at DESC, name” (really only used with GROUP BY calculations).

• :group - An attribute name by which the result should be grouped. Uses the GROUP BY SQL-clause.

• :select - 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.

• :distinct - 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")