Flowdock
method

calculate

Importance_3
calculate(operation, column_name, options = {}) public

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 :group 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.
  • :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'
  Person.minimum(:age, :having => 'min(age) > 17', :group => :last_name) # Selects the minimum age for any family without any minors
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January 18, 2011
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Grouping

Person.sum(:age, :group => “sex”) # =>[[“male”,2500],[“female”,2200]]