Module ActiveRecord::Calculations::ClassMethods

Calculates average value on a given column. The value is returned as a float. See calculate for examples with options.

Person.average('age')

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

Count operates using three different approaches.

• Count all: By not passing any parameters to count, it will return a count of all the rows for the model.

• Count by conditions or joins: This API has been deprecated and will be removed in Rails 2.0

• Count using options will find the row count matched by the options used.

The last approach, count using options, accepts an option hash as the only parameter. The options are:

• :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.

• :include: Named associations that should be loaded alongside using LEFT OUTER JOINs. The symbols named refer to already defined associations. When using named associations count returns the number DISTINCT items for the model you're counting. See eager loading under Associations.

• :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 for counting all:

Person.count         # returns the total count of all people

Examples for count by conditions and joins (this has been deprecated):

Person.count("age > 26")  # returns the number of people older than 26
Person.find("age > 26 AND job.salary > 60000", "LEFT JOIN jobs on jobs.person_id = person.id") # returns the total number of rows matching the conditions and joins fetched by SELECT COUNT(*).

Examples for count with options:

Person.count(:conditions => "age > 26")
Person.count(:conditions => "age > 26 AND job.salary > 60000", :include => :job) # because of the named association, it finds the DISTINCT count using LEFT OUTER JOIN.
Person.count(:conditions => "age > 26 AND job.salary > 60000", :joins => "LEFT JOIN jobs on jobs.person_id = person.id") # finds the number of rows matching the conditions and joins. 
Person.count('id', :conditions => "age > 26") # Performs a COUNT(id)
Person.count(:all, :conditions => "age > 26") # Performs a COUNT(*) (:all is an alias for '*')

Note: Person.count(:all) will not work because it will use :all as the condition. Use Person.count instead.

Calculates the maximum value on a given column. The value is returned with the same data type of the column.. See calculate for examples with options.

Person.maximum('age')

Calculates the minimum value on a given column. The value is returned with the same data type of the column.. See calculate for examples with options.

Person.minimum('age')

Calculates the sum value on a given column. The value is returned with the same data type of the column.. See calculate for examples with options.

Person.sum('age')