Calculates the average value on a given column. The value is returned as a float, or nil if there‘s no row. See calculate for examples with options.

  Person.average('age') # => 35.8

[Source]

    # File vendor/rails/activerecord/lib/active_record/calculations.rb, line 56
56:       def average(column_name, options = {})
57:         calculate(:avg, column_name, options)
58:       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 :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'
  Person.minimum(:age, :having => 'min(age) > 17', :group => :last_name) # Selects the minimum age for any family without any minors
  Person.sum("2 * age")

[Source]

     # File vendor/rails/activerecord/lib/active_record/calculations.rb, line 125
125:       def calculate(operation, column_name, options = {})
126:         validate_calculation_options(operation, options)
127:         column_name     = options[:select] if options[:select]
128:         column_name     = '*' if column_name == :all
129:         column          = column_for column_name
130:         catch :invalid_query do
131:           if options[:group]
132:             return execute_grouped_calculation(operation, column_name, column, options)
133:           else
134:             return execute_simple_calculation(operation, column_name, column, options)
135:           end
136:         end
137:         0
138:       end

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 using column: By passing a column name to count, it will return a count of all the rows for the model with supplied column present
• Count using options will find the row count matched by the options used.

The third 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 to ActiveRecord::Base.
• :joins: Either an SQL fragment for additional joins like "LEFT JOIN comments ON comments.post_id = id" (rarely needed) or named associations in the same form used for the :include option, which will perform an INNER JOIN on the associated table(s). If the value is a string, then the records will be returned read-only since they will have attributes that do not correspond to the table‘s columns. Pass :readonly => false to override.
• :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 of 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) …
• :from - By default, this is the table name of the class, but can be changed to an alternate table name (or even the name of a database view).

Examples for counting all:

  Person.count         # returns the total count of all people

Examples for counting by column:

  Person.count(:age)  # returns the total count of all people whose age is present in database

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.

[Source]

    # File vendor/rails/activerecord/lib/active_record/calculations.rb, line 47
47:       def count(*args)
48:         calculate(:count, *construct_count_options_from_args(*args))
49:       end

Calculates the maximum value on a given column. The value is returned with the same data type of the column, or nil if there‘s no row. See calculate for examples with options.

  Person.maximum('age') # => 93

[Source]

    # File vendor/rails/activerecord/lib/active_record/calculations.rb, line 74
74:       def maximum(column_name, options = {})
75:         calculate(:max, column_name, options)
76:       end

Calculates the minimum value on a given column. The value is returned with the same data type of the column, or nil if there‘s no row. See calculate for examples with options.

  Person.minimum('age') # => 7

[Source]

    # File vendor/rails/activerecord/lib/active_record/calculations.rb, line 65
65:       def minimum(column_name, options = {})
66:         calculate(:min, column_name, options)
67:       end

Calculates the sum of values on a given column. The value is returned with the same data type of the column, 0 if there‘s no row. See calculate for examples with options.

  Person.sum('age') # => 4562

[Source]

    # File vendor/rails/activerecord/lib/active_record/calculations.rb, line 83
83:       def sum(column_name, options = {})
84:         calculate(:sum, column_name, options)
85:       end

[Source]

     # File vendor/rails/activerecord/lib/active_record/calculations.rb, line 141
141:         def construct_count_options_from_args(*args)
142:           options     = {}
143:           column_name = :all
144:           
145:           # We need to handle
146:           #   count()
147:           #   count(:column_name=:all)
148:           #   count(options={})
149:           #   count(column_name=:all, options={})
150:           case args.size
151:           when 1
152:             args[0].is_a?(Hash) ? options = args[0] : column_name = args[0]
153:           when 2
154:             column_name, options = args
155:           else
156:             raise ArgumentError, "Unexpected parameters passed to count(): #{args.inspect}"
157:           end if args.size > 0
158:           
159:           [column_name, options]
160:         end