module ActiveRecord::Calculations

Public Instance Methods

average(column_name) Show source

# File activerecord/lib/active_record/relation/calculations.rb, line 47
def average(column_name)
  calculate(:average, column_name)
end

Calculates the average value on a given column. Returns nil if there's no row. See calculate for examples with options.

Person.average(:age) # => 35.8

calculate(operation, column_name) Show source

# File activerecord/lib/active_record/relation/calculations.rb, line 110
def calculate(operation, column_name)
  if column_name.is_a?(Symbol) && attribute_alias?(column_name)
    column_name = attribute_alias(column_name)
  end

  if has_include?(column_name)
    relation = construct_relation_for_association_calculations
    relation = relation.distinct if operation.to_s.downcase == "count"

    relation.calculate(operation, column_name)
  else
    perform_calculation(operation, column_name)
  end
end

This calculates aggregate values in the given column. Methods for count, sum, average, minimum, and maximum have been added as shortcuts.

Person.calculate(:count, :all) # The same as Person.count
Person.average(:age) # SELECT AVG(age) FROM people...

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

Person.sum("2 * age")

There are two basic forms of output:

Single aggregate value: The single value is type cast to Integer 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. It takes either a column name, or the name of a belongs_to association.

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

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

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

count(column_name = nil) Show source

# File activerecord/lib/active_record/relation/calculations.rb, line 39
def count(column_name = nil)
  calculate(:count, column_name)
end

Count the records.

Person.count
# => the total count of all people

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

Person.count(:all)
# => performs a COUNT(*) (:all is an alias for '*')

Person.distinct.count(:age)
# => counts the number of different age values

If count is used with Relation#group, it returns a Hash whose keys represent the aggregated column, and the values are the respective amounts:

Person.group(:city).count
# => { 'Rome' => 5, 'Paris' => 3 }

If count is used with Relation#group for multiple columns, it returns a Hash whose keys are an array containing the individual values of each column and the value of each key would be the count.

Article.group(:status, :category).count
# =>  {["draft", "business"]=>10, ["draft", "technology"]=>4,
       ["published", "business"]=>0, ["published", "technology"]=>2}

If count is used with Relation#select, it will count the selected columns:

Person.select(:age).count
# => counts the number of different age values

Note: not all valid Relation#select expressions are valid count expressions. The specifics differ between databases. In invalid cases, an error from the database is thrown.

ids() Show source

# File activerecord/lib/active_record/relation/calculations.rb, line 181
def ids
  pluck primary_key
end

Pluck all the ID's for the relation using the table's primary key

Person.ids # SELECT people.id FROM people
Person.joins(:companies).ids # SELECT people.id FROM people INNER JOIN companies ON companies.person_id = people.id

maximum(column_name) Show source

# File activerecord/lib/active_record/relation/calculations.rb, line 65
def maximum(column_name)
  calculate(:maximum, column_name)
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

minimum(column_name) Show source

# File activerecord/lib/active_record/relation/calculations.rb, line 56
def minimum(column_name)
  calculate(:minimum, column_name)
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

pluck(*column_names) Show source

# File activerecord/lib/active_record/relation/calculations.rb, line 160
def pluck(*column_names)
  if loaded? && (column_names.map(&:to_s) - @klass.attribute_names - @klass.attribute_aliases.keys).empty?
    return records.pluck(*column_names)
  end

  if has_include?(column_names.first)
    construct_relation_for_association_calculations.pluck(*column_names)
  else
    relation = spawn
    relation.select_values = column_names.map { |cn|
      @klass.has_attribute?(cn) || @klass.attribute_alias?(cn) ? arel_attribute(cn) : cn
    }
    result = klass.connection.select_all(relation.arel, nil, bound_attributes)
    result.cast_values(klass.attribute_types)
  end
end

Use pluck as a shortcut to select one or more attributes without loading a bunch of records just to grab the attributes you want.

Person.pluck(:name)

instead of

Person.all.map(&:name)

Pluck returns an Array of attribute values type-casted to match the plucked column names, if they can be deduced. Plucking an SQL fragment returns String values by default.

Person.pluck(:name)
# SELECT people.name FROM people
# => ['David', 'Jeremy', 'Jose']

Person.pluck(:id, :name)
# SELECT people.id, people.name FROM people
# => [[1, 'David'], [2, 'Jeremy'], [3, 'Jose']]

Person.distinct.pluck(:role)
# SELECT DISTINCT role FROM people
# => ['admin', 'member', 'guest']

Person.where(age: 21).limit(5).pluck(:id)
# SELECT people.id FROM people WHERE people.age = 21 LIMIT 5
# => [2, 3]

Person.pluck('DATEDIFF(updated_at, created_at)')
# SELECT DATEDIFF(updated_at, created_at) FROM people
# => ['0', '27761', '173']