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

# File activerecord/lib/active_record/relation/calculations.rb, line 41
def average(column_name, options = {})
  # TODO: Remove options argument as soon we remove support to
  # activerecord-deprecated_finders.
  calculate(:average, column_name, options)
end

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

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. 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

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")

# File activerecord/lib/active_record/relation/calculations.rb, line 109
def calculate(operation, column_name, options = {})
  # TODO: Remove options argument as soon we remove support to
  # activerecord-deprecated_finders.
  if column_name.is_a?(Symbol) && attribute_alias?(column_name)
    column_name = attribute_alias(column_name)
  end

  if has_include?(column_name)
    construct_relation_for_association_calculations.calculate(operation, column_name, options)
  else
    perform_calculation(operation, column_name, options)
  end
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 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 select, it will count the selected columns:

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

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

# File activerecord/lib/active_record/relation/calculations.rb, line 30
def count(column_name = nil, options = {})
  # TODO: Remove options argument as soon we remove support to
  # activerecord-deprecated_finders.
  column_name, options = nil, column_name if column_name.is_a?(Hash)
  calculate(:count, column_name, options)
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

# File activerecord/lib/active_record/relation/calculations.rb, line 195
def ids
  pluck primary_key
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

# File activerecord/lib/active_record/relation/calculations.rb, line 63
def maximum(column_name, options = {})
  # TODO: Remove options argument as soon we remove support to
  # activerecord-deprecated_finders.
  calculate(:maximum, column_name, options)
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

# File activerecord/lib/active_record/relation/calculations.rb, line 52
def minimum(column_name, options = {})
  # TODO: Remove options argument as soon we remove support to
  # activerecord-deprecated_finders.
  calculate(:minimum, column_name, options)
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(:id)
# SELECT people.id FROM people
# => [1, 2, 3]

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

Person.pluck('DISTINCT 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']

# File activerecord/lib/active_record/relation/calculations.rb, line 156
def pluck(*column_names)
  column_names.map! do |column_name|
    if column_name.is_a?(Symbol) && attribute_alias?(column_name)
      attribute_alias(column_name)
    else
      column_name.to_s
    end
  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|
      columns_hash.key?(cn) ? arel_table[cn] : cn
    }
    result = klass.connection.select_all(relation.arel, nil, bind_values)
    columns = result.columns.map do |key|
      klass.column_types.fetch(key) {
        result.column_types.fetch(key) { result.identity_type }
      }
    end

    result = result.rows.map do |values|
      values = result.columns.zip(values).map do |column_name, value|
        single_attr_hash = { column_name => value }
        klass.initialize_attributes(single_attr_hash).values.first
      end

      columns.zip(values).map { |column, value| column.type_cast value }
    end
    columns.one? ? result.map!(&:first) : result
  end
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

# File activerecord/lib/active_record/relation/calculations.rb, line 74
def sum(*args)
  calculate(:sum, *args)
end