Same as average, but performs the query asynchronously and returns an ActiveRecord::Promise.

# File activerecord/lib/active_record/relation/calculations.rb, line 122
def async_average(column_name)
  async.average(column_name)
end

Same as count, but performs the query asynchronously and returns an ActiveRecord::Promise.

# File activerecord/lib/active_record/relation/calculations.rb, line 108
def async_count(column_name = nil)
  async.count(column_name)
end

Same as ids, but performs the query asynchronously and returns an ActiveRecord::Promise.

# File activerecord/lib/active_record/relation/calculations.rb, line 408
def async_ids
  async.ids
end

Same as maximum, but performs the query asynchronously and returns an ActiveRecord::Promise.

# File activerecord/lib/active_record/relation/calculations.rb, line 152
def async_maximum(column_name)
  async.maximum(column_name)
end

Same as minimum, but performs the query asynchronously and returns an ActiveRecord::Promise.

# File activerecord/lib/active_record/relation/calculations.rb, line 137
def async_minimum(column_name)
  async.minimum(column_name)
end

Same as pick, but performs the query asynchronously and returns an ActiveRecord::Promise.

# File activerecord/lib/active_record/relation/calculations.rb, line 362
def async_pick(*column_names)
  async.pick(*column_names)
end

Same as pluck, but performs the query asynchronously and returns an ActiveRecord::Promise.

# File activerecord/lib/active_record/relation/calculations.rb, line 333
def async_pluck(*column_names)
  async.pluck(*column_names)
end

Same as sum, but performs the query asynchronously and returns an ActiveRecord::Promise.

# File activerecord/lib/active_record/relation/calculations.rb, line 181
def async_sum(identity_or_column = nil)
  async.sum(identity_or_column)
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

# File activerecord/lib/active_record/relation/calculations.rb, line 116
def average(column_name)
  calculate(:average, column_name)
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

# File activerecord/lib/active_record/relation/calculations.rb, line 216
def calculate(operation, column_name)
  operation = operation.to_s.downcase

  if @none
    case operation
    when "count", "sum"
      result = group_values.any? ? Hash.new : 0
      return @async ? Promise::Complete.new(result) : result
    when "average", "minimum", "maximum"
      result = group_values.any? ? Hash.new : nil
      return @async ? Promise::Complete.new(result) : result
    end
  end

  if has_include?(column_name)
    relation = apply_join_dependency

    if operation == "count"
      unless distinct_value || distinct_select?(column_name || select_for_count)
        relation.distinct!
        relation.select_values = Array(model.primary_key || table[Arel.star])
      end
      # PostgreSQL: ORDER BY expressions must appear in SELECT list when using DISTINCT
      relation.order_values = [] if group_values.empty?
    end

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

When given a block, calls the block with each record in the relation and returns the number of records for which the block returns a truthy value.

Person.count { |person| person.age > 21 }
# => counts the number of people older that 21

If the relation hasn’t been loaded yet, calling count with a block will load all records in the relation. If there are a lot of records in the relation, loading all records could result in performance issues.

# File activerecord/lib/active_record/relation/calculations.rb, line 94
def count(column_name = nil)
  if block_given?
    unless column_name.nil?
      raise ArgumentError, "Column name argument is not supported when a block is passed."
    end

    super()
  else
    calculate(:count, column_name)
  end
end

Returns the base model’s ID’s for the relation using the table’s primary key

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

# File activerecord/lib/active_record/relation/calculations.rb, line 370
def ids
  primary_key_array = Array(primary_key)

  if loaded?
    result = records.map do |record|
      if primary_key_array.one?
        record._read_attribute(primary_key_array.first)
      else
        primary_key_array.map { |column| record._read_attribute(column) }
      end
    end
    return @async ? Promise::Complete.new(result) : result
  end

  if has_include?(primary_key)
    relation = apply_join_dependency.group(*primary_key_array)
    return relation.ids
  end

  columns = arel_columns(primary_key_array)
  relation = spawn
  relation.select_values = columns

  result = if relation.where_clause.contradiction?
    ActiveRecord::Result.empty
  else
    skip_query_cache_if_necessary do
      model.with_connection do |c|
        c.select_all(relation, "#{model.name} Ids", async: @async)
      end
    end
  end

  result.then { |result| type_cast_pluck_values(result, columns) }
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 146
def maximum(column_name)
  calculate(:maximum, 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

# File activerecord/lib/active_record/relation/calculations.rb, line 131
def minimum(column_name)
  calculate(:minimum, column_name)
end

Pick the value(s) from the named column(s) in the current relation. This is short-hand for relation.limit(1).pluck(*column_names).first, and is primarily useful when you have a relation that’s already narrowed down to a single row.

Just like pluck, pick will only load the actual value, not the entire record object, so it’s also more efficient. The value is, again like with pluck, typecast by the column type.

Person.where(id: 1).pick(:name)
# SELECT people.name FROM people WHERE id = 1 LIMIT 1
# => 'David'

Person.where(id: 1).pick(:name, :email_address)
# SELECT people.name, people.email_address FROM people WHERE id = 1 LIMIT 1
# => [ 'David', 'david@loudthinking.com' ]

# File activerecord/lib/active_record/relation/calculations.rb, line 351
def pick(*column_names)
  if loaded? && all_attributes?(column_names)
    result = records.pick(*column_names)
    return @async ? Promise::Complete.new(result) : result
  end

  limit(1).pluck(*column_names).then(&:first)
end

Use pluck as a shortcut to select one or more attributes without loading an entire record object per row.

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]

Comment.joins(:person).pluck(:id, person: :id)
# SELECT comments.id, person.id FROM comments INNER JOIN people person ON person.id = comments.person_id
# => [[1, 2], [2, 2]]

Comment.joins(:person).pluck(:id, person: [:id, :name])
# SELECT comments.id, person.id, person.name FROM comments INNER JOIN people person ON person.id = comments.person_id
# => [[1, 2, 'David'], [2, 2, 'David']]

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

See also ids.

# File activerecord/lib/active_record/relation/calculations.rb, line 290
def pluck(*column_names)
  if @none
    if @async
      return Promise::Complete.new([])
    else
      return []
    end
  end

  if loaded? && all_attributes?(column_names)
    result = records.pluck(*column_names)
    if @async
      return Promise::Complete.new(result)
    else
      return result
    end
  end

  if has_include?(column_names.first)
    relation = apply_join_dependency
    relation.pluck(*column_names)
  else
    model.disallow_raw_sql!(flattened_args(column_names))
    relation = spawn
    columns = relation.arel_columns(column_names)
    relation.select_values = columns
    result = skip_query_cache_if_necessary do
      if where_clause.contradiction?
        ActiveRecord::Result.empty(async: @async)
      else
        model.with_connection do |c|
          c.select_all(relation.arel, "#{model.name} Pluck", async: @async)
        end
      end
    end
    result.then do |result|
      type_cast_pluck_values(result, columns)
    end
  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

When given a block, calls the block with each record in the relation and returns the sum of initial_value_or_column plus the block return values:

Person.sum { |person| person.age } # => 4562
Person.sum(1000) { |person| person.age } # => 5562

If the relation hasn’t been loaded yet, calling sum with a block will load all records in the relation. If there are a lot of records in the relation, loading all records could result in performance issues.

# File activerecord/lib/active_record/relation/calculations.rb, line 171
def sum(initial_value_or_column = 0, &block)
  if block_given?
    map(&block).sum(initial_value_or_column)
  else
    calculate(:sum, initial_value_or_column)
  end
end