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

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

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

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

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

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

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

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

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

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 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, loads all records in the relation, if the relation hasn’t been loaded yet. Calls the block with each record in the relation. 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

Note: If there are a lot of records in the relation, loading all records could result in performance issues.

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

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

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

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' ]

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, people.id FROM comments INNER JOIN people on comments.person_id = people.id
# => [[1, 2], [2, 2]]

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

See also ids.

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, loads all records in the relation, if the relation hasn’t been loaded yet. Calls the block with each record in the relation. Returns the sum of initial_value_or_column and the block return values:

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

Note: If there are a lot of records in the relation, loading all records could result in performance issues.