Instance Public methods
average(column_name) Link
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) Link
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 129 def calculate(operation, column_name) if has_include?(column_name) relation = apply_join_dependency if operation.to_s.downcase == "count" unless distinct_value || distinct_select?(column_name || select_for_count) relation.distinct! relation.select_values = [ klass.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(column_name = nil) Link
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.
# File activerecord/lib/active_record/relation/calculations.rb, line 43 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
ids() Link
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) Link
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) Link
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(*column_names) Link
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' ]
pluck(*column_names) Link
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(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 183 def pluck(*column_names) if loaded? && all_attributes?(column_names) return records.pluck(*column_names) end if has_include?(column_names.first) relation = apply_join_dependency relation.pluck(*column_names) else klass.disallow_raw_sql!(column_names) columns = arel_columns(column_names) relation = spawn relation.select_values = columns result = skip_query_cache_if_necessary do if where_clause.contradiction? ActiveRecord::Result.new([], []) else klass.connection.select_all(relation.arel, nil) end end type_cast_pluck_values(result, columns) end end
sum(column_name = nil) Link
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 86 def sum(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(:sum, column_name) end end