count

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}[rdoc-ref:QueryMethods#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}[rdoc-ref:QueryMethods#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}[rdoc-ref:QueryMethods#select], it will count the selected columns:

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

Note: not all valid {Relation#select}[rdoc-ref:QueryMethods#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