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, 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.