class ActiveRecord::Associations::CollectionProxy

def size

# collection.size and no SQL count query is executed.
# Because the collection is already loaded, this will behave like
person.pets.size # => 3

# ]
# #
# #,
# #,
# => [
person.pets # This will execute a SELECT * FROM query

# executes something like SELECT COUNT(*) FROM "pets" WHERE "pets"."person_id" = 1
person.pets.size # => 3

end
has_many :pets
class Person < ActiveRecord::Base

+length+ will take one less query. Otherwise +size+ is more efficient.
equivalent. If not and you are going to need the records anyway
If the collection has been already loaded +size+ and +length+ are

it executes a SELECT COUNT(*) query. Else it calls collection.size.
Returns the size of the collection. If the collection hasn't been loaded, 
def size
  @association.size
end