@landlord = Landlord.first

# generates a SQL query for the associated properties collection
@landlord.properties.size # => 30, or whatever

@landlord.properties.size # => 30; no additional SQL query generated

# generates the following SQL in Rails 2.3.5:
# SELECT count(*) AS count_all FROM `properties` WHERE (`properties`.landlord_id = 1)
@landlord.properties.count # => 30