You want to know how many Items are currently in the Store and you’ve got this in your Store definition:
has_many :items

If the Items for this Store have already been read in and objects have been mapped, you simply want to ask for the count:
flagship_store.items.size

but if they haven’t already been mapped, you want to just do a quick select count(*):
Item.count(:conditions => ["store_id = ?", flagship_store.id])

The answer?

For many of us it’s:

item_count = flagship_store.instance_variable_get(:@items) ? @items.size : Item.count(:conditions => ["store_id = ?", flagship_store.id])


And for those of you living in the future it’s even more straightforward:

item_count = flagship_store.instance_variable_defined?(:@items) ? @items.size : Item.count(:conditions => ["store_id = ?", flagship_store.id])