Refactor my code to eliminate separate DB queries

Question

Here's my existing code. I could use some guidance on how to optimize this. Essentially, I don't want to have to run a SQL query more than once -- self.active_item?(r). What I'm thinking I can do instead of making separate DB queries, is retrieve an array of all the user's active inventory items (e.g., self.active_inventory_items), turn that into an array of items, then query the array to see if related_item == item. What do you think?

    # User has many inventory_items.
    # Item has many inventory_items.
    # InventoryItem belongs to user, item.

# in User Model
def retrieve_owned_items(purchasing_item)
  related_items = purchasing_item.related_items # returns an array of Items (related to item user is going to purchase)

  owned_items = []
  unless related_items.nil?
    related_items.each do |r| # iterate through each related item
      if self.active_item?(r) then # runs a SQL query every time to check if the related item is currently an active item in the inventory
        owned_items << InventoryItem.find_by_item_id(r.id) # create an array of related_items already owned
      end
    end
  end

  return (owned_items.blank? ? nil : owned_items)
end

Answer 1

def retrieve_owned_items(purchasing_item)
  if related_items = purchasing_item.related_items
    InventoryItem.all(:conditions => {:item_id => related_items.map(&:id)})
  end
end