Determining who owns a transferrable item in MySQL - mysql

Question

I am working on a game where users can buy virtual items with virtual currency, and also give those items away to other users. We need to be able to track the history of the item (who bought it, who it was gifted to, etc) and the current owner. I'm struggling with some of the table design.
My thinking is to have this (with table design simplified slightly and constraints/keys/etc omitted for space):
TABLE order
id INT NOT NULL AUTO_INCREMENT,
buyer_id INT NOT NULL,
paid INT UNSIGNED NOT NULL,
owned_item_id INT NOT NULL,
------------------------------------------------------
TABLE owned_item
id INT NOT NULL AUTO_INCREMENT,
store_product_id INT NOT NULL,
owner_id INT NOT NULL,
------------------------------------------------------
TABLE gift
id INT NOT NULL,
giver_id INT NOT NULL,
receiver_id INT NOT NULL,
owned_item_id INT NOT NULL,
With the idea being that when an item is purchased, both an order and an owned_item are created for that item. If the item is gifted, then a new entry in the gift table is created and the owner_id field is updated.
This approach makes it very easy to determine who currently owns a given item. However, it has redundant data and leaves room for problems with data integrity. If the 'owner_id' field was set incorrectly, we might end up with records that an item that was purchased by A and gifted to B but is now inexplicably owned by C.
How should such a structure be normalized? I've considered eliminating the owned_item table:
TABLE order
id INT NOT NULL AUTO_INCREMENT,
buyer_id INT NOT NULL,
paid INT UNSIGNED NOT NULL,
product_id INT NOT NULL,
------------------------------------------------------
TABLE gift
id INT NOT NULL,
giver_id INT NOT NULL,
receiver_id INT NOT NULL,
order_id INT NOT NULL,
I don't like this solution because finding all of the items that some person owns becomes a very complex operation (find every gift record X where the recipient is A and no later gift record exists for the same order, combined with find every order record Y where the buyer is A and no gift records exist for that order) but if that's the correct solution then I'll make do.

Answer 1

Something like this would be a good 3nf schema for what you're looking to do.
Entities and transactions are kept generic in order to allow all entity / transaction relationships to be simplified.
-- An Entity is a person or business, as a party in a transaction
CREATE TABLE entity (
id INT NOT NULL AUTO_INCREMENT,
entity_type ENUM('store', 'person') NOT NULL
name VARCHAR NOT NULL
);
-- Unique item types - items are each an instance of an item-type
-- e.g. the item "Steve's broom" may be an item of type "broom"
CREATE TABLE item_type (
id INT NOT NULL AUTO_INCREMENT,
name VARCHAR NOT NULL
);
-- Non-unique item, instance of an item-type owned by an entity
CREATE TABLE item (
id INT NOT NULL AUTO_INCREMENT,
-- optionally include owner_id as a quick-reference to the current owner of the item
owner_id INT NULL REFERENCES entity (id),
-- FK to unique item types, e.g. "broom"
item_type_id INT NOT NULL REFERENCES item_type (id),
-- possible description, e.g. "Steve's broom"
description VARCHAR NOT NULL
);
-- A transaction is a sale, gift, or other method of transferrence
-- of an item between entities. Transaction is a bad name, because
-- it's a reserved word. That's why it's encased in ticks.
-- You'd probably be better off choosing a different generic name
CREATE TABLE `transaction` (
id
-- transaction_type can be NULL in cases of origination with entity
transaction_type ENUM('sale', 'gift') NULL,
-- NULL in cases of origination with an entity
from_entity_id INT NULL REFERENCES entity (id),
to_entity_id INT NOT NULL REFERENCES entity (id),
-- amount can be 0 in cases of gifts
amount DECIMAL(9,2) UNSIGNED NOT NULL DEFAULT 0
);
A "gift" transaction would have an amount of 0 (or NULL, if you wanted to make it nullable).
An "origin" transaction (e.g. something was made, or found) would have no transaction_type, and 0 amount.
To know who the current owner of an item is, use a view that retrieves the last "to_entity_id" for that item in the transaction table, e.g.:
SELECT
e.name
FROM entity AS e
INNER JOIN `transaction` AS t ON e.id = t.to_entity_id
INNER JOIN
(SELECT MAX(id) AS id
FROM `transaction`
WHERE item_id = 5) AS tx ON tx.id = t.id
Alternatively, you could also store the owner_id in the item table (see note above in the schema). This would be a little redundant, and would require updating that table on each transaction, but would save a lot of expensive queries to know who owns what.