How to properly handle rental inventory via database

Question

I am building a rental inventory app that handles the tracking of rental reservations. I think I might be over thinking it but I am getting stuck on trying to figure out the model or schema of inventory scheduling part. In the attached image, I have 4 copies of one movie. On the first only one copy is scheduled to be out; on the 5th, 3 copies are out; on the 6th and 10th all copies are out. Now, I would like to design the database in a way in which I can look up a date OR date-range to see how much inventory is available that day(s). The challenge is that there might NOT be individual sku's or tracking for each individual rental item. So I can't treat Movie_1 as if it has movie_1_a,movie_1_b,movie_1_c,movie_1_d. Instead I have to treat it like Movie_1 has 4 copies and on the 5th, 3 copies are out but we don't know which ones.

Can anyone give any suggestions on how to write the schema. How would a sample query look like to search for availability?

Answer 1

There are two, (three if you include sales/delivery) aspects to this.

Something like

Originals (StockId etc)

CopySchedule(CopyId, StockID, DeliveryDate, NumberOfCopiesRequired, NumberOFCopiesAchieved, Status, etc)

Copies (CopyId, Quantity, etc)

So when you set a CopyScheduleStatus to Done, you add a record to Copies.

Then your projected stock level would be from Now? To Whenever would be

NumberInStock - NumberScheduledtoDeliver( between now and whenever) + NumberScheduledToCopy(between now and whenever)

Don't try to do all of your needs in one model.

Answer 2

In your movie table you shoud have movie_copies field, then create a table for [Movie_flow] with this schema:

[PK]movement_id (could be an identity or an pair year/movement number if you wanna reset  movements counter every year)
movie_id
movement_datetime
movement_type (I-in/O-out)
quantity

When you want to know movie availability at a specific date you can simply read al movements by movie_id, adding quantity value for movement_type "I" and decreasing quantity for movement_type "O" until the given date: movie_copies - this count gives you the movie copies available.

Keep clean that table to optimize performance, you can archive older movements if you want to keep an history