I have a problem, which I expect is very common, but I am not sure what to search for, as it can be explained in many different ways.
I am designing a database structure for managing invoices, products, prices, currencies, etc.
I am able to manage prices and names of the products (these are expected to change every now and then).
On invoices, products are added by quantity, with discount, vat, unicorns, squirrels, and so on, and then the total of the invoice is calculated.
Now, when a price is updated in the products table, the invoice is not supposed to change. It needs to stay the same forever.
When thinking about this, to me it seems like the product list is a list of "class type" objects, which describe what a product looks like. Or we could call it a "template" or "base" or "abstract", depending on what abstraction we think of.
The products that are added to the invoice are "instances", "implementations", "children", "versioned records" or whatever we'd call them.
How would I structure this in the database? I could split them into two different tables "AbstractProduct" and "Product", but they would have most (possibly all) of their columns in common. In that regard it would seem appropriate to keep them in the same table. But then I would need an extra column to signify whether it is "class" or "instance". That is fine too. This could also be a foreignkey to the class it was created from (or NULL for classes).
I have tried different approaches, but all seems to have annoying caveats.
Is this a known pattern? What is it called? Are there best practices for solving this issue?