I have an application with a lot of database relationships that depend on each other to successfully operate the application. The hinge in the application is a model called the Schedule, but the schedule will pull Blocks, an Employee, a JobTitle, and an Assignment (in addition to that, every Block will pull an assignment from the database along with it as well) to assemble an employees schedule throughout the day.
When I built the app, I put a lot of emphasis on validations that would ensure that all of the pieces had to be in place before everything was saved to the database. This has worked out fantastically so far, and the app has been live and pounded on for almost 6 months, serving approximately 150,000 requests a month with no hiccups or errors. Until last week.
Last week, while someone was altering a schedule, it looks like the database erred, and a Schedule was saved to the database with its Assignment missing. Because the association is called in every view, whenever this schedule was called from the database, the application would throw an NoMethod error for calling on nil.
When designing an application in the way that I state, do you guard against a possible failure on the part of the database/validations? And if so how do you programatically defend against it? Do you check every relationship to make sure that it is not nil before sending it to the view?
I know this question is awash in generality, and if I can be more specific in what I mean, please let me know in the comments.
I would recommend adding database-enforced foreign key constraints and wrapping important groups of operations into transactions.
If there is a foreign-key between Schedule and Assignment somewhere, a database-enforced foreign key constraint would have prevented the errant insert. Additionally, if you wrap the particular action in a transaction, you can be sure that either the entire stream of inserts/updates/deletes happens or fails, reverting to a clean state.