Car Rental Management System

The problem

A rental company's data is all constraints: a car cannot be on two contracts at the same time, a contract cannot end before it starts, a customer needs a valid licence. Enforce those only in C# and they hold until the day someone writes to the database another way.

What I did & decided

• Wrote down the business rules (19 of them, in docs/business_rules.md) and modelled the domain in a normalised 12-table PostgreSQL schema: people, customers, employees, licences, cars and categories, contracts, payments, discounts, penalties and returns.
• Put the overlap check in the database: a BEFORE INSERT and a BEFORE UPDATE trigger call fn_car_available and reject any contract whose dates collide with an existing one for the same car.
• Added an after-return trigger that updates the car's mileage and maintenance state, plus stored functions for a customer's active contracts and a car's age.
• Built seven MVC modules on top (cars, customers, employees, contracts, payments, licences, accounts) with ASP.NET Core Identity and role-based access.
• Committed the SQL scripts in order (01_tables → 05_seed_data) with an ERD and a requirements document, so the schema can be reviewed without reading any C#.

What came of it

• The rules survive a direct write: anything that bypasses the app still meets the same triggers.
• A double booking fails with a readable error naming the car and the dates, instead of becoming a support call.
• A reviewer can read the schema on its own, which is how a reviewer actually reads.