My ChatGPT-mate says :
You're on a great path already, and this is a very classic scheduling/booking problem with edge-case wrinkles like same-day check-out/in — so you're right to think carefully at the high level before getting lost in looping logic or complex rules.
Here’s a clean, high-level strategy to catch double bookings for units — one that scales and stays relatively tidy:
✅ High-Level Approach: Think in Terms of Date Ranges Per Unit
Instead of checking reservations or units individually, build a structure that represents each reservation as a date range per unit, and then look for overlaps.
🧠 Step-by-Step Strategy:
Build a Reservation Timeline per Unit
For each Unit, get all Reservations that are linked to it.
For each Reservation, create a "stay window" from CheckInDate to CheckOutDate exclusive (i.e., guest checks out on the last date, so that date is available to others).
This way, check-out and check-in on the same date won’t count as a clash.
Sort Reservations by CheckInDate
Within each Unit's reservation list, sort by CheckInDate.
Iterate Through the Sorted List and Compare Overlaps
Go through the sorted list of reservations per unit, and check each reservation against the next one in the list.
If next.CheckInDate < current.CheckOutDate → that’s an overlap (a double booking).
If next.CheckInDate == current.CheckOutDate → this is valid, no overlap (check-out/check-in allowed).
This pairwise comparison avoids the O(n²) mess of comparing every reservation with every other one and gives you clear, easy-to-flag cases.
🧱 BO Structure Assumptions:
BO Reservation:
CheckInDate
CheckOutDate
Reference to BO Unit
BO Unit:
One-to-many to BO Reservation
BO UnitStatus:
You can use this for visualization or planning, but it’s not needed for detecting double bookings, since reservation date ranges carry the necessary info.
✨ Optional Enhancements:
After detecting overlaps, create a DoubleBooking BO with links to both reservations and the conflicting unit, so you can easily display/report them.
Allow exceptions (e.g., admin override or maintenance bookings) with flags in Reservation.
🧩 Why the Other Approaches Got Messy
Looping through Reservations and querying overlapping ones dynamically creates combinatorial chaos, especially when many Units or date ranges are involved.
Trying to use UnitStatus per date can feel intuitive, but it leads to massive loops or logic-heavy checks that miss the elegance of sorted date ranges.
