Hello everyone, 

Our company supplies GPS tracking for vehicles, with a subscription plan (1 to 2-years contract) and with GPS tracking devices lent to our customers.

So, I have two custom modules:

- Vehicles (keeps record of all vehicle we are tracking)

- Tracking Devices (keeps record of all devices we have)

Every entry on Vehicles module have a lookup field to tracking device module, to record the tracking device installed in each car/vehicle.

I trying to find a way where i can know how many tracking devices we still have available for installation, by cross-checking two modules data, and not by adding a new field to tracking device module entries like "status: installed/available"

My question is:

Is there any way i can know how many and which tracking device hasn't yet being installed ( are not related to any vehicle)? 

Thank you in advance!

Rafael