I can't find a solution to use the value of a lookup column in a list, can anyone help?

Question

I am trying to create a kind of registration tool with a Sharepoint list (next step could be to create an app), but I am struggling to get the value of a lookup column. I read a lot of forums and tried a few things but I must be doing something wrong and I can't figure out what it is...

I have a list called "Yoga Schedule", where the goal is to show the dates for the classes and how many slots are still available for registration.

Then I have a second list for registration, where the person will need to enter the date they want to register, which is a lookup column from the list "Yoga Schedule", and their name.

So to show the remaining slots for that date in list "Yoga Schedule", I created a column called "Registered", which is a lookup (Count Related) of the date in list "Yoga Registration"

My next step would be to calculate "Max Slots" minus "Registered", but I know lookup columns can't be used in calculated columns, so I tried to work it out with Microsoft flows by creating a mirror of my "Registered" column that can be used in a calculation for the "Remaining Slots". My flow seems to work but I don't get any value in column "Registered Mirror", it's all empty.

Am I doing something wrong?

Answer 1

Sample test demo:

Show Parent item id in lookup column(so you could use it in Flow).

Call SharePoint rest api to get the lookup count so you could use to calculate.

Answer 2

Thank you Lee_MSFT, I think you are onto something. Unfortunately I don't have that much experience with programming so even using an API can be tricky for me. I did some reading and it looks like it can work, as you said, but I think I am not doing it 100% right because it still doesn't work even after trying to add the parenting ID in the lookup. Can you see if I am doing something wrong?