Your welcome 
Then how about doing like this.( the idea that came to my mind now)
You can create a relationship property (Many to Many) between the Exercise collection and the Users collection.( I’m naming this property as Completed Users.) And you can filter one list as Completed Users>All>Does not contain>Logged in user.( Not completed exercises). And other list as Completed Users> All>Contains>Logged in user.( Completed Exercises). And you can add a update action to the button to add the logged in user to the Completed Users property of the current exercise.
Here’s how I did.
Hope it helps!
Thank you