Sometimes one needs to be sure that the value of the property in collection is unique for each record. As an example - you need to be sure that name of the company / restaurant / event / etc. is unique and record with this name can’t be added for the 2nd time.
UPDATE: added a second video how to make a modal window with warnings. Thanks for your comments inspiring that!
Here is the short tutorial how to prevent from creating “duplicate” records:
Second video with modal warning: