Hi!

We can use Expression constraint on the object's property and object itself will be accessible from inside the expression via this keyword. However, when validating plain arrays instead of objects, we can't access array itself, in this case this has a null value. I think we should also allow entire array to be accessed from inside the expression.

Here's example of how it can be used with form builder:
https://gist.github.com/slavafomin/805d36fe671fbdc3aced