Introduced here -> https://github.com/3scale/porta/pull/2364/changes#diff-a863929d619dcebce8978d6057036c1b6a17ed95c870d98d2db470997cfc0553R8

The predicate would alsways be false (conparing string and symbol).

However, it looks like the type of fields definitions is always builtin.

On the other hand, Fields::FormBuilder#user_defined_form do generate que correct type.