Inserted data from a controller to another


I’m having a problem regarding some data’s inserts.

I have 2 different controllers, and I would need a shared data_field, I
tried then to copy-paste the _form, but I always receive the same error

“undefined method `model_name’ for NilClass:Class”

Briefly, how could I do a shared text_field , in the way that when I
insert the datas from the second controller, I’ll have it also on the
first one?