Q:

Field collection: Can I make a field mandatory depending on another field's value?

I'm trying to set up a custom solution using Field Collection consisting of:

term reference (autocomplete tagging style)
plain text.

What I need to achieve is to react to Javascript event of autocomplete (but only for that field) and - if the selected term title contains a string placeholder %1 - make the plain text mandatory, so that user is force to enter the substitution text.
Is that technically possible or would it require setting up a custom collection from scratch?

A:

The Conditional Fields module can help you to achieve this. 
You will probably need to create an explicit list of taxonomy terms for the case when you want the plain text field to appear and to be required instead of checking the existence of the aforementioned placeholder.

