[TYPO3-mvc] Validate form field depending on another passed objects property

[Jan Kornblum]

Hi newsgroup,

i have to validate a form property depending on the value of another 
passed objects property. The two objects passed to the form 
(createAction) are:

- $booking (the object to be created)
- $event.

The property "$booking->extTermsAndConditions" should be validated as 
boolean (and must be TRUE) when "$event->extTermsAndConditonsRequired" 
is TRUE.

In case of "$event->extTermsAndConditionsRequired" is FALSE the 
property "$booking->extTermsAndConditions" should not be validated at 
all! So, i cannot set the validation directly in the model.

I want the validation results to be available in the form 
("f:form.validationResults"), too.

How can i accomplish this?

Kind regards, Jan