Email Template - The template does not have a controller that can be selected for preview

Question

When I include the following code inside the "subject" of my visualforce email template I can't use "send test and verify merge fields" - the message 'The template does not have a controller that can be selected for preview.' appears.

{! IF( AND(relatedTo.event__r.start_date__c != null, relatedTo.event__r.end_date__c != null, 
(DAY(relatedTo.event__r.start_date__c) > DAY(relatedTo.event__r.end_date__c))), 
CASE(MONTH(relatedTo.event__r.start_date__c),
 1, ' January',
 2, ' February',
 3, ' March',
 4, ' April',
 5, ' May',
 6, ' June',
 7, ' July',
 8, ' August',
 9, ' September',
 10, ' October',
 11, ' November',
 12, ' December',
 ''
)
,'')}

In the email subject I want to get:

Confirmation - Event, 14-15 September, City, Country 

(if event date is 14-15 september)

OR if the dates go over the month 29 Sep - 1 October

Confirmation - Event, 29 September - 1 October, City, County.

Full Code (only the above seems to be causing an issue):

<messaging:emailTemplate subject="Confirmation - {! relatedTo.event__r.name }, 
{! DAY(relatedTo.event__r.start_date__c)}
{! IF( AND(relatedTo.event__r.start_date__c != null, relatedTo.event__r.end_date__c != null, 
(DAY(relatedTo.event__r.start_date__c) > DAY(relatedTo.event__r.end_date__c))), 
CASE(MONTH(relatedTo.event__r.start_date__c),
 1, ' January',
 2, ' February',
 3, ' March',
 4, ' April',
 5, ' May',
 6, ' June',
 7, ' July',
 8, ' August',
 9, ' September',
 10, ' October',
 11, ' November',
 12, ' December',
 ''
)
,'')}

-{!DAY(relatedTo.event__r._end_date__c)} 
{!CASE( MONTH(relatedTo.event__r.end_date__c),
 1, 'January',
 2, 'February',
 3, 'March',
 4, 'April',
 5, 'May',
 6, 'June',
 7, 'July',
 8, 'August',
 9, 'September',
 10, 'October',
 11, 'November',
 12, 'December',
 ''
)
}, 
{! relatedTo.event__r.city__c },
{! relatedTo.event__r.country__c }
" recipientType="User" relatedToType="Participant__c" >

Answer 1

There seems to be something a bit funky about date comparison, and possibly short circuit evaluation, when you put that formula into the subject - (works fine in body of email).

If you just put in

IF( DAY(relatedTo.date_field_1__c) >
DAY(relatedTo.date_field_2__c), 'good', 'bad')

It gives error -

The value 'null' is not valid for operator '>'.

With the null check in the AND()

IF(AND(relatedTo.date_field_1__c != null,
relatedTo.date_field_2__c != null,
DAY(relatedTo.date_field_1__c) >
DAY(relatedTo.date_field_2__c)), 'good', 'bad')

Works fine in body, but when in subject it gives the above error.

If you put in:

IF(AND(false, DAY(relatedTo.date_field_1__c) >
DAY(relatedTo.date_field_2__c)), 'good', 'bad') 

You still get the error which suggests to me it's something to do with the day comparison getting evaluated somewhere.

One solution - create a checkbox formula field on the object that compares the day value of the two date fields

DAY(date_field_1__c) > DAY(date_field_2__c)

and reference that in the visualforce IF formula