How to implement validation rule on multi select picklist

Question

Suppose i have a multi select picklist which has 30 items. Now i have to put a validation rule that "user can't choose all the items from picklist. Atleast one item should be available in available list."

How can i implement this validation.

I know i can do like " AND(INCLUDES(item_c,'1'),INCLUDES(item_c,'2'),INCLUDES(item__c,'3'),.... etc) for all values but i don't want to follow this approach as i will have to hard code all the values.

Answer 1

What you're asking can't be accomplished through a validation rule, without hand coding every value like you suggested.

On the other hand this could very easily be accomplished with a before update trigger that looks something like this:

trigger MyTrigger on MyObject__c (before update)
{
    //this variable could also be gotten through use of the system describe methods
    Integer totalPicklistItems = 30;

    for(MyObject__c obj : Trigger.new)
    {
        if(obj.MultiSelectField__c != null && obj.MultiSelectField__c != '')
        {
            //split picklist into list
            String[] selectedValues = obj.MultiSelectField__c.split(';', 0);

            //check if the selected selected values are less than 1 or equal to the
            //total picklist items to add an error
            if(selectedValues.size() < 1 || selectedValues.size() == totalPicklistItems)
                obj.MultiSelectField__c.addError('You must select at least 1 value but not all values.');
        }
        else
        {
            obj.MultiSelectField__c.addError('You must select at least 1 value.');
        }
    }
}

Answer 2

I've seen similar issues solved by doing an addition of If statement outputs.

Essentially, one if() statement per picklist value

If(includes(picklist, value1), 1, 0) +
If(includes(picklist, value1), 1, 0) +
... value n

Then wrap it in an if statement saying if total = n, fail to validate, else, record is valid.