A Mesosphere rule to make sure a check box array has at least one field checked.

exact_checked_rule

Mesosphere.registerRule("exactChecked", function(fieldValue, ruleValue, fieldName, formFieldsObject, fieldRequirements){
    if(_(formFieldsObject[fieldName]).isArray() && formFieldsObject[fieldName].length == ruleValue){
        return true;
    }
    return false;
});