Require a field only if a checkbox is checked CakePHP 2 Validation - validation

Question

Heres the problem
I want to require a field (litters_per_year) only if another field that is a checkbox is checked. When I do this, cake is trying to force me to put a value into the field and I don't know why. I have tried setting required & allowEmpty to false & true respectively, but then my custom rule does not run.
Heres the code
NOTE: The details of the following code aren't that important - they are here to provide a scenario.
I have the following code in my VIEW which works fine:
echo $this->Form->input('litters_per_year', array(
'label' => 'Litters per year (average)'
));
I have the following code in my MODEL's public $validate:
'litters_per_year' => array(
'isNeeded' => array(
'rule' => array('isNeeded', 'litters_per_year'),
'message' => 'Please enter the litters per year average'
)
)
which is calling the custom validation method
public function isNeeded($field) {
// Check if a checkbox is checked right here
// Assume it is not... return false
return false;
}
It returns false for simplicity to solve this issue.
Let's assume that the checkbox field is named 'the_checkbox'.

Answer 1

At the moment your field should always fail validation, since you return false from isNeeded.
To make it work as you expect, do something like this:
(Note: replace 'ModelName' with your model name)
public function isNeeded($field) {
if ($this->data['ModelName']['the_checkbox']) {
// Checkbox is checked, so we have to require litters per year
if (empty($field)) {
// They have checked the box but have NOT entered litters_per_year
// so we have a problem. NOT VALID!
return false;
} else {
// They have checked the box, and entered a litters_per_year
// value, so all good! Everything is valid!
return true;
}
} else {
// Checkbox is not checked, so we don't care what
// the field contains - it is always valid.
return true;
}
}
Or, without the needless verbosity, this should work:
public function isNeeded($field) {
if ($this->data['ModelName']['the_checkbox']) {
return $field;
} else {
return true;
}
}
In this example, if the checkbox is checked, validation will pass if $field is truthy, and fail if it is falsey.