How do i save the initial time a boolean went from false to true

I have a boolean column in my table and a datetime related to it. I want to be able to track down when the boolean initially turns true. In the model I have tried

def example
if boolean_field == true
datetime_field = time.now
end
end
but the problem with that is when the boolean is set to true, the datetime_field will keep updating everytime i reload the page or database since the boolean will always be in a constant state of true. I have also try to use rails dirty on it with:

boolean_field_changed?
but that method doesn't seems to update the datetime column at all. P.S i do have a before_save callback on the method.

any help on how i can save the initial time once the boolean_field changes from false to true will be much appreciated. thanks in advance

Hey Dim,

Just check to see if the time field is blank

def example
  if boolean_field == true && datetime_field.blank?
      datetime_field = Time.now
  end
end

Hey Jacob, Thank you so much, can't believe this was going over my head. hahah much appreciated man