groovy.lang.MissingPropertyException on reading property from a domain sub classs

[swzaidi]

I have the following domain classes in my test app

---

class PricePlan {

String id
double price

static mapping = {
    id type: 'string', generator: 'assigned'
    discriminator column: 'purchase_type'
}

}

---

class RecurringPricePlan extends PricePlan {

int durationInMonths

static mapping = {
    discriminator 'Subscription'
}

}

class SubscriptionPlan {

PricePlan pricePlan

static constraints = {
    pricePlan nullable: true

}

int getDurationInMonths() {

    pricePlan.durationInMonths
}

}

Steps to reproduce

1. save RecurringPricePlan
2. Assign the saved RecurringPricePlan to new SubscriptionPlan
3. save SubscriptionPlan
4. Find SubscriptionPlan by id
5. invoke subscriptionPlan.getDurationInMonths()
6. getting following exception
   groovy.lang.MissingPropertyException: No such property: durationInMonths for class: RecurringPricePlan
   Possible solutions: durationInMonths

Can you please let me know why i am seeing this error?