Preheat oven to 400F.
Mix everything but the rhubarb together, and then add the rhubarb at the end. Add filling to a 9" pie crust.
Bake at 400 for 15 minutes. Reduce heat to 350 and bake for another 25 minutes. Remove the pie, and sprinkle the crumb topping on top. Bake at 350 for another 15 minutes or until the topping is brown.
For the topping, simply mix the ingredients together until crumbly.