Beat the soft butter and sugar. Add baking soda into the rye flour. Add the flour-soda mixture into the butter-sugar mixture. Set aside about 1/3 of the pastry for the lid of the pie. Use the rest (about 2/3) of the pastry to cover the pie dish. Mix the ingredients for the filling and put into the pie dish. Roll the remaining pastry into a lid on top of the blueberries.Bake for 1-1½ hours in a preheated oven (180 °C).Let the pie cool a while, and serve with vanilla ice cream or vanilla sauce. In winter, instead of fresh bilberries, use frozen bilberries. In this case the cooking time should be longer.