Preheat the oven to 350 degrees F (175 degrees C).

Grease and flour a 9x13 inch baking dish.

In a large bowl, cream together the softened butter and white sugar.

Add the baking soda, salt, and all-purpose flour to the butter mixture and mix well.

Stir in the beaten eggs and sour cream until the batter is smooth.

Gently fold in the diced rhubarb.

Pour the batter into the prepared baking dish and spread it evenly.

In a smaller bowl, mix together the brown sugar, white sugar, softened butter, all-purpose flour, and ground cinnamon until the topping is crumbly.

Sprinkle the crumbly topping mixture evenly over the cake batter.

Sprinkle the chopped walnuts evenly over the topping.

Bake in the preheated oven for 45 minutes, or until a toothpick inserted into the center comes out clean.