search for: demand1

Hi, I have two products which are substitudes. I try to fix a system as below to mydata. Demand1 = A1 -B1*Price1 + C1*Price2 Demand2 = A2 +B2*Price1 - C2*Price2 I would expect C1 & B2 to be symmetric, If they are truly substitude. How can I enforce this symmetry when creating a system of equations via SystemFit ? -- View this message in context: http://www.nabble.com/SystemFit-tp2458825...

Hi, I am trying to use method `validates_presence_of'' for validating my input fields on form . I m using it in my action class as follows:- ======================= validates_presence_of(params[:product][:quantity]) ====================== But when I am running my application i m getting error like:- ========================= undefined method `validates_presence_of'' for