I think Alan is right. Rails is expected a date, but you are passing it an array.
Use date_select in your form instead of an array of values. (ActionView::Helpers::DateHelper)
That is the correct usage, but your controller is trying to pull the date submitted apart and force it into the database. Since the date fields are named with the database field name you can just use:
Exhibition.create(params[:details])
instead of:
@exhibition.open_date = params[:details][:open_date] @exhibition.save
If you want some error checking, you might want to try something like:
@exhibition = Exhibition.new(params[:details]) if @exhibition.save flash[:notice] = "Exhibition created" redirect_to :action => :index and return