creates setter association for budget investments

app/models/budget/investment.rb

@@ -18,6 +18,8 @@ class Budget
     has_many :valuators, through: :valuator_assignments
     has_many :comments, as: :commentable
 
+    delegate :budget, to: :heading
+
     validates :title, presence: true
     validates :author, presence: true
     validates :description, presence: true
@@ -118,6 +120,10 @@ class Budget
       heading.group.budget
     end
 
+    def budget=(resource)
+      heading.group.budget = resource
+    end
+
     def undecided?
       feasibility == "undecided"
     end