Add original_heading_id to investments

Investments can be reclassified to a different heading during the participatory budget process.

Whilst we are recording this change of heading in the `previous_heading_id` attribute, we are only keeping the _last_ heading. If there are multiple reclassifications we lose this chain of reclassifications.

In this commit we are adding an `original_heading_id` attribute, that will only be set once, when creating the investment, and will not get lost with multiple reclassificaitons of an investment.

app/models/budget/investment.rb

@@ -104,6 +104,7 @@ class Budget
 
     scope :for_render, -> { includes(:heading) }
 
+    before_create :set_original_heading_id
     before_save :calculate_confidence_score
     after_save :recalculate_heading_winners
     before_validation :set_responsible_name
@@ -404,6 +405,10 @@ class Budget
         self.budget_id ||= heading&.group&.budget_id
       end
 
+      def set_original_heading_id
+        self.original_heading_id = heading_id
+      end
+
       def change_log
         self.changed.each do |field|
           unless field == "updated_at"

db/migrate/20190531173316_add_original_heading_id_to_investments.rb

@@ -0,0 +1,5 @@
+class AddOriginalHeadingIdToInvestments < ActiveRecord::Migration[5.0]
+  def change
+    add_column :budget_investments, :original_heading_id, :integer
+  end
+end

db/schema.rb

@@ -294,6 +294,7 @@ ActiveRecord::Schema.define(version: 20190607160900) do
     t.datetime "ignored_flag_at"
     t.integer  "flags_count",                                 default: 0
     t.integer  "tracker_assignments_count"
+    t.integer  "original_heading_id"
     t.index ["administrator_id"], name: "index_budget_investments_on_administrator_id", using: :btree
     t.index ["author_id"], name: "index_budget_investments_on_author_id", using: :btree
     t.index ["community_id"], name: "index_budget_investments_on_community_id", using: :btree

spec/models/budget/investment_spec.rb

@@ -56,6 +56,29 @@ describe Budget::Investment do
     expect(investment.group_id).to eq group_2.id
   end
 
+  it "logs previous heading value if it is changed" do
+    budget = create(:budget, phase: "balloting")
+
+    group = create(:budget_group, budget: budget)
+
+    heading_1 = create(:budget_heading, group: group)
+    heading_2 = create(:budget_heading, group: group)
+
+    investment = create(:budget_investment, heading: heading_1)
+
+    expect(investment.previous_heading_id).to eq nil
+
+    investment.update(heading: heading_2)
+
+    expect(investment.previous_heading_id).to eq heading_1.id
+  end
+
+  it "stores original heading id" do
+    investment = create(:budget_investment)
+
+    expect(investment.original_heading_id).to eq investment.heading_id
+  end
+
   describe "#unfeasibility_explanation blank" do
     it "is valid if valuation not finished" do
       investment.unfeasibility_explanation = ""