Add headings_voted_by_user

This method was used only in Madrid’s fork, but it is now needed to
complete the backport for voting in multiple headings

There wasn’t a test in Madrid, so here goes one too. Even though, the
responsibility should probably be moved soon to the `Budget::Heading`.
For consistency with the related methods and tests it has been left in
the investment_spec

app/models/budget/investment.rb

@@ -239,6 +239,10 @@ class Budget
       headings_voted_by_user(user).count < group.max_votable_headings
     end
 
+    def headings_voted_by_user(user)
+      user.votes.for_budget_investments(budget.investments.where(group: group)).votables.map(&:heading_id).uniq
+    end
+
     def voted_in?(heading_ids, user)
       heading_ids.include? heading_voted_by_user?(user)
     end

spec/models/budget/investment_spec.rb

@@ -673,6 +673,35 @@ describe Budget::Investment do
     end
   end
 
+  describe "#headings_voted_by_user" do
+    it "returns the headings voted by a user" do
+      user1 = create(:user)
+      user2 = create(:user)
+
+      budget = create(:budget)
+      group = create(:budget_group, budget: budget)
+
+      new_york = create(:budget_heading, group: group)
+      san_franciso = create(:budget_heading, group: group)
+      another_heading = create(:budget_heading, group: group)
+
+      new_york_investment = create(:budget_investment, heading: new_york)
+      san_franciso_investment = create(:budget_investment, heading: san_franciso)
+      another_investment = create(:budget_investment, heading: san_franciso)
+
+      create(:vote, votable: new_york_investment, voter: user1)
+      create(:vote, votable: san_franciso_investment, voter: user1)
+
+      expect(another_investment.headings_voted_by_user(user1)).to include(new_york.id)
+      expect(another_investment.headings_voted_by_user(user1)).to include(san_franciso.id)
+      expect(another_investment.headings_voted_by_user(user1)).to_not include(another_heading.id)
+
+      expect(another_investment.headings_voted_by_user(user2)).to_not include(new_york.id)
+      expect(another_investment.headings_voted_by_user(user2)).to_not include(san_franciso.id)
+      expect(another_investment.headings_voted_by_user(user2)).to_not include(another_heading.id)
+    end
+  end
+
   describe "Order" do
     describe "#sort_by_confidence_score" do