Extract "supported headings" logic to User method

In preparation to use this method from views where
it doesn't make sense for it to be associated with
a specific investment.

app/models/budget/investment.rb

@@ -257,7 +257,7 @@ class Budget
     end
 
     def can_vote_in_another_heading?(user)
-      headings_voted_by_user(user).count < group.max_votable_headings
+      user.headings_voted_within_group(group).count < group.max_votable_headings
     end
 
     def headings_voted_by_user(user)
@@ -265,7 +265,7 @@ class Budget
     end
 
     def voted_in?(heading, user)
-      headings_voted_by_user(user).include?(heading.id)
+      user.headings_voted_within_group(group).where(id: heading.id).exists?
     end
 
     def ballotable_by?(user)

app/models/user.rb

@@ -127,6 +127,11 @@ class User < ActiveRecord::Base
     votes.for_budget_investments(Budget::Investment.where(group: group)).exists?
   end
 
+  def headings_voted_within_group(group)
+    voted_investments = votes.for_budget_investments(Budget::Investment.by_group(group.id)).votables
+    Budget::Heading.where(id: voted_investments.map(&:heading_id).uniq)
+  end
+
   def administrator?
     administrator.present?
   end

spec/models/user_spec.rb

@@ -2,6 +2,35 @@ require 'rails_helper'
 
 describe User do
 
+  describe '#headings_voted_within_group' 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(user1.headings_voted_within_group(group)).to include(new_york)
+      expect(user1.headings_voted_within_group(group)).to include(san_franciso)
+      expect(user1.headings_voted_within_group(group)).to_not include(another_heading)
+
+      expect(user2.headings_voted_within_group(group)).to_not include(new_york)
+      expect(user2.headings_voted_within_group(group)).to_not include(san_franciso)
+      expect(user2.headings_voted_within_group(group)).to_not include(another_heading)
+    end
+  end
+
   describe "#debate_votes" do
     let(:user) { create(:user) }