diff --git a/app/models/budget/ballot.rb b/app/models/budget/ballot.rb
index bf7851883..9b9a089d6 100644
--- a/app/models/budget/ballot.rb
+++ b/app/models/budget/ballot.rb
@@ -11,12 +11,12 @@ class Budget
       investments.sum(:price).to_i
     end
 
-    def amount_spent(heading)
-      investments.by_heading(heading).sum(:price).to_i
+    def amount_spent(heading_id)
+      investments.by_heading(heading_id).sum(:price).to_i
     end
 
     def amount_available(heading)
-      budget.heading_price(heading) - amount_spent(heading)
+      budget.heading_price(heading) - amount_spent(heading.try(:id))
     end
   end
 end
diff --git a/app/models/budget/investment.rb b/app/models/budget/investment.rb
index 32ea9ad92..329c2fce2 100644
--- a/app/models/budget/investment.rb
+++ b/app/models/budget/investment.rb
@@ -42,6 +42,7 @@ class Budget
     scope :undecided,              -> { where(feasibility: "undecided") }
     scope :with_supports,          -> { where('cached_votes_up > 0') }
 
+    scope :by_heading,  -> (heading_id)  { where(heading_id: heading_id) }
     scope :by_budget,   -> (budget_id)   { where(budget_id: budget_id) }
     scope :by_admin,    -> (admin_id)    { where(administrator_id: admin_id) }
     scope :by_tag,      -> (tag_name)    { tagged_with(tag_name) }
diff --git a/spec/models/budget/ballot_spec.rb b/spec/models/budget/ballot_spec.rb
index df31c9eae..ee6be74f1 100644
--- a/spec/models/budget/ballot_spec.rb
+++ b/spec/models/budget/ballot_spec.rb
@@ -16,9 +16,7 @@ describe Budget::Ballot do
 
       expect(ballot.total_amount_spent).to eq 30000
     end
-  end
 
-  describe "#amount_spent by heading" do
     it "returns the amount spent on all investments assigned to a specific heading" do
       heading = create(:budget_heading)
       inv1 = create(:budget_investment, :feasible, price: 10000, heading: heading)
@@ -40,4 +38,31 @@ describe Budget::Ballot do
     end
   end
 
-end
\ No newline at end of file
+  describe "#amount_available" do
+    it "returns how much is left after taking some investments" do
+      budget = create(:budget, price: 200000)
+      heading = create(:budget_heading, budget: budget)
+      inv1 = create(:budget_investment, :feasible, price: 10000, heading: heading)
+      inv2 = create(:budget_investment, :feasible, price: 20000, heading: create(:budget_heading))
+      inv3 = create(:budget_investment, :feasible, price: 25000)
+      inv4 = create(:budget_investment, :feasible, price: 40000, heading: heading)
+
+      inv1 = create(:budget_investment, :feasible, price: 10000)
+      inv2 = create(:budget_investment, :feasible, price: 20000)
+
+      ballot = create(:budget_ballot, budget: budget)
+      ballot.investments << inv1
+      ballot.investments << inv2
+
+      expect(ballot.amount_available(heading)).to eq 1000000
+      expect(ballot.amount_available(nil)).to eq 170000
+
+      ballot.investments << inv3
+      ballot.investments << inv4
+
+      expect(ballot.amount_available(heading)).to eq 960000
+      expect(ballot.amount_available(nil)).to eq 145000
+    end
+  end
+
+end
diff --git a/spec/models/budget_spec.rb b/spec/models/budget_spec.rb
index d874148a1..398bb1cff 100644
--- a/spec/models/budget_spec.rb
+++ b/spec/models/budget_spec.rb
@@ -1,15 +1,52 @@
 require 'rails_helper'
 
 describe Budget do
-  it "validates the phase" do
-    budget = create(:budget)
-    Budget::VALID_PHASES.each do |phase|
-      budget.phase = phase
-      expect(budget).to be_valid
+  describe "phase" do
+    let(:budget) { create(:budget) }
+
+    it "is validated" do
+      Budget::VALID_PHASES.each do |phase|
+        budget.phase = phase
+        expect(budget).to be_valid
+      end
+
+      budget.phase = 'inexisting'
+      expect(budget).to_not be_valid
     end
 
-    budget.phase = 'inexisting'
-    expect(budget).to_not be_valid
+    it "produces auxiliary methods" do
+      budget.phase = "on_hold"
+      expect(budget).to be_on_hold
+
+      budget.phase = "accepting"
+      expect(budget).to be_accepting
+
+      budget.phase = "selecting"
+      expect(budget).to be_selecting
+
+      budget.phase = "balloting"
+      expect(budget).to be_balloting
+
+      budget.phase = "finished"
+      expect(budget).to be_finished
+    end
+  end
+
+  describe "heading_price" do
+    let(:budget) { create(:budget, price: 1000) }
+
+    it "returns the budget price if no heading is provided" do
+      expect(budget.heading_price(nil)).to eq(1000)
+    end
+
+    it "returns the heading price if the heading provided is part of the budget" do
+      heading = create(:budget_heading, price: 100, budget: budget)
+      expect(budget.heading_price(heading)).to eq(100)
+    end
+
+    it "returns -1 if the heading provided is not part of the budget" do
+      expect(budget.heading_price(create(:budget_heading))).to eq(-1)
+    end
   end
 end