add payment tests

spec/factories/payments.rb

@@ -0,0 +1,9 @@
+FactoryGirl.define do
+  factory :payment do
+    user
+    conference
+    last4 4242
+    authorization_code 1234567890
+    amount 10
+  end
+end

spec/models/payment_spec.rb

@@ -0,0 +1,44 @@
+require 'spec_helper'
+
+describe Payment do
+
+  context 'new payment' do
+    let(:payment) { create(:payment) }
+    it 'sets status to "unpaid" by default' do
+      expect(payment.status).to eq('unpaid')
+    end
+  end
+
+  describe 'validations' do
+    it 'has a valid factory' do
+      expect(build(:payment)).to be_valid
+    end
+
+    it { is_expected.to validate_presence_of(:last4) }
+
+    it { is_expected.to validate_presence_of(:amount) }
+
+    it { is_expected.to validate_presence_of(:authorization_code) }
+
+    it { is_expected.to validate_presence_of(:status) }
+
+    it { is_expected.to validate_presence_of(:user_id) }
+
+    it { is_expected.to validate_presence_of(:conference_id) }
+
+    it 'is not valid with a amount equals zero' do
+      should_not allow_value(0).for(:amount)
+    end
+
+    it 'is not valid with a amount smaller than zero' do
+      should_not allow_value(-1).for(:amount)
+    end
+
+    it 'is valid with a amount greater than zero' do
+      should allow_value(1).for(:amount)
+    end
+
+  end
+
+  describe 'self#purchase'
+end