Ensure unique EPP session id

#700

app/models/epp_session.rb

@@ -2,5 +2,5 @@ class EppSession < ActiveRecord::Base
   belongs_to :user, required: true
   belongs_to :registrar
 
-  validates :session_id, presence: true
+  validates :session_id, uniqueness: true, presence: true
 end

test/models/epp_session_test.rb

@@ -20,4 +20,18 @@ def test_invalid_without_user
     @epp_session.validate
     assert @epp_session.invalid?
   end
+
+  def test_invalid_if_persisted_record_with_the_same_session_id_exists
+    epp_session = EppSession.new(session_id: @epp_session.session_id, user: @epp_session.user)
+    epp_session.validate
+    assert epp_session.invalid?
+  end
+
+  def test_database_session_id_unique_constraint
+    epp_session = EppSession.new(session_id: @epp_session.session_id, user: @epp_session.user)
+
+    assert_raises ActiveRecord::RecordNotUnique do
+      epp_session.save(validate: false)
+    end
+  end
 end