enforce nonzero price in ratechecker API

Passing a 'price' parameter of 0 causes a DivisionByZero exception.
This change modifies the API to change price=0 to price=1. This solely
prevents this case in the API and has no effect on the application
frontend which already enforces a minimum price.

CHANGELOG.md

@@ -2,6 +2,7 @@ All notable changes to this project will be documented in this file.
 We follow the [Semantic Versioning 2.0.0](http://semver.org/) format.
 
 ## Unreleased
+- Enforce non-zero price in ratechecker API.
 
 ## 0.9.94 - 2017-02-02
 - Add a monitor to watch for changes in census county values

ratechecker/ratechecker_parameters.py

@@ -106,10 +106,14 @@ def validate(self, attrs):
 
     def validate_price(self, value):
         """
-        Validate price and convert to positive if negative
+        Validate price, convert to positive if negative, and enforce a
+        minimum value of 0.
         """
         if value < 0:
             value = abs(value)
+        elif value == 0:
+            value = Decimal('1')
+
         return value
 
     def validate_lock(self, value):

ratechecker/tests/test_views_ratecheckerparameters.py

@@ -160,12 +160,24 @@ def test_is_valid__loan_amount_negative(self):
         self.assertTrue(serializer.is_valid())
         self.assertEqual(serializer.validated_data.get('loan_amount'), Decimal('10000'))
 
+    def test_is_valid__price(self):
+        self.data['price'] = 1000
+        serializer = ParamsSerializer(data=self.data)
+        self.assertTrue(serializer.is_valid())
+        self.assertEqual(serializer.validated_data['price'], Decimal('1000'))
+
     def test_is_valid__price_negative(self):
         self.data['price'] = -10000
         serializer = ParamsSerializer(data=self.data)
         self.assertTrue(serializer.is_valid())
         self.assertEqual(serializer.validated_data.get('price'), Decimal('10000'))
 
+    def test_is_valid__price_zero(self):
+        self.data['price'] = 0
+        serializer = ParamsSerializer(data=self.data)
+        self.assertTrue(serializer.is_valid())
+        self.assertEqual(serializer.validated_data['price'], Decimal('1'))
+
     def test_is_valid__state_invalid(self):
         self.data['state'] = 123
         serializer = ParamsSerializer(data=self.data)