add with_diffs to MeasureManager; it allows to generate a dynamic property which generate diff value between two instances.

heima/sherlock/models.py

@@ -1,3 +1,4 @@
+from datetime import date
 from django.db import models
 
 
@@ -25,6 +26,16 @@ class DiffMeasure(object):
         self.timedelta = abs(measure_1.moment - measure_2.moment)
 
 
+def diff_from_measures(measure1, measure2):
+    """Generate the diff between the two measures.
+
+    Returns:
+        The daily difference between the two measures.
+    """
+    number_of_days = measure2.moment - measure1.moment
+    return (measure2.value - measure1.value) / number_of_days
+
+
 class MeasureManager(models.Manager):
     def create_with_deps(self, moment, value, gauge_sn, category_label):
         cat, cat_created = Category.objects.get_or_create(name=category_label)
@@ -33,6 +44,12 @@ class MeasureManager(models.Manager):
 
         return measure
 
+    def with_diffs(self):
+        default_measure = None
+        for measure in self.get_queryset().iterator():
+            if default_measure:
+                setattr(Measure, 'diff', diff_from_measures(default_measure, measure))
+
 
 class Measure(models.Model):
     moment = models.DateTimeField()

heima/sherlock/tests.py

@@ -85,6 +85,8 @@ class TestMeasureManager(TestCase):
         self.assertEqual(cat, m2.category)
         self.assertEqual(gauge, m2.gauge)
 
+    def test_with_diffs(self):
+        raise NotImplementedError('Not implemented yet')
 
 class TestMeasureModel(TestCase):