Add LmsCompatibilityMixin to OpenAssessmentBlock.

edx-platform expects certain things during the grade calculation
process. This adds a few fields and tests for that purpose.

[TIM-232]

apps/openassessment/xblock/lms_mixin.py

+from xblock.core import XBlock
+from xblock.fields import Float, Scope
+
+class LmsCompatibilityMixin(object):
+    """Extra methods we tack on because the LMS expects them for grading."""
+    weight = Float(
+        display_name="Problem Weight",
+        help=("Defines the number of points each problem is worth. "
+              "If the value is not set, the problem is worth the sum of the "
+              "option point values."),
+        values={"min": 0, "step": .1},
+        scope=Scope.settings
+    )
+
+    def has_dynamic_children(self):
+        """Do we dynamically determine our children? No, we don't have any.
+
+        The LMS wants to know this to see if it has to instantiate our module
+        and query it to find the children, or whether it can just trust what's
+        in the static (cheaper) children listing.
+        """
+        return False
+
+    @property
+    def has_score(self):
+        """Are we a scored type (read: a problem). Yes.
+
+        For LMS Progress page/grades download purposes, we're always going to
+        have a score, even if it's just 0 at the start.
+        """
+        return True
+
+    def max_score(self):
+        """The maximum raw score of our problem.
+
+        Called whenever the LMS knows that something is scorable, but finds no
+        recorded raw score for it (i.e. the student hasn't done it). In that
+        case, the LMS knows that the earned score is 0, but it doesn't know what
+        to put in the denominator. So we supply it with the total number of
+        points that it is possible for us to earn -- the sum of the highest
+        pointed options from each criterion.
+
+        Note that if we have already recorded a score in submissions, this
+        method will never be called. So it's perfectly possible for us to have
+        10/10 on the progress page and a 12 returning from this method if our
+        10/10 score was earned in the past and the problem has changed since
+        then.
+        """
+        return sum(
+            max(option["points"] for option in criterion["options"])
+            for criterion in self.rubric_criteria
+        )

apps/openassessment/xblock/openassessmentblock.py

@@ -16,6 +16,7 @@ from xblock.fragment import Fragment
 from openassessment.xblock.grade_mixin import GradeMixin
 
 from openassessment.xblock.peer_assessment_mixin import PeerAssessmentMixin
+from openassessment.xblock.lms_mixin import LmsCompatibilityMixin
 from openassessment.xblock.self_assessment_mixin import SelfAssessmentMixin
 from openassessment.xblock.submission_mixin import SubmissionMixin
 from openassessment.xblock.studio_mixin import StudioMixin
@@ -165,7 +166,8 @@ class OpenAssessmentBlock(
     SelfAssessmentMixin,
     StudioMixin,
     GradeMixin,
-    WorkflowMixin):
+    WorkflowMixin,
+    LmsCompatibilityMixin):
     """Displays a question and gives an area where students can compose a response."""
 
     start = String(
@@ -503,4 +505,4 @@ class OpenAssessmentBlock(
         """
         for assessment in self.rubric_assessments:
             if assessment["name"] == mixin_name:
-                return assessment
\ No newline at end of file
+                return assessment

apps/openassessment/xblock/test/test_lms.py

+"""
+Tests for the LMS compatibility mixin for the OpenAssessment block.
+"""
+from ddt import ddt
+from .base import scenario, XBlockHandlerTestCase
+
+
+@ddt
+class LmsMixinTest(XBlockHandlerTestCase):
+    """Test the simple LMS-specific attributes used during grading."""
+
+    @scenario('data/basic_scenario.xml')
+    def test_simple_methods(self, xblock):
+        self.assertTrue(xblock.has_score)
+        self.assertFalse(xblock.has_dynamic_children())
+        self.assertTrue(hasattr(xblock, 'weight'))
+
+    @scenario('data/basic_scenario.xml')
+    def test_max_score(self, xblock):
+        self.assertEqual(xblock.max_score(), 20)