fix: some tests still use old mongo and weren't passing

xmodule/modulestore/mongo/base.py

@@ -177,6 +177,11 @@ class CachingDescriptorSystem(MakoDescriptorSystem, EditInfoRuntimeMixin):  # li
             [str(key) for key in self.module_data.keys()],
             self.default_class,
         ))
+    
+    # This CachingDescriptorSystem runtime sets block._field_data on each block via construct_xblock_from_class(),
+    # rather than the newer approach of providing a "field-data" service via runtime.service(). As a result, during
+    # bind_for_student() we can't just set ._bound_field_data; we must overwrite block._field_data.
+    uses_deprecated_field_data = True
 
     def __init__(self, modulestore, course_key, module_data, default_class, **kwargs):
         """
@@ -303,6 +308,18 @@ class CachingDescriptorSystem(MakoDescriptorSystem, EditInfoRuntimeMixin):  # li
                     error_msg=exc_info_to_str(sys.exc_info())
                 )
 
+    def service(self, block, service_name):
+        """
+        Return a service, or None.
+        Services are objects implementing arbitrary other interfaces.
+        """
+        # A very minimal shim for compatibility with the new API for how we access field data in split mongo:
+        if service_name == 'field-data-unbound':
+            return block._field_data  # pylint: disable=protected-access
+        elif service_name == 'field-data':
+            return block._bound_field_data if hasattr(block, "_bound_field_data") else block._field_data
+        return super().service(block, service_name)
+
     def _convert_reference_to_key(self, ref_string):
         """
         Convert a single serialized UsageKey string in a ReferenceField into a UsageKey.

xmodule/x_module.py

@@ -669,6 +669,10 @@ class XModuleMixin(XModuleFields, XBlock):
             for wrapper in wrappers:
                 wrapped_field_data = wrapper(wrapped_field_data)
             self._bound_field_data = wrapped_field_data
+            if getattr(self.runtime, "uses_deprecated_field_data", False):
+                # This approach is deprecated but old mongo's CachingDescriptorSystem still requires it.
+                # For Split mongo's CachingDescriptor system, don't set ._field_data this way.
+                self._field_data = wrapped_field_data
 
     @property
     def non_editable_metadata_fields(self):