Navigation: Related field support to SourceColumn

Add support to the SourceColumn class to resolve related fields
using the double underscore as separator. Columns that use related
no longer have to use throw away lambdas.

Signed-off-by: Roberto Rosario <Roberto.Rosario@mayan-edms.com>

mayan/apps/navigation/classes.py

@@ -8,14 +8,15 @@ from furl import furl
 from django.apps import apps
 from django.conf import settings
 from django.contrib.admin.utils import label_for_field
-from django.core.exceptions import PermissionDenied
+from django.core.exceptions import FieldDoesNotExist, PermissionDenied
+from django.db.models.constants import LOOKUP_SEP
 from django.shortcuts import resolve_url
 from django.template import VariableDoesNotExist, Variable
 from django.template.defaulttags import URLNode
 from django.urls import Resolver404, resolve
 from django.utils.encoding import force_str, force_text
 
-from mayan.apps.common.utils import return_attrib
+from mayan.apps.common.utils import resolve_attribute
 from mayan.apps.permissions import Permission
 
 logger = logging.getLogger(__name__)
@@ -469,6 +470,26 @@ class Separator(Link):
 class SourceColumn(object):
     _registry = {}
 
+    @staticmethod
+    def get_attribute_recursive(attribute, model):
+        """
+        Walk over the double underscore (__) separated path to the last
+        field. Returns the field name and the corresponding model class.
+        Used to introspect the label or short_description of a model's
+        attribute.
+        """
+        last_model = model
+        for part in attribute.split(LOOKUP_SEP):
+            last_model = model
+            try:
+                field = model._meta.get_field(part)
+            except FieldDoesNotExist:
+                break;
+            else:
+                model = field.related_model or field.model
+
+        return part, last_model
+
     @staticmethod
     def sort(columns):
         return sorted(columns, key=lambda x: x.order)
@@ -507,11 +528,13 @@ class SourceColumn(object):
 
     @property
     def label(self):
-        # TODO: Add support for related fields (dotted or double underscore attributes)
         if not self._label:
             if self.attribute:
+                name, model = SourceColumn.get_attribute_recursive(
+                    attribute=self.attribute, model=self.source._meta.model
+                )
                 self._label = label_for_field(
-                    name=self.attribute, model=self.source._meta.model
+                    name=name, model=model
                 )
             else:
                 self._label = 'Function'
@@ -520,7 +543,7 @@ class SourceColumn(object):
 
     def resolve(self, context):
         if self.attribute:
-            result = return_attrib(context['object'], self.attribute)
+            result = resolve_attribute(context['object'], self.attribute)
         elif self.func:
             result = self.func(context=context)