Backwards compatible approach to HyperlinkedIdentityField URLs. Closes #892

rest_framework/relations.py

@@ -518,8 +518,6 @@ class HyperlinkedIdentityField(Field):
         request = self.context.get('request', None)
         format = self.context.get('format', None)
         view_name = self.view_name or self.parent.opts.view_name
-        lookup_field = getattr(obj, self.lookup_field)
-        kwargs = {self.lookup_field: lookup_field}
 
         if request is None:
             warnings.warn("Using `HyperlinkedIdentityField` without including the "
@@ -539,23 +537,26 @@ class HyperlinkedIdentityField(Field):
         if format and self.format and self.format != format:
             format = self.format
 
+        lookup_field = getattr(obj, self.lookup_field)
+        kwargs = {self.lookup_field: lookup_field}
         try:
             return reverse(view_name, kwargs=kwargs, request=request, format=format)
         except NoReverseMatch:
             pass
 
-        slug = getattr(obj, self.slug_field, None)
-
-        if not slug:
-            raise Exception('Could not resolve URL for field using view name "%s"' % view_name)
-
-        kwargs = {self.slug_url_kwarg: slug}
+        if self.pk_url_kwarg != 'pk':
+            # Only try pk lookup if it has been explicitly set.
+            # Otherwise, the default `lookup_field = 'pk'` has us covered.
+            kwargs = {self.pk_url_kwarg: obj.pk}
             try:
                 return reverse(view_name, kwargs=kwargs, request=request, format=format)
             except NoReverseMatch:
                 pass
 
-        kwargs = {self.pk_url_kwarg: obj.pk, self.slug_url_kwarg: slug}
+        slug = getattr(obj, self.slug_field, None)
+        if slug:
+            # Only use slug lookup if a slug field exists on the model
+            kwargs = {self.slug_url_kwarg: slug}
             try:
                 return reverse(view_name, kwargs=kwargs, request=request, format=format)
             except NoReverseMatch: