glamr/views: Use ChainedQuerySet to export sample-based readabundance

Closes #56.

mibios/glamr/tables.py

@@ -339,7 +339,13 @@ class Meta:
         exclude = ['id']
 
     def customize_queryset(self, qs):
-        if not self.is_for_export():
+        if self.is_for_export():
+            if isinstance(qs, ChainedQuerySet):
+                if qs.fk_field is self._meta.model._meta.get_field('sample'):
+                    # use sample+ref uniq constraint index, so existing order
+                    # by ref_id
+                    qs.iterate_sortkey = 'ref_id'
+        else:
             # For normal HTML table get the function names, don't need or want
             # those for export
             qs = qs.prefetch_related('ref__function_names')

mibios/glamr/views.py

@@ -194,6 +194,12 @@ def get_export_queryset(self, export_option):
             # more intricate options would need to be implemented by inheriting
             # views.
             remote_field = self.get_export_remote_field(export_option)
+            if self.model is Sample:
+                return remote_field.model.objects.all().split_by_fk(
+                    remote_field,
+                    self.get_queryset(),
+                )
+
             f = {remote_field.name + '__in': self.get_queryset()}
             return remote_field.model.objects.filter(**f)