diff --git a/dynamic_rest/filters.py b/dynamic_rest/filters.py
index eb3617b9..e38fdeb3 100644
--- a/dynamic_rest/filters.py
+++ b/dynamic_rest/filters.py
@@ -462,6 +462,15 @@ def _get_queryset(self, queryset=None, serializer=None):
     def _serializer_filter(self, serializer=None, queryset=None):
         return serializer.filter_queryset(queryset)
 
+    def _get_id_fields(self, serializer, fields, is_root):
+        """ Return FK field names, with `_id` appended. """
+        get_id_fields = getattr(serializer, 'get_id_fields', lambda a: [])
+        if not is_root:
+            # For sideloads, return all ID fields they may be needed
+            # to related them to parent objects.
+            fields = None
+        return get_id_fields(fields)
+
     def _build_queryset(
         self,
         serializer=None,
@@ -538,7 +547,7 @@ def _build_queryset(
             not self.view.is_update() and
             not self.view.is_delete()
         ):
-            id_fields = getattr(serializer, 'get_id_fields', lambda: [])()
+            id_fields = self._get_id_fields(serializer, fields, is_root_level)
             # only include local model fields
             only = [
                 field for field in set(
@@ -546,6 +555,11 @@ def _build_queryset(
                 ) if is_model_field(model, field) and
                 not is_field_remote(model, field)
             ]
+            '''
+            print "Requirements: %s" % requirements
+            print "Fields: %s" % fields.keys()
+            print "Only: %s" % only
+            '''
             queryset = queryset.only(*only)
 
         # add request filters
@@ -622,6 +636,10 @@ def _serializer_filter(self, serializer=None, queryset=None):
         )
         return queryset
 
+    def _get_id_fields(self, serializer, fields, is_root_level):
+        get_id_fields = getattr(serializer, 'get_id_fields', lambda a: [])
+        return get_id_fields(fields)
+
 
 class DynamicSortingFilter(OrderingFilter):
 
diff --git a/dynamic_rest/prefetch.py b/dynamic_rest/prefetch.py
index 2660c5f0..af983cd0 100644
--- a/dynamic_rest/prefetch.py
+++ b/dynamic_rest/prefetch.py
@@ -173,10 +173,8 @@ def prefetch_related(self, *args):
         return self
 
     def only(self, *fields):
-        # TODO: support this for realz
-        '''
-        self.fields = set(self.fields) + set(fields)
-        '''
+        self.fields |= set(fields)
+        self.queryset = self.queryset.only(*fields)
         return self
 
     def exclude(self, *args, **kwargs):
@@ -251,7 +249,7 @@ def __init__(self, queryset):
         self.queryset = queryset
         self.model = queryset.model
         self.prefetches = {}
-        self.fields = None
+        self.fields = set()
         self.pk_field = queryset.model._meta.pk.attname
         self._data = None
         self._my_ids = None
@@ -267,7 +265,10 @@ def execute(self):
         use_fastquery = getattr(self.model, 'USE_FASTQUERY', True)
 
         if use_fastquery:
-            data = list(qs.values())
+            # print "Fields: %s" % self.fields
+            # print qs.query.__str__()
+            data = list(qs.values(*list(self.fields)))
+            # print data[0] if data else ''
 
             self.merge_prefetch(data)
             self._data = FastList(
@@ -412,7 +413,10 @@ def merge_o2or(self, data, field, prefetch, m2o_mode=False):
         filter_args = {remote_filter_key: my_ids}
 
         # Fetch remote objects
-        remote_objects = prefetch.query.filter(**filter_args).execute()
+        query = prefetch.query
+        if query.fields:
+            query = query.only(remote_field)
+        remote_objects = query.filter(**filter_args).execute()
         id_map = self._make_id_map(data, pk_field=self.pk_field)
 
         field_name = prefetch.field
diff --git a/dynamic_rest/serializers.py b/dynamic_rest/serializers.py
index be24225e..7c22608c 100644
--- a/dynamic_rest/serializers.py
+++ b/dynamic_rest/serializers.py
@@ -626,7 +626,10 @@ def to_representation(self, instance):
             Otherwise, a tagged data dict representation.
         """
         if self.id_only():
-            return instance.pk
+            try:
+                return instance.pk
+            except AttributeError:
+                return instance
         else:
             if self.enable_optimization:
                 representation = self._faster_to_representation(instance)
@@ -720,7 +723,7 @@ class WithDynamicModelSerializerMixin(WithDynamicSerializerMixin):
     def get_model(cls):
         return getattr(cls.Meta, 'model', None)
 
-    def get_id_fields(self):
+    def get_id_fields(self, fields=None):
         """
         Called to return a list of fields consisting of, at minimum,
         the PK field name. The output of this method is used to
@@ -732,6 +735,11 @@ def get_id_fields(self):
 
         out = [model._meta.pk.name]  # get PK field name
 
+        if fields is not None:
+            sources = [f.source for f in fields.values()]
+        else:
+            sources = None  # return all
+
         # If this is being called, it means it
         # is a many-relation  to its parent.
         # Django wants the FK to the parent,
@@ -741,6 +749,8 @@ def get_id_fields(self):
         # TODO: We also might need to return all non-nullable fields,
         #    or else it is possible Django will issue another request.
         for field in model._meta.fields:
+            if sources is not None and field.name not in sources:
+                continue
             if isinstance(field, models.ForeignKey):
                 out.append(field.name + '_id')