added lazy unique together validator

rest_serializers/validators.py

@@ -0,0 +1,21 @@
+from rest_framework import serializers
+
+
+class LazyUniqueTogetherValidator(serializers.UniqueTogetherValidator):
+    """
+    A unique together validator that will only check if all fields exist.
+
+    This allows a missing field being added programmatically after the
+    initial validate.
+    """
+
+    def has_missing_fields(self, attrs):
+        for field_name in self.fields:
+            if field_name not in attrs:
+                return True
+
+    def __call__(self, attrs):
+        # only allow it to continue if all fields are present
+        if self.has_missing_fields(attrs):
+            return
+        super().__call__(attrs)