[code][serializers] Implement code field handling PUT

This includes both save and to_representation.

beat/web/code/serializers.py

@@ -66,6 +66,23 @@ class CodeModSerializer(ContributionModSerializer):
 
         return super().save(**kwargs)
 
+    def filter_representation(self, representation):
+        def add_code(representation):
+            # This is inspired from the source of Django REST Framework
+            field = self._declared_fields["code"]
+            representation["code"] = field.to_representation(self.instance.source_code)
+
+        request = self.context["request"]
+        fields = request.query_params.get("fields", None)
+        if fields is not None:
+            fields = fields.split(",")
+            if "code" in fields:
+                add_code(representation)
+        else:
+            add_code(representation)
+
+        return super().filter_representation(representation)
+
 
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