Merge branch '5.3' into 5.4

* 5.3:
  Minor tweak
  Inserted a warning about a limitation model transformers

Author: javiereguiluz

form/data_transformers.rst

@@ -478,6 +478,20 @@ Which transformer you need depends on your situation.
 
 To use the view transformer, call ``addViewTransformer()``.
 
+.. caution::
+
+    Be careful with model transformers and
+    :doc:`Collection </reference/forms/types/collection>` field types.
+    Collection's children are created early at ``PRE_SET_DATA`` by its
+    ``ResizeFormListener`` and their data is populated later from the normalized
+    data. So your model transformer cannot reduce the number of items within the
+    Collection (i.e. filtering out some items), as in that case the collection
+    ends up with some empty children.
+    
+    A possible workaround for that limitation could be not using the underlying
+    object directly, but a DTO (Data Transfer Object) instead, that implements
+    the transformation of such incompatible data structures.
+
 So why Use the Model Transformer?
 ---------------------------------