minor #11436 Added a note about UniqueEntity and collections (javiereguiluz)

This PR was merged into the 3.4 branch.

Discussion
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Added a note about UniqueEntity and collections

Fixes #10942.

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13a6a8e Added a note about UniqueEntity and collections

Author: javiereguiluz

reference/constraints/UniqueEntity.rst

@@ -114,6 +114,12 @@ your user table:
     this validation has passed and before this entity is actually persisted in
     the database.
 
+.. caution::
+
+    This constraint cannot deal with duplicates found in a collection of items
+    that haven't been persisted as entities yet. You'll need to create your own
+    validator to handle that case.
+
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