minor #10051 Added a caution note about race conditions in EntityType (javiereguiluz)

This PR was merged into the 2.8 branch.

Discussion
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Added a caution note about race conditions in EntityType

Fixes #7305.

Commits
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e0ecfd3 Added a caution note about race conditions in EntityType

Author: javiereguiluz

reference/constraints/UniqueEntity.rst

@@ -109,6 +109,13 @@ your user table:
             }
         }
 
+.. caution::
+
+    This constraint doesn't provide any protection against `race conditions`_.
+    They may occur when another entity is persisted by an external process after
+    this validation has passed and before this entity is actually persisted in
+    the database.
+
 Options
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@@ -262,3 +269,5 @@ If set to ``false``, only one ``null`` value is allowed - if a second entity
 also has a ``null`` value, validation would fail.
 
 .. include:: /reference/constraints/_payload-option.rst.inc
+
+.. _`race conditions`: https://en.wikipedia.org/wiki/Race_condition