minor symfony#11019 Added a note about N+1 problem and form collections (javiereguiluz)

This PR was squashed before being merged into the 3.4 branch (closes symfony#11019).

Discussion
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Added a note about N+1 problem and form collections

Fixes symfony#3374 and symfony#4323.

Commits
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6240853 Added a note about N+1 problem and form collections

Author: javiereguiluz

doctrine/associations.rst

@@ -355,6 +355,8 @@ to the given ``Category`` object via their ``category_id`` value.
     all at once (via a *join*), Doctrine will return the *true* ``Category``
     object, since nothing needs to be lazily loaded.
 
+.. _doctrine-associations-join-query:
+
 Joining Related Records
 -----------------------
 

reference/forms/types/entity.rst

@@ -99,6 +99,13 @@ the `query_builder`_ option::
         'choice_label' => 'username',
     ]);
 
+.. note::
+
+    Using form collections may result in making too many database requests to
+    fetch related entities. This is known as the *"N + 1 query problem"* and it
+    can be solved by :ref:`joining related records <doctrine-associations-join-query>`
+    when querying for Doctrine associations.
+
 .. _reference-forms-entity-choices:
 
 Using Choices