Here is solutions explain with details :
https://groups.google.com/forum/#!topic/doctrine-user/fkIaKxifDqc
“fetch” in the mapping is a hint, that is, if it is possible
Doctrine does that, but if its not possible, obviously it does not.
Proxying for lazy-loading is simply not always possible, technically.
The situations where its not possible are:1) one-to-one from inverse to owning side (appears only in
bidirectional one-to-one associations). Precondition a) above can not
be met. 2) one-to-one/many-to-one association to a hierarchy and the
targeted class has subclasses (is not a leaf in the class hierarchy).
Precondition b) above can not be met.In these cases, proxying is technically not possible.
Your options to avoid this n+1 problem:
1) fetch-join via DQL: “select c,ca from Customer join c.cart ca”.
Single query but join, however, joins on to-one associations are
relatively cheap.2) force partial objects. No additional queries but
also no lazy-load: $query->setHint(Query::HINT_FORCE_PARTIAL_LOAD,
true)3) if an alternative result format (i.e. getArrayResult()) is
sufficient for a use-case, these also avoid this problem.Benjamin had some ideas about automatic batching of these loads to
avoid n+1 queries but this does not change the fact that proxying is
not always possible.