Mike Bayer  committed 2309db1

- repair "map to selectable" example, place a caveat that this isn't
something people should be pursuing

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  • Parent commits eba2fe7

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File doc/build/orm/mapper_config.rst

-    customer_select = select([customers,subq]).\
-                where(customers.c.customer_id==subq.c.customer_id)
+    customer_select = select([customers, subq]).\
+                select_from(
+                    join(customers, subq,
+                   == subq.c.customer_id)
+                ).alias()
     class Customer(Base):
         __table__ = customer_select
 will only emit an INSERT into a table for which it has mapped the primary
+.. note::
+    The practice of mapping to arbitrary SELECT statements, especially
+    complex ones as above, is
+    almost never needed; it necessarily tends to produce complex queries
+    which are often less efficient than that which would be produced
+    by direct query construction.   The practice is to some degree
+    based on the very early history of SQLAlchemy where the :func:`.mapper`
+    construct was meant to represent the primary querying interface;
+    in modern usage, the :class:`.Query` object can be used to construct
+    virtually any SELECT statement, including complex composites, and should
+    be favored over the "map-to-selectable" approach.
 Multiple Mappers for One Class