tutorial 2.0 WIPreview/mike_bayer/tutorial20

Add SelectBase.exists() method as it seems strange this is
not available already.  The Exists construct itself does
not provide full SELECT-building capabilities so it makes
sense this should be used more like a scalar_subquery.

Make sure stream_results is getting set up when yield_per
is used, for 2.0 style statements as well.  this was
hardcoded inside of Query.yield_per() and is now moved
to take place within QueryContext.

Change-Id: Icafcd4fd9b708772343d56edf40995c9e8f835d6

1 files changed, 8 insertions, 4 deletions

diff --git a/lib/sqlalchemy/engine/row.py b/lib/sqlalchemy/engine/row.py
index 288f08e29..60954fcec 100644
--- a/lib/sqlalchemy/engine/row.py
+++ b/lib/sqlalchemy/engine/row.py
@@ -500,10 +500,14 @@ class RowMapping(BaseRow, collections_abc.Mapping):
     """A ``Mapping`` that maps column names and objects to :class:`.Row` values.
 
     The :class:`.RowMapping` is available from a :class:`.Row` via the
-    :attr:`.Row._mapping` attribute and supplies Python mapping (i.e.
-    dictionary) access to the  contents of the row.   This includes support
-    for testing of containment of specific keys (string column names or
-    objects), as well as iteration of keys, values, and items::
+    :attr:`.Row._mapping` attribute, as well as from the iterable interface
+    provided by the :class:`.MappingResult` object returned by the
+    :meth:`_engine.Result.mappings` method.
+
+    :class:`.RowMapping` supplies Python mapping (i.e. dictionary) access to
+    the  contents of the row.   This includes support for testing of
+    containment of specific keys (string column names or objects), as well
+    as iteration of keys, values, and items::
 
         for row in result:
             if 'a' in row._mapping: