Add additional documentation for ORM fetched defaults

Add additional examples to the section first added as part
of #4317 to cover the use cases requested in #3921.

Fixes: #3921
Change-Id: I6ec283aa0a6fbabedef40bb4320751ab4cd990ea

doc/build/orm/persistence_techniques.rst

@@ -339,31 +339,102 @@ An INSERT for the above table on SQL Server looks like:
 Case 4: primary key, RETURNING or equivalent is not supported
 --------------------------------------------------------------
 
-In this area we are generating rows for a database such as SQLite or
-more typically MySQL where some means of generating a default is occurring
-on the server, but is outside of the database's usual autoincrement routine.
-In this case, we have to make sure SQLAlchemy can "pre-execute" the default,
-which means it has to be an explicit SQL expression.   Again using the example
-of TIMESTAMP for MySQL, we unfortunately need to use our own explicit default::
+In this area we are generating rows for a database such as SQLite or MySQL
+where some means of generating a default is occurring on the server, but is
+outside of the database's usual autoincrement routine. In this case, we have to
+make sure SQLAlchemy can "pre-execute" the default, which means it has to be an
+explicit SQL expression.
+
+.. note::  This section will will illustrate multiple recipes involving
+   datetime values for MySQL and SQLite, since the datetime datatypes on these
+   two  backends have additional idiosyncratic requirements that are useful to
+   illustrate.  Keep in mind however that SQLite and MySQL require an explicit
+   "pre-executed" default generator for *any* auto-generated datatype used as
+   the primary key other than the usual single-column autoincrementing integer
+   value.
+
+MySQL with DateTime primary key
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+Using the example of a :class:`.DateTime` column for MySQL, we add an explicit
+pre-execute-supported default using the "NOW()" SQL function::
 
     class MyModel(Base):
         __tablename__ = 'my_table'
 
-        timestamp = Column(TIMESTAMP(), default=func.cast(func.now(), Binary), primary_key=True)
+        timestamp = Column(DateTime(), default=func.now(), primary_key=True)
 
-Where above, we select the "NOW()" function and also cast to binary to
-be used with MySQL's TIMESTAMP column, that is in fact a binary datatype.
-The SQL generated by the above is:
+Where above, we select the "NOW()" function to deliver a datetime value
+to the column.  The SQL generated by the above is:
+
+.. sourcecode:: sql
+
+    SELECT now() AS anon_1
+    INSERT INTO my_table (timestamp) VALUES (%s)
+    ('2018-08-09 13:08:46',)
+
+MySQL with TIMESTAMP primary key
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+When using the :class:`.TIMESTAMP` datatype with MySQL, MySQL ordinarily
+associates a server-side default with this datatype automatically.  However
+when we use one as a primary key, the Core cannot retrieve the newly generated
+value unless we execute the function ourselves.  As :class:`.TIMESTAMP` on
+MySQL actually stores a binary value, we need to add an additional "CAST" to our
+usage of "NOW()" so that we retrieve a binary value that can be persisted
+into the column::
+
+    from sqlalchemy import cast, Binary
+
+    class MyModel(Base):
+        __tablename__ = 'my_table'
+
+        timestamp = Column(
+            TIMESTAMP(),
+            default=cast(func.now(), Binary),
+            primary_key=True)
+
+Above, in addition to selecting the "NOW()" function, we additionally make
+use of the :class:`.Binary` datatype in conjunction with :func:`.cast` so that
+the returned value is binary.  SQL rendered from the above within an
+INSERT looks like:
 
 .. sourcecode:: sql
 
     SELECT CAST(now() AS BINARY) AS anon_1
     INSERT INTO my_table (timestamp) VALUES (%s)
     (b'2018-08-09 13:08:46',)
 
-The Core currently does not support a means of fetching the timestamp value
-after the fact without using RETURNING, so on MySQL must run a SELECT ahead of
-time to pre-select the value.
+SQLite with DateTime primary key
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+For SQLite, new timestamps can be generated using the SQL function
+``datetime('now', 'localtime')`` (or specify ``'utc'`` for UTC),
+however making things more complicated is that this returns a string
+value, which is then incompatible with SQLAlchemy's :class:`.DateTime`
+datatype (even though the datatype converts the information back into a
+string for the SQLite backend, it must be passed through as a Python datetime).
+We therefore must also specify that we'd like to coerce the return value to
+:class:`.DateTime` when it is returned from the function, which we achieve
+by passing this as the ``type_`` parameter::
+
+    class MyModel(Base):
+        __tablename__ = 'my_table'
+
+        timestamp = Column(
+            DateTime,
+            default=func.datetime('now', 'localtime', type_=DateTime),
+            primary_key=True)
+
+The above mapping upon INSERT will look like:
+
+.. sourcecode:: sql
+
+    SELECT datetime(?, ?) AS datetime_1
+    ('now', 'localtime')
+    INSERT INTO my_table (timestamp) VALUES (?)
+    ('2018-10-02 13:37:33.000000',)
+
 
 .. seealso::