Support for generated columns

Added DDL support for "computed columns"; these are DDL column
specifications for columns that have a server-computed value, either upon
SELECT (known as "virtual") or at the point of which they are INSERTed or
UPDATEd (known as "stored").  Support is established for Postgresql, MySQL,
Oracle SQL Server and Firebird. Thanks to Federico Caselli for lots of work
on this one.

ORM round trip tests included.  The ORM makes use of existing
FetchedValue support and no additional ORM logic is present for
the basic feature.

It has been observed that Oracle RETURNING does not return the
new value of a computed column upon UPDATE; it returns the
prior value.  As this is very dangerous, a warning is emitted
if a computed column is rendered into the RETURNING clause
of an UPDATE statement.

Fixes: #4894
Closes: #4928
Pull-request: https://github.com/sqlalchemy/sqlalchemy/pull/4928
Pull-request-sha: d39c521d5ac6ebfb4fb5b53846451de79752e64c

Change-Id: I2610b2999a5b1b127ed927dcdaeee98b769643ce

1 files changed, 67 insertions, 0 deletions

diff --git a/test/dialect/oracle/test_dialect.py b/test/dialect/oracle/test_dialect.py
index 62926700e..20c6336b8 100644
--- a/test/dialect/oracle/test_dialect.py
+++ b/test/dialect/oracle/test_dialect.py
@@ -3,6 +3,7 @@
 import re
 
 from sqlalchemy import bindparam
+from sqlalchemy import Computed
 from sqlalchemy import create_engine
 from sqlalchemy import exc
 from sqlalchemy import Float
@@ -258,6 +259,72 @@ class EncodingErrorsTest(fixtures.TestBase):
             )
 
 
+class ComputedReturningTest(fixtures.TablesTest):
+    __only_on__ = "oracle"
+    __backend__ = True
+
+    @classmethod
+    def define_tables(cls, metadata):
+        Table(
+            "test",
+            metadata,
+            Column("id", Integer, primary_key=True),
+            Column("foo", Integer),
+            Column("bar", Integer, Computed("foo + 42")),
+        )
+
+        Table(
+            "test_no_returning",
+            metadata,
+            Column("id", Integer, primary_key=True),
+            Column("foo", Integer),
+            Column("bar", Integer, Computed("foo + 42")),
+            implicit_returning=False,
+        )
+
+    def test_computed_insert(self):
+        test = self.tables.test
+        with testing.db.connect() as conn:
+            result = conn.execute(
+                test.insert().return_defaults(), {"id": 1, "foo": 5}
+            )
+
+            eq_(result.returned_defaults, (47,))
+
+            eq_(conn.scalar(select([test.c.bar])), 47)
+
+    def test_computed_update_warning(self):
+        test = self.tables.test
+        with testing.db.connect() as conn:
+            conn.execute(test.insert(), {"id": 1, "foo": 5})
+
+            with testing.expect_warnings(
+                "Computed columns don't work with Oracle UPDATE"
+            ):
+                result = conn.execute(
+                    test.update().values(foo=10).return_defaults()
+                )
+
+                # returns the *old* value
+                eq_(result.returned_defaults, (47,))
+
+            eq_(conn.scalar(select([test.c.bar])), 52)
+
+    def test_computed_update_no_warning(self):
+        test = self.tables.test_no_returning
+        with testing.db.connect() as conn:
+            conn.execute(test.insert(), {"id": 1, "foo": 5})
+
+            result = conn.execute(
+                test.update().values(foo=10).return_defaults()
+            )
+
+            # no returning
+            eq_(result.returned_defaults, None)
+
+            eq_(conn.scalar(select([test.c.bar])), 52)
+
+
 class OutParamTest(fixtures.TestBase, AssertsExecutionResults):
     __only_on__ = "oracle+cx_oracle"
     __backend__ = True