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, 22 insertions, 0 deletions

diff --git a/test/dialect/mssql/test_compiler.py b/test/dialect/mssql/test_compiler.py
index 00a8a08fc..6b3244c30 100644
--- a/test/dialect/mssql/test_compiler.py
+++ b/test/dialect/mssql/test_compiler.py
@@ -1,5 +1,6 @@
 # -*- encoding: utf-8
 from sqlalchemy import Column
+from sqlalchemy import Computed
 from sqlalchemy import delete
 from sqlalchemy import extract
 from sqlalchemy import func
@@ -1193,6 +1194,27 @@ class CompileTest(fixtures.TestBase, AssertsCompiledSQL):
             "SELECT TRY_CAST (t1.id AS INTEGER) AS id FROM t1",
         )
 
+    @testing.combinations(
+        ("no_persisted", "", "ignore"),
+        ("persisted_none", "", None),
+        ("persisted_true", " PERSISTED", True),
+        ("persisted_false", "", False),
+        id_="iaa",
+    )
+    def test_column_computed(self, text, persisted):
+        m = MetaData()
+        kwargs = {"persisted": persisted} if persisted != "ignore" else {}
+        t = Table(
+            "t",
+            m,
+            Column("x", Integer),
+            Column("y", Integer, Computed("x + 2", **kwargs)),
+        )
+        self.assert_compile(
+            schema.CreateTable(t),
+            "CREATE TABLE t (x INTEGER NULL, y AS (x + 2)%s)" % text,
+        )
+
 
 class SchemaTest(fixtures.TestBase):
     def setup(self):