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, 13 insertions, 0 deletions

diff --git a/lib/sqlalchemy/sql/compiler.py b/lib/sqlalchemy/sql/compiler.py
index 546fffc6c..85c1750b7 100644
--- a/lib/sqlalchemy/sql/compiler.py
+++ b/lib/sqlalchemy/sql/compiler.py
@@ -3173,6 +3173,9 @@ class DDLCompiler(Compiled):
         if default is not None:
             colspec += " DEFAULT " + default
 
+        if column.computed is not None:
+            colspec += " " + self.process(column.computed)
+
         if not column.nullable:
             colspec += " NOT NULL"
         return colspec
@@ -3314,6 +3317,16 @@ class DDLCompiler(Compiled):
             text += " MATCH %s" % constraint.match
         return text
 
+    def visit_computed_column(self, generated):
+        text = "GENERATED ALWAYS AS (%s)" % self.sql_compiler.process(
+            generated.sqltext, include_table=False, literal_binds=True
+        )
+        if generated.persisted is True:
+            text += " STORED"
+        elif generated.persisted is False:
+            text += " VIRTUAL"
+        return text
+
 
 class GenericTypeCompiler(TypeCompiler):
     def visit_FLOAT(self, type_, **kw):