Add support for computed columns

Added support for rendering of "computed" elements on :class:`.Column`
objects, supported in SQLAlchemy via the new :class:`.Computed` element
introduced in version 1.3.11. Pull request courtesy Federico Caselli.

Note that there is currently no support for ALTER COLUMN to add, remove, or
modify the "GENERATED ALWAYS AS" element from a column;  at least for
PostgreSQL, it does not seem to be supported by the database. Additionally,
SQLAlchemy does not currently reliably reflect the "GENERATED ALWAYS AS"
phrase from an existing column, so there is also no autogenerate support
for addition or removal of the :class:`.Computed` element to or from an
existing column, there is only support for adding new columns that include
the :class:`.Computed` element.  In the case that the :class:`.Computed`
element is removed from the :class:`.Column` object in the table metadata,
PostgreSQL and Oracle currently reflect the "GENERATED ALWAYS AS"
expression as the "server default" which will produce an op that tries to
drop the element as a default.

In order to facilitate using testing combinations with elements that
are not necessarily present, the support for lambda combinations
from SQLAlchemy Ia63a510f9c1d08b055eef62cf047f1f427f0450c is also
ported here, as well as a vendoring in of the current
sqlalchemy.testing.exclusions module where we need the additional
combinations support added in I15d2839954d77a252bab5aaf6e3fd9f388c99dd5.

Fixes: #624
Closes: #631
Pull-request: https://github.com/sqlalchemy/alembic/pull/631
Pull-request-sha: 9c45651295626edf5f172ec827a5ced1440818cf

Change-Id: Ifd27c2f541b22fb7a78de1afaa36dbf509ff6d3f

1 files changed, 31 insertions, 2 deletions

diff --git a/tests/requirements.py b/tests/requirements.py
index e07a517..7eb528e 100644
--- a/tests/requirements.py
+++ b/tests/requirements.py
@@ -1,5 +1,4 @@
-from sqlalchemy.testing import exclusions
-
+from alembic.testing import exclusions
 from alembic.testing.requirements import SuiteRequirements
 from alembic.util import sqla_compat
 
@@ -136,6 +135,36 @@ class DefaultRequirements(SuiteRequirements):
         return exclusions.only_on("postgresql", "mysql")
 
     @property
+    def computed_columns(self):
+        # TODO: in theory if these could come from SQLAlchemy dialects
+        # that would be helpful
+        return self.computed_columns_api + exclusions.only_on(
+            ["postgresql >= 12", "oracle", "mssql", "mysql >= 5.7"]
+        )
+
+    @property
+    def computed_reflects_as_server_default(self):
+        # note that this rule will go away when SQLAlchemy correctly
+        # supports reflection of the "computed" construct; the element
+        # will consistently be present as both column.computed and
+        # column.server_default for all supported backends.
+        return self.computed_columns + exclusions.only_if(
+            ["postgresql", "oracle"],
+            "backend reflects computed construct as a server default",
+        )
+
+    @property
+    def computed_doesnt_reflect_as_server_default(self):
+        # note that this rule will go away when SQLAlchemy correctly
+        # supports reflection of the "computed" construct; the element
+        # will consistently be present as both column.computed and
+        # column.server_default for all supported backends.
+        return self.computed_columns + exclusions.skip_if(
+            ["postgresql", "oracle"],
+            "backend reflects computed construct as a server default",
+        )
+
+    @property
     def check_constraint_reflection(self):
         return exclusions.fails_on_everything_except(
             "postgresql",