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

2 files changed, 44 insertions, 7 deletions

diff --git a/alembic/autogenerate/compare.py b/alembic/autogenerate/compare.py
index cacedee..71f947e 100644
--- a/alembic/autogenerate/compare.py
+++ b/alembic/autogenerate/compare.py
@@ -910,6 +910,18 @@ def _compare_server_default(
     conn_col_default = conn_col.server_default
     if conn_col_default is None and metadata_default is None:
         return False
+
+    if sqla_compat.has_computed and isinstance(
+        metadata_default, sa_schema.Computed
+    ):
+        # return False in case of a computed column as the server
+        # default. Note that DDL for adding or removing "GENERATED AS" from
+        # an existing column is not currently known for any backend.
+        # Once SQLAlchemy can reflect "GENERATED" as the "computed" element,
+        # we would also want to ignore and/or warn for changes vs. the
+        # metadata (or support backend specific DDL if applicable).
+        return False
+
     rendered_metadata_default = _render_server_default_for_compare(
         metadata_default, metadata_col, autogen_context
     )
diff --git a/alembic/autogenerate/render.py b/alembic/autogenerate/render.py
index d9aba00..99c15c3 100644
--- a/alembic/autogenerate/render.py
+++ b/alembic/autogenerate/render.py
@@ -593,13 +593,20 @@ def _render_column(column, autogen_context):
     if rendered is not False:
         return rendered
 
+    args = []
     opts = []
+
     if column.server_default:
-        rendered = _render_server_default(
-            column.server_default, autogen_context
-        )
-        if rendered:
-            opts.append(("server_default", rendered))
+        if sqla_compat._server_default_is_computed(column):
+            rendered = _render_computed(column.computed, autogen_context)
+            if rendered:
+                args.append(rendered)
+        else:
+            rendered = _render_server_default(
+                column.server_default, autogen_context
+            )
+            if rendered:
+                opts.append(("server_default", rendered))
 
     if (
         column.autoincrement is not None
@@ -618,10 +625,11 @@ def _render_column(column, autogen_context):
         opts.append(("comment", "%r" % comment))
 
     # TODO: for non-ascii colname, assign a "key"
-    return "%(prefix)sColumn(%(name)r, %(type)s, %(kwargs)s)" % {
+    return "%(prefix)sColumn(%(name)r, %(type)s, %(args)s%(kwargs)s)" % {
         "prefix": _sqlalchemy_autogenerate_prefix(autogen_context),
         "name": _ident(column.name),
         "type": _repr_type(column.type, autogen_context),
+        "args": ", ".join([str(arg) for arg in args]) + ", " if args else "",
         "kwargs": (
             ", ".join(
                 ["%s=%s" % (kwname, val) for kwname, val in opts]
@@ -640,7 +648,9 @@ def _render_server_default(default, autogen_context, repr_=True):
     if rendered is not False:
         return rendered
 
-    if isinstance(default, sa_schema.DefaultClause):
+    if sqla_compat.has_computed and isinstance(default, sa_schema.Computed):
+        return _render_computed(default, autogen_context)
+    elif isinstance(default, sa_schema.DefaultClause):
         if isinstance(default.arg, compat.string_types):
             default = default.arg
         else:
@@ -654,6 +664,21 @@ def _render_server_default(default, autogen_context, repr_=True):
     return default
 
 
+def _render_computed(computed, autogen_context):
+    text = _render_potential_expr(
+        computed.sqltext, autogen_context, wrap_in_text=False
+    )
+
+    kwargs = {}
+    if computed.persisted is not None:
+        kwargs["persisted"] = computed.persisted
+    return "%(prefix)sComputed(%(text)s, %(kwargs)s)" % {
+        "prefix": _sqlalchemy_autogenerate_prefix(autogen_context),
+        "text": text,
+        "kwargs": (", ".join("%s=%s" % pair for pair in kwargs.items())),
+    }
+
+
 def _repr_type(type_, autogen_context):
     rendered = _user_defined_render("type", type_, autogen_context)
     if rendered is not False: