.transform() preserves check constraints, refs #762

docs/changelog.rst

@@ -10,6 +10,7 @@ Unreleased
 ----------
 
 - New ``table.checks``, ``table.column_checks`` and ``table.table_checks`` introspection properties expose column-level and table-level ``CHECK`` constraints. (:issue:`834`)
+- ``table.transform()`` now preserves ``CHECK`` constraints, including comments within their expressions. Renaming a column rewrites identifier references in them without changing string literals or function names. Dropping a column drops a check owned by that column, and raises ``TransformError`` if a remaining check depends on it. (:issue:`762`)
 - ``table.transform()`` now works for tables that are referenced by views. Previously the ``ALTER TABLE ... RENAME TO`` step raised ``no such table`` if a view referenced the table being transformed. View definitions are left unchanged - see :ref:`python_api_transform_views`. This also fixes a bug where ``transform(keep_table=...)`` silently rewrote dependent views to point at the frozen backup table instead of the live one. (:issue:`831`)
 
 .. _v3_39_1:

docs/python-api.rst

@@ -1986,6 +1986,15 @@ A bare column name drops any foreign key that column participates in, including
 
 Renaming a column with ``rename=`` updates any foreign keys that use it, and dropping a column with ``drop=`` also drops any foreign keys it participates in - for a compound foreign key this removes the whole constraint.
 
+.. _python_api_transform_check_constraints:
+
+CHECK constraints
+-----------------
+
+``.transform()`` preserves both column-level and table-level ``CHECK`` constraints. If a column is renamed, references to that column in the check expression are renamed too.
+
+A column-level check is removed if its owning column is dropped. Dropping a column referenced by any remaining check raises ``TransformError`` instead of creating an invalid or unexpectedly weakened schema.
+
 .. _python_api_transform_views:
 
 Tables referenced by views