Allow preserve comment on column rename in MySQL

[randallpittman]

Versions:
Python: 3.7.4 (Linux/x64/conda)
Alembic: 1.0.11
SQLAlchemy: 1.3.5
MariaDB: 10.3.13

Steps to reproduce

1. Have a table storage_drive with Integer column drive_partition, nullable, and comment If more than one partition, the 1-based partition index.
2. In alembic upgrade() migration function, issue the following command to change the column name to partition_:

op.alter_column(
    "storage_drive",
    "drive_partition",
    existing_type=sa.Integer(),
    existing_comment="If more than one partition, the 1-based partition index.",
    new_column_name="partition_",
)

Expected Result:
Column is renamed to partition_ and the comment is preserved.

Actual Result:
Column is renamed to partition_ and the comment disappears.

Investigation

• I used my interactive debugger to step through the alter_table command, and eventually got to alembic/ddl/mysql.py line 47.
• Since name is not None evaluated to True , the call to MySQLChangeColumn at line 51 was executed.
• Notably, this call does not include the comment keyword argument (vs. the MySQLModifyColumn call at line 77), so existing_comment was dropped.

Workaround
Add another alter_column call to reinstate the comment.

op.alter_column(
    "storage_drive",
    "partition_",
    existing_type=sa.Integer(),
    comment="If more than one partition, the 1-based partition index.",
)

[randallpittman]

Note: In MySQL, the following two commands:

ALTER TABLE storage_drive CHANGE drive_partition partition_ INTEGER NULL;
ALTER TABLE storage_drive MODIFY partition_ INTEGER NULL COMMENT 'If more than one partition, the 1-based partition index.';

Can be combined into one:

ALTER TABLE storage_drive CHANGE drive_partition partition_ INTEGER NULL COMMENT 'If more than one partition, the 1-based partition index.';

[randallpittman]

Really, I'd think preserving comments would be default behavior, but it isn't default behavior in the database so I can see why it isn't default in alembic.

[sqla-tester]

Mike Bayer has proposed a fix for this issue in the master branch:

Include existing_comment in MySQLChangeColumn https://gerrit.sqlalchemy.org/1431

[zzzeek]

Can you confirm this patch solves your issue:

diff --git a/alembic/ddl/mysql.py b/alembic/ddl/mysql.py
index 17ae2de..e919599 100644
--- a/alembic/ddl/mysql.py
+++ b/alembic/ddl/mysql.py
@@ -64,6 +64,9 @@ class MySQLImpl(DefaultImpl):
                     autoincrement=autoincrement
                     if autoincrement is not None
                     else existing_autoincrement,
+                    comment=comment
+                    if comment is not False
+                    else existing_comment,
                 )
             )
         elif (

[randallpittman]

Yes, your patch does indeed solve the issue!