Alembic keeps detecting JSON change

[WilliamDEdwards]

Describe the bug

MariaDB's JSON type is an alias for LONGTEXT with a JSON_VALID check. When defining a column of type sqlalchemy.dialects.mysql.JSON, Alembic keeps generating a migration of existing_type=mysql.LONGTEXT(charset='utf8mb4', collation='utf8mb4_bin') to type_=mysql.JSON().

Expected behavior

I would expect Alembic to not detect any changes between LONGTEXT with JSON_VALID check and sqlalchemy.dialects.mysql.JSON.

To Reproduce

• Add an sqlalchemy.dialects.mysql.JSON column
• Create a migration & upgrade
• Create another migration. This will cause another migration with an operation like the one below to be created.

op.alter_column('api_users', 'trusted_ip_networks',
    existing_type=mysql.LONGTEXT(charset='utf8mb4', collation='utf8mb4_bin'),
    type_=mysql.JSON(),
    existing_nullable=False,
    existing_server_default=sa.text("'[]'")
)

Error

None.

Versions.

• OS: macOS 11.6
• Python: 3.8.9
• Alembic: 1.4.2
• SQLAlchemy: 1.3.16
• Database: MariaDB 10.5.9
• DBAPI: -

Additional context

None.

[sqla-tester]

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

add type synonym for mysql.LONGTEXT / JSON https://gerrit.sqlalchemy.org/c/sqlalchemy/alembic/+/3306