Usage of JSON(none_as_null=True) with CockroachDB causes infinite diffs in Alembic

[NotJustPizza]

Description

Trying to set JSON(none_as_null=True) for SQLAlchemy models causes constant diffs in Alembic.

FYA: @gordthompson

Reproduction:

Setup:

from sqlalchemy import Column, DateTime, JSON
from sqlalchemy.orm import declarative_base

Base = declarative_base()

class TestModel(Base):
    __tablename__ = "test"

    id = Column(Integer, primary_key=True)
    mydata = Columnt(JSON(none_as_null=True))

Run commands:
alembic revision -m init --autogenerate
alembic upgrade head
alembic check

Alembic output:

INFO     Context impl CockroachDBImpl.                                                                     
INFO     Will assume non-transactional DDL.                                                               
INFO     Detected type change from JSONB(astext_type=Text()) to JSON(none_as_null=True) on 'test.mydata'   compare.py:1003
ERROR    New upgrade operations detected: [[('modify_type', None, 'test', 'mydata', {'existing_nullable':  messaging.py:71
         True, 'existing_server_default': False, 'existing_comment': None}, JSONB(astext_type=Text()),                                   
         JSON(none_as_null=True))]]          

Versions

OS: Windows 11 OR Docker Image python:3.13.11-slim-trixie
Python: 3.13.11
Alembic: 1.17.2
SQLAlchemy: 2.0.46 (latest)
Database: Cockroach 25.4.2 or Docker Image cockroachdb/cockroach:v25.4.2
DBAPI: sqlalchemy-cockroachdb 2.0.4.dev0 from main branch (sha: d57d176be810c75696678efc04493f51fdecfa85)

[gordthompson]

This one is complicated because CRDB doesn't actually have a json type. It accepts json in DDL, but the column is created as jsonb:

root@localhost:26257/defaultdb> create table test (id varchar(50) primary key, mydata json);                                      
CREATE TABLE

Time: 34ms total (execution 12ms / network 22ms)

root@localhost:26257/defaultdb> \d test                                                                                           
Table "public.test"
  Column |         Type          | Collation | Nullable | Default
---------+-----------------------+-----------+----------+----------
  id     | character varying(50) |           | not null |
  mydata | jsonb                 |           |          |
(2 rows)
                 Indexes
-----------------------------------------
  test_pkey PRIMARY KEY, btree (id ASC)
(1 row)

Note that if your model defines the column as JSONB then Alembic stops complaining

from sqlalchemy.dialects.postgresql import JSONB

…

    mydata = Column(JSONB(none_as_null=True))

See also the comment on the PR for JSONB reflection

#277 (comment)

[gordthompson]

Alternatively, this also appears to avoid the issue:

    mydata = Column(JSON(none_as_null=True).with_variant(JSONB(none_as_null=True), "cockroachdb"))

[NotJustPizza]

@gordthompson Thanks for quick response, I will use JSONB columns directly as you suggested, but imho JSON field should just work as abstraction and map JSON to JSONB.