AttributeError: 'PGDDLCompiler' object has no attribute 'visit_clause'

Hi, I have this issue when I try to run upgrade on a migration that will change length of a VARCHAR column. This is traceback:

Traceback

/.../env/lib/python3.7/site-packages/flask_sqlalchemy/__init__.py:794: FSADeprecationWarning: SQLALCHEMY_TRACK_MODIFICATIONS adds significant overhead and will be disabled by default in the future.  Set it to True or False to suppress this warning.
  'SQLALCHEMY_TRACK_MODIFICATIONS adds significant overhead and '
INFO  [alembic.runtime.migration] Context impl CockroachDBImpl.
INFO  [alembic.runtime.migration] Will assume non-transactional DDL.
INFO  [alembic.runtime.migration] Running upgrade 57cc0f0c7862 -> 30684b5c7f5a, empty message
Traceback (most recent call last):
  File "/.../env/lib/python3.7/site-packages/sqlalchemy/sql/visitors.py", line 77, in _compiler_dispatch
    meth = getter(visitor)
AttributeError: 'PGDDLCompiler' object has no attribute 'visit_clause'

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/.../env/lib/python3.7/site-packages/sqlalchemy/ext/compiler.py", line 426, in _wrap_existing_dispatch
    return existing_dispatch(element, compiler, **kw)
  File "/.../env/lib/python3.7/site-packages/sqlalchemy/sql/visitors.py", line 79, in _compiler_dispatch
    raise exc.UnsupportedCompilationError(visitor, cls)
sqlalchemy.exc.UnsupportedCompilationError: Compiler <sqlalchemy.dialects.postgresql.base.PGDDLCompiler object at 0x108b3ba90> can't render element of type <class 'sqlalchemy.sql.elements.ClauseElement'>

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "manage.py", line 26, in <module>
    manager.run()
  File "/.../env/lib/python3.7/site-packages/flask_script/__init__.py", line 417, in run
    result = self.handle(argv[0], argv[1:])
  File "/.../env/lib/python3.7/site-packages/flask_script/__init__.py", line 386, in handle
    res = handle(*args, **config)
  File "/.../env/lib/python3.7/site-packages/flask_script/commands.py", line 216, in __call__
    return self.run(*args, **kwargs)
  File "/.../env/lib/python3.7/site-packages/flask_migrate/__init__.py", line 95, in wrapped
    f(*args, **kwargs)
  File "/.../env/lib/python3.7/site-packages/flask_migrate/__init__.py", line 280, in upgrade
    command.upgrade(config, revision, sql=sql, tag=tag)
  File "/.../env/lib/python3.7/site-packages/alembic/command.py", line 254, in upgrade
    script.run_env()
  File "/.../env/lib/python3.7/site-packages/alembic/script/base.py", line 427, in run_env
    util.load_python_file(self.dir, 'env.py')
  File "/.../env/lib/python3.7/site-packages/alembic/util/pyfiles.py", line 81, in load_python_file
    module = load_module_py(module_id, path)
  File "/.../env/lib/python3.7/site-packages/alembic/util/compat.py", line 145, in load_module_py
    spec.loader.exec_module(module)
  File "<frozen importlib._bootstrap_external>", line 728, in exec_module
  File "<frozen importlib._bootstrap>", line 219, in _call_with_frames_removed
  File "migrations/env.py", line 88, in <module>
    run_migrations_online()
  File "migrations/env.py", line 81, in run_migrations_online
    context.run_migrations()
  File "<string>", line 8, in run_migrations
  File "/.../env/lib/python3.7/site-packages/alembic/runtime/environment.py", line 836, in run_migrations
    self.get_context().run_migrations(**kw)
  File "/.../env/lib/python3.7/site-packages/alembic/runtime/migration.py", line 330, in run_migrations
    step.migration_fn(**kw)
  File "/.../test/migrations/versions/30684b5c7f5a_.py", line 24, in upgrade
    existing_nullable=True)
  File "<string>", line 8, in alter_column
  File "<string>", line 3, in alter_column
  File "/.../env/lib/python3.7/site-packages/alembic/operations/ops.py", line 1444, in alter_column
    return operations.invoke(alt)
  File "/.../env/lib/python3.7/site-packages/alembic/operations/base.py", line 319, in invoke
    return fn(self, operation)
  File "/.../env/lib/python3.7/site-packages/alembic/operations/toimpl.py", line 53, in alter_column
    **operation.kw
  File "/.../env/lib/python3.7/site-packages/alembic/ddl/postgresql.py", line 101, in alter_column
    existing_nullable=existing_nullable,
  File "/.../env/lib/python3.7/site-packages/alembic/ddl/impl.py", line 115, in _exec
    return conn.execute(construct, *multiparams, **params)
  File "/.../env/lib/python3.7/site-packages/sqlalchemy/engine/base.py", line 948, in execute
    return meth(self, multiparams, params)
  File "/.../env/lib/python3.7/site-packages/sqlalchemy/sql/ddl.py", line 68, in _execute_on_connection
    return connection._execute_ddl(self, multiparams, params)
  File "/.../env/lib/python3.7/site-packages/sqlalchemy/engine/base.py", line 1003, in _execute_ddl
    if not self.schema_for_object.is_default else None)
  File "<string>", line 1, in <lambda>
  File "/.../env/lib/python3.7/site-packages/sqlalchemy/sql/elements.py", line 442, in compile
    return self._compiler(dialect, bind=bind, **kw)
  File "/.../env/lib/python3.7/site-packages/sqlalchemy/sql/ddl.py", line 26, in _compiler
    return dialect.ddl_compiler(dialect, self, **kw)
  File "/.../env/lib/python3.7/site-packages/sqlalchemy/sql/compiler.py", line 219, in __init__
    self.string = self.process(self.statement, **compile_kwargs)
  File "/.../env/lib/python3.7/site-packages/sqlalchemy/sql/compiler.py", line 245, in process
    return obj._compiler_dispatch(self, **kwargs)
  File "/.../env/lib/python3.7/site-packages/sqlalchemy/ext/compiler.py", line 435, in <lambda>
    lambda *arg, **kw: existing(*arg, **kw))
  File "/.../env/lib/python3.7/site-packages/sqlalchemy/ext/compiler.py", line 474, in __call__
    return fn(element, compiler, **kw)
  File "/.../env/lib/python3.7/site-packages/sqlalchemy/ext/compiler.py", line 430, in _wrap_existing_dispatch
    "compilation handler." % type(element))
sqlalchemy.exc.CompileError: <class 'alembic.ddl.postgresql.PostgresqlColumnType'> construct has no default compilation handler.

Environment

python: v3.7.0
cockroachdb: v2.1.4

Packages

alembic==1.0.5
cockroachdb==0.3.0
SQLAlchemy==1.2.11
psycopg2==2.7.5
psycopg2-binary==2.7.5
Flask-SQLAlchemy==2.3.2

Note: Adding this line to alembic.ini doesn’t change anything:

[sqlalchemy.dialects]
cockroachdb = cockroachdb.sqlalchemy.dialect:CockroachDBDialect

---

Detailed Exception Reproducing Scenario:

1. Create these files:

db-docker-compose.yml:

version: "3"

services:
  roach1:
      image: cockroachdb/cockroach:v2.1.4
      command: start --insecure
      ports:
        - 26257:26257
        - 8080:8080
      volumes:
        - cockroach1-data:/cockroach/cockroach-data
      networks:
        - roachnet


networks:
  roachnet:
    driver: bridge

volumes:
  cockroach1-data:

setup_db.sh:

#!/usr/bin/env bash
set -ex
docker-compose -f db-docker-compose.yml up -d

docker-compose -f db-docker-compose.yml exec roach1 ./cockroach sql --insecure -e 'DROP DATABASE IF EXISTS testdb'
docker-compose -f db-docker-compose.yml exec roach1 ./cockroach sql --insecure -e 'CREATE DATABASE testdb'

manage.py

from flask import Flask
from flask_sqlalchemy import SQLAlchemy
from flask_script import Manager
from flask_migrate import Migrate, MigrateCommand
from sqlalchemy.dialects import postgresql

import cockroachdb

app = Flask(__name__)
app.config['SQLALCHEMY_DATABASE_URI'] = 'cockroachdb://root@localhost:26257/testdb?sslmode=disable'

db = SQLAlchemy(app)
migrate = Migrate(app, db)

manager = Manager(app)
manager.add_command('db', MigrateCommand)


class User(db.Model):
    id = db.Column(db.INTEGER, primary_key=True)
    name = db.Column(db.VARCHAR(length=64))


if __name__ == '__main__':
    manager.run()

1. ./setup_db.sh

2. python manage.py db init (python v3.7.0), and in the migrations/env.py file, add compare_type=True to context.configure(...) arguments, becuase we want alembic compare columns type.

3. python manage.py db migrate, will create this file: migrations/versions/57cc0f0c7862_.py:

"""empty message

Revision ID: 57cc0f0c7862
Revises: 
Create Date: 2019-01-27 11:40:29.338727

"""
from alembic import op
import sqlalchemy as sa


# revision identifiers, used by Alembic.
revision = '57cc0f0c7862'
down_revision = None
branch_labels = None
depends_on = None


def upgrade():
    # ### commands auto generated by Alembic - please adjust! ###
    op.create_table('user',
    sa.Column('id', sa.INTEGER(), nullable=False),
    sa.Column('name', sa.VARCHAR(length=64), nullable=True),
    sa.PrimaryKeyConstraint('id')
    )
    # ### end Alembic commands ###


def downgrade():
    # ### commands auto generated by Alembic - please adjust! ###
    op.drop_table('user')
    # ### end Alembic commands ###

1. python manage.py db upgrade, will create the table on DB.
2. change string column length in the manage.py file: name = db.Column(db.VARCHAR(length=128))
3. python manage.py db migrate, will create another file: migrations/versions/30684b5c7f5a_.py:

"""empty message

Revision ID: 30684b5c7f5a
Revises: 57cc0f0c7862
Create Date: 2019-01-27 11:41:15.854357

"""
from alembic import op
import sqlalchemy as sa


# revision identifiers, used by Alembic.
revision = '30684b5c7f5a'
down_revision = '57cc0f0c7862'
branch_labels = None
depends_on = None


def upgrade():
    # ### commands auto generated by Alembic - please adjust! ###
    op.alter_column('user', 'name',
               existing_type=sa.VARCHAR(length=64),
               type_=sa.VARCHAR(length=128),
               existing_nullable=True)
    # ### end Alembic commands ###


def downgrade():
    # ### commands auto generated by Alembic - please adjust! ###
    op.alter_column('user', 'name',
               existing_type=sa.VARCHAR(length=128),
               type_=sa.VARCHAR(length=64),
               existing_nullable=True)
    # ### end Alembic commands ###

1. python manage.py db upgrade. This commnad will produce the exception and will not change the db.

[drdee]

@mohsenasm – where you able to resolve this?

[rafiss]

We just published a new version: https://pypi.org/project/sqlalchemy-cockroachdb/1.3.0/

If you’re able to try it could you let me know if this is fixed?