ENUM with metadata is not translated to sql CREATE TYPE in autogenerate

Hi,

I know that ENUM autogeneration is not entirely complete and polished, but I think I might have encountered not something not working, but working incorrectly :)

Describe the bug

1. Autogenerate for postgres ENUM with metadata=... generates a migration with Metadata(bind=None) without importing Metadata, resulting in NameError when running migration.
2. The enum type is not created in sql if specified with metadata=..., even though it seems like a well-documented use-case in sqlalchemy docs.

Expected behavior

• from sqlalchemy import MetaData
• Create the enum type in the database

To Reproduce

from enum import Enum

from sqlalchemy import BigInteger, Column, MetaData, Table
from sqlalchemy.dialects.postgresql import ENUM

metadata = MetaData(schema="myschema")

class SomeEnum(str, Enum):
    OPEN = "OPEN"
    CLOSED = "CLOSED"

some_enum = ENUM(
    SomeEnum,
    metadata=metadata,
    schema=metadata.schema,
)

some_table = Table(
    "some_table",
    metadata,
    Column("id", BigInteger, primary_key=True),
    Column("some_enum_value", some_enum),
)

Resulting migration file:

"""initial

Revision ID: 653c623403f4
Revises: 
Create Date: 2021-06-24 18:15:52.750889

"""
from alembic import op
import sqlalchemy as sa
from sqlalchemy.dialects import postgresql

# revision identifiers, used by Alembic.
revision = '653c623403f4'
down_revision = None
branch_labels = None
depends_on = None


def upgrade():
    # ### commands auto generated by Alembic - please adjust! ###
    op.create_table('some_table',
    sa.Column('id', sa.BigInteger(), nullable=False),
    sa.Column('some_enum_value', postgresql.ENUM('OPEN', 'CLOSED', name='someenum', schema='myschema', metadata=MetaData(bind=None)), nullable=True),
    sa.PrimaryKeyConstraint('id'),
    schema='myschema'
    )
    # ### end Alembic commands ###


def downgrade():
    # ### commands auto generated by Alembic - please adjust! ###
    op.drop_table('some_table', schema='myschema')
    # ### end Alembic commands ###

Error

18:15 $ alembic upgrade head --sql
INFO  [alembic.runtime.migration] Context impl PostgresqlImpl.
INFO  [alembic.runtime.migration] Generating static SQL
INFO  [alembic.runtime.migration] Will assume transactional DDL.
BEGIN;

CREATE TABLE myschema.alembic_version (
    version_num VARCHAR(32) NOT NULL, 
    CONSTRAINT alembic_version_pkc PRIMARY KEY (version_num)
);

INFO  [alembic.runtime.migration] Running upgrade  -> 653c623403f4, initial
-- Running upgrade  -> 653c623403f4

Traceback (most recent call last):
  File "/home/.../bin/alembic", line 8, in <module>
    sys.exit(main())
  File "/home/.../lib/python3.8/site-packages/alembic/config.py", line 559, in main
    CommandLine(prog=prog).main(argv=argv)
  File "/home/.../lib/python3.8/site-packages/alembic/config.py", line 553, in main
    self.run_cmd(cfg, options)
  File "/home/.../lib/python3.8/site-packages/alembic/config.py", line 530, in run_cmd
    fn(
  File "/home/.../lib/python3.8/site-packages/alembic/command.py", line 293, in upgrade
    script.run_env()
  File "/home/.../lib/python3.8/site-packages/alembic/script/base.py", line 490, in run_env
    util.load_python_file(self.dir, "env.py")
  File "/home/.../lib/python3.8/site-packages/alembic/util/pyfiles.py", line 97, in load_python_file
    module = load_module_py(module_id, path)
  File "/home/.../lib/python3.8/site-packages/alembic/util/compat.py", line 184, in load_module_py
    spec.loader.exec_module(module)
  File "<frozen importlib._bootstrap_external>", line 783, in exec_module
  File "<frozen importlib._bootstrap>", line 219, in _call_with_frames_removed
  File "migrations/env.py", line 90, in <module>
    run_migrations_offline()
  File "migrations/env.py", line 60, in run_migrations_offline
    context.run_migrations()
  File "<string>", line 8, in run_migrations
  File "/home/.../lib/python3.8/site-packages/alembic/runtime/environment.py", line 813, in run_migrations
    self.get_context().run_migrations(**kw)
  File "/home/.../lib/python3.8/site-packages/alembic/runtime/migration.py", line 561, in run_migrations
    step.migration_fn(**kw)
  File "/home/.../migrations/versions/2021_06_24_653c623403f4_initial.py", line 23, in upgrade
    sa.Column('some_enum_value', postgresql.ENUM('OPEN', 'CLOSED', name='someenum', schema='myschema', metadata=MetaData(bind=None)), nullable=True),
NameError: name 'MetaData' is not defined

Note that adding the missing import (from sqlalchemy import MetaData) results in a proper sql, but without the CREATE TYPE:

alembic upgrade head --sql
INFO  [alembic.runtime.migration] Context impl PostgresqlImpl.
INFO  [alembic.runtime.migration] Generating static SQL
INFO  [alembic.runtime.migration] Will assume transactional DDL.
BEGIN;

CREATE TABLE myschema.alembic_version (
    version_num VARCHAR(32) NOT NULL, 
    CONSTRAINT alembic_version_pkc PRIMARY KEY (version_num)
);

INFO  [alembic.runtime.migration] Running upgrade  -> 653c623403f4, initial
-- Running upgrade  -> 653c623403f4

CREATE TABLE myschema.some_table (
    id BIGSERIAL NOT NULL, 
    some_enum_value myschema.someenum, 
    PRIMARY KEY (id)
);

INSERT INTO myschema.alembic_version (version_num) VALUES ('653c623403f4');

COMMIT;

Versions.

• OS: Ubuntu 20.04.2
• Python: 3.8.5
• Alembic: 1.6.5
• SQLAlchemy: 1.3.24 (I can't upgrade to 1.4 yet)
• Database: PostgreSQL 12.7
• DBAPI: psycopg2-binary 2.8.6

Additional context

Note that removing the metadata argument makes the issue disappear:

some_enum = ENUM(
    SomeEnum,
    # metadata=metadata,
    schema=metadata.schema,
)

gives no metadata=Metadata(bind=None), so no NameError is given as a result:

sa.Column('some_enum_value', postgresql.ENUM('OPEN', 'CLOSED', name='someenum', schema='myschema'), nullable=True),

Also, the resulting SQL is as expected:

-- Running upgrade  -> c2d662ec840a

CREATE TYPE myschema.someenum AS ENUM ('OPEN', 'CLOSED');

CREATE TABLE myschema.some_table (
    id BIGSERIAL NOT NULL, 
    some_enum_value myschema.someenum, 
    PRIMARY KEY (id)
);

According to the sqlalchemy docs:

To use a common enumerated type between multiple tables, the best practice is to declare the Enum or ENUM independently, and associate it with the MetaData object itself:
...
If we specify checkfirst=True, the individual table-level create operation will check for the ENUM and create if not exists:

So I guess in scenario where I provided the metadata argument, it is expected that the enum would not be created by sqlalchemy table create unless asked to with checkfirst=True.
However I think that alembic should either produce the same SQL statements in both cases (metadata argument to enum specified or not), or at least inform in the docs how to enforce the checkfirst-like behaviour.

Reproduce the issue with the provided MetaData, PostgreSQL ENUM, and generated migration, then inspect Alembic's autogenerate handling of the resulting migration. Verify the generated migration can run without a missing MetaData import and that offline SQL includes CREATE TYPE before CREATE TABLE.