generate schemas optionally

[daeun-ws]

Hello, thank you for your work on this. I've been using your project very helpfully.
I have one feature request : generate schemas optionally.

I'm using a db user that does not have permission to execute DDL for tortoise-orm and executing shell command configured with the user occurs OperationalError.

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/dapi-venv/bin/tortoise-cli", line 8, in <module>
    sys.exit(main())
  File "/dapi-venv/lib/python3.9/site-packages/tortoise_cli/cli.py", line 76, in main
    cli()
  File "/dapi-venv/lib/python3.9/site-packages/click/core.py", line 1130, in __call__
    return self.main(*args, **kwargs)
  File "/dapi-venv/lib/python3.9/site-packages/click/core.py", line 1055, in main
    rv = self.invoke(ctx)
  File "/dapi-venv/lib/python3.9/site-packages/click/core.py", line 1654, in invoke
    super().invoke(ctx)
  File "/dapi-venv/lib/python3.9/site-packages/click/core.py", line 1404, in invoke
    return ctx.invoke(self.callback, **ctx.params)
  File "/dapi-venv/lib/python3.9/site-packages/click/core.py", line 760, in invoke
    return __callback(*args, **kwargs)
  File "/dapi-venv/lib/python3.9/site-packages/click/decorators.py", line 26, in new_func
    return f(get_current_context(), *args, **kwargs)
  File "/dapi-venv/lib/python3.9/site-packages/tortoise_cli/cli.py", line 21, in wrapper
    loop.run_until_complete(f(*args, **kwargs))
  File "/usr/local/lib/python3.9/asyncio/base_events.py", line 642, in run_until_complete
    return future.result()
  File "/dapi-venv/lib/python3.9/site-packages/tortoise_cli/cli.py", line 55, in cli
    await Tortoise.generate_schemas(safe=True)
  File "/dapi-venv/lib/python3.9/site-packages/tortoise/__init__.py", line 662, in generate_schemas
    await generate_schema_for_client(connection, safe)
  File "/dapi-venv/lib/python3.9/site-packages/tortoise/utils.py", line 31, in generate_schema_for_client
    await generator.generate_from_string(schema)
  File "/dapi-venv/lib/python3.9/site-packages/tortoise/backends/base/schema_generator.py", line 448, in generate_from_string
    await self.client.execute_script(creation_string)
  File "/dapi-venv/lib/python3.9/site-packages/tortoise/backends/mysql/client.py", line 52, in translate_exceptions_
    raise OperationalError(exc)
tortoise.exceptions.OperationalError: (1142, "CREATE command denied to user 'user_rw'@'10.240.35.55' for table 'city'")

So I think the command will be more useful if it allows to generate schema optionally like below.

@click.group(context_settings={"help_option_names": ["-h", "--help"]})
@click.version_option(__version__, "-V", "--version")
@click.option(
    "-c",
    "--config",
    help="TortoiseORM config dictionary path, like settings.TORTOISE_ORM",
)
@click.option(
    "--generate-schema",
    show_default=True,
    default=True,
    help="Generate database schema.",
)
@click.pass_context
@coro
async def cli(ctx: click.Context, config: Optional[str], generate_schema: bool):
    ...
    if generate_schema:
        print("generate schema automatically..")
        tortoise_config = utils.get_tortoise_config(ctx, config)
        await Tortoise.init(config=tortoise_config)
        await Tortoise.generate_schemas(safe=True)

• with generate-schema param

$ tortoise-cli --generate-schema false -c settings.TORTOISE_ORM shell

• without generate-schema param (generate schemas by default)

$ tortoise-cli -c settings.TORTOISE_ORM shell 

I'll be appreciated if you consider my request and give me an opportunity to create PR for this.