flask: migrate检测不到server_default值的变化

解决：

修改migrations/env.py

在

conf_args = current_app.extensions['migrate'].configure_args

下方增加一行:

conf_args['compare_server_default'] = True

如下:

    conf_args = current_app.extensions['migrate'].configure_args
    conf_args['compare_server_default'] = True
    if conf_args.get("process_revision_directives") is None:
        conf_args["process_revision_directives"] = process_revision_directives