Flask数据迁移保留现有的表

在使用flask-migrate 进行数据迁移的过程中，会删除掉数据库中所有的表。因为在alembic的设定中：如果目标数据库存在不属于元数据的表，自动生成过程通常会假定这些表是数据库中的无用，并对每个表执行 Operations.drop _ table ()操作。

为了防止这种情况的出现，使用 EnvironmentContext.configure.include _ name 钩子进行过滤：

（The EnvironmentContext.configure.include_name hook is also most appropriate to limit the names of tables in the target database to be considered. If a target database has many tables that are not part of the MetaData, the autogenerate process will normally assume these are extraneous tables in the database to be dropped, and it will generate a Operations.drop_table() operation for each. To prevent this, the EnvironmentContext.configure.include_name hook may be used to search for each name within the tables collection of the MetaData object and ensure names which aren’t present are not included:）

from flask import current_app
'''
target_metadata 获取当前项目中所有的表数据
在 MetaData 对象的表集合中搜索每个名称，并确保不包含不存在的名称
'''
target_metadata = current_app.extensions['migrate'].db.metadata

def include_name(name, type_, parent_names):
    if type_ == "table":
        return name in target_metadata.tables
    else:
        return True

#默认会执行 run_migrations_online()函数，在context.configure配置以下几项
context.configure(
    # ...
    target_metadata = target_metadata,
    include_name = include_name,
    include_schemas = False
)

 env.py

from __future__ import with_statement

import logging
from logging.config import fileConfig

from flask import current_app

from alembic import context

# this is the Alembic Config object, which provides
# access to the values within the .ini file in use.
config = context.config

# Interpret the config file for Python logging.
# This line sets up loggers basically.
fileConfig(config.config_file_name)
logger = logging.getLogger('alembic.env')

# add your model's MetaData object here
# for 'autogenerate' support
# from myapp import mymodel
# target_metadata = mymodel.Base.metadata
config.set_main_option(
    'sqlalchemy.url',
    str(current_app.extensions['migrate'].db.engine.url).replace('%', '%%'))
target_metadata = current_app.extensions['migrate'].db.metadata

# other values from the config, defined by the needs of env.py,
# can be acquired:
# my_important_option = config.get_main_option("my_important_option")
# ... etc.


def run_migrations_offline():
    """Run migrations in 'offline' mode.

    This configures the context with just a URL
    and not an Engine, though an Engine is acceptable
    here as well.  By skipping the Engine creation
    we don't even need a DBAPI to be available.

    Calls to context.execute() here emit the given string to the
    script output.

    """
    url = config.get_main_option("sqlalchemy.url")
    context.configure(
        url=url, target_metadata=target_metadata, literal_binds=True
    )

    with context.begin_transaction():
        context.run_migrations()
def include_name(name, type_, parent_names):
    if type_ == "table":
        return name in target_metadata.tables
    else:
        return True


def run_migrations_online():
    """Run migrations in 'online' mode.

    In this scenario we need to create an Engine
    and associate a connection with the context.

    """

    # this callback is used to prevent an auto-migration from being generated
    # when there are no changes to the schema
    # reference: http://alembic.zzzcomputing.com/en/latest/cookbook.html
    def process_revision_directives(context, revision, directives):
        if getattr(config.cmd_opts, 'autogenerate', False):
            script = directives[0]
            if script.upgrade_ops.is_empty():
                directives[:] = []
                logger.info('No changes in schema detected.')

    connectable = current_app.extensions['migrate'].db.engine

    with connectable.connect() as connection:
        context.configure(
            connection=connection,
            compare_type=True,compare_server_default=True,
            target_metadata=target_metadata,
            process_revision_directives=process_revision_directives,
            **current_app.extensions['migrate'].configure_args,
            include_name = include_name,
            include_schemas = False
        )

        with context.begin_transaction():
            context.run_migrations()


if context.is_offline_mode():
    run_migrations_offline()
else:
    run_migrations_online()