一、問題描述

flask-migrate插件是對Alembic的簡單封裝，當程序使用SQLite數據庫作為backend的時候，使用 flask migrate命令生成自動遷移腳本，使用flask upgrade命令進行數據庫更新，會出現以下問題：

1、op.alter_column、op.alter_table會報錯，因為SQlite不支持替換alter column等操作。

2、對于column的一些constraint,比如：unique、foreignkey，這些操作自動腳本生成的時候無法自動探測，必須手動添加。

3、報錯：ValueError: Constraint must have a name

類似下面

Traceback (most recent call last):File "/home/openlab/flasky/venv/bin/flask", line 11, in <module>sys.exit(main())File "/home/openlab/flasky/venv/lib/python3.6/site-packages/flask/cli.py", line 894, in maincli.main(args=args, prog_name=name)File "/home/openlab/flasky/venv/lib/python3.6/site-packages/flask/cli.py", line 557, in mainreturn super(FlaskGroup, self).main(*args, **kwargs)File "/home/openlab/flasky/venv/lib/python3.6/site-packages/click/core.py", line 697, in mainrv = self.invoke(ctx)File "/home/openlab/flasky/venv/lib/python3.6/site-packages/click/core.py", line 1066, in invokereturn _process_result(sub_ctx.command.invoke(sub_ctx))File "/home/openlab/flasky/venv/lib/python3.6/site-packages/click/core.py", line 1066, in invokereturn _process_result(sub_ctx.command.invoke(sub_ctx))File "/home/openlab/flasky/venv/lib/python3.6/site-packages/click/core.py", line 895, in invokereturn ctx.invoke(self.callback, **ctx.params)File "/home/openlab/flasky/venv/lib/python3.6/site-packages/click/core.py", line 535, in invokereturn callback(*args, **kwargs)File "/home/openlab/flasky/venv/lib/python3.6/site-packages/click/decorators.py", line 17, in new_funcreturn f(get_current_context(), *args, **kwargs)File "/home/openlab/flasky/venv/lib/python3.6/site-packages/flask/cli.py", line 412, in decoratorreturn __ctx.invoke(f, *args, **kwargs)File "/home/openlab/flasky/venv/lib/python3.6/site-packages/click/core.py", line 535, in invokereturn callback(*args, **kwargs)File "/home/openlab/flasky/venv/lib/python3.6/site-packages/flask_migrate/cli.py", line 134, in upgrade_upgrade(directory, revision, sql, tag, x_arg)File "/home/openlab/flasky/venv/lib/python3.6/site-packages/flask_migrate/__init__.py", line 95, in wrappedf(*args, **kwargs)File "/home/openlab/flasky/venv/lib/python3.6/site-packages/flask_migrate/__init__.py", line 280, in upgradecommand.upgrade(config, revision, sql=sql, tag=tag)File "/home/openlab/flasky/venv/lib/python3.6/site-packages/alembic/command.py", line 254, in upgradescript.run_env()File "/home/openlab/flasky/venv/lib/python3.6/site-packages/alembic/script/base.py", line 427, in run_envutil.load_python_file(self.dir, 'env.py')File "/home/openlab/flasky/venv/lib/python3.6/site-packages/alembic/util/pyfiles.py", line 81, in load_python_filemodule = load_module_py(module_id, path)File "/home/openlab/flasky/venv/lib/python3.6/site-packages/alembic/util/compat.py", line 82, in load_module_pyspec.loader.exec_module(module)File "<frozen importlib._bootstrap_external>", line 678, in exec_moduleFile "<frozen importlib._bootstrap>", line 219, in _call_with_frames_removedFile "migrations/env.py", line 88, in <module>run_migrations_online()File "migrations/env.py", line 81, in run_migrations_onlinecontext.run_migrations()File "<string>", line 8, in run_migrationsFile "/home/openlab/flasky/venv/lib/python3.6/site-packages/alembic/runtime/environment.py", line 836, in run_migrationsself.get_context().run_migrations(**kw)File "/home/openlab/flasky/venv/lib/python3.6/site-packages/alembic/runtime/migration.py", line 330, in run_migrationsstep.migration_fn(**kw)File "/home/openlab/flasky/flasky/migrations/versions/bb9a3eb8a5fb_second_migrate.py", line 30, in upgradebatch_op.create_unique_constraint(None, ['name'])File "/usr/lib/python3.6/contextlib.py", line 88, in __exit__next(self.gen)File "/home/openlab/flasky/venv/lib/python3.6/site-packages/alembic/operations/base.py", line 300, in batch_alter_tableimpl.flush()File "/home/openlab/flasky/venv/lib/python3.6/site-packages/alembic/operations/batch.py", line 76, in flushfn(*arg, **kw)File "/home/openlab/flasky/venv/lib/python3.6/site-packages/alembic/operations/batch.py", line 343, in add_constraintraise ValueError("Constraint must have a name") ValueError: Constraint must have a name

二、解決方案

步驟1：新增插件flask-migrate以及flask_sqlalchemy的初始化實參

from flask_sqlalchemy import SQLAlchemy from sqlalchemy import MetaData from flask_migrate import Migrate###初始化插件 #定義命名慣例，不需要改 naming_convention = {"ix": 'ix_%(column_0_label)s',"uq": "uq_%(table_name)s_%(column_0_name)s","ck": "ck_%(table_name)s_%(column_0_name)s","fk": "fk_%(table_name)s_%(column_0_name)s_%(referred_table_name)s","pk": "pk_%(table_name)s" } #初始化db,將命名慣例naming_convention傳給SQL_Alchemy,解決“ValueError: Constraint must have a name"的問題 db = SQLAlchemy(app=app,metadata=MetaData(naming_convention=naming_convention)) #使用batch操作替換普通操作，因為普通操作不支持表名，列名的改變！ migrate = Migrate(app,db,render_as_batch=True)

當然，前提是安裝了flask-migrate；工廠函數初始化的方式亦然；

步驟2：生成自動化遷移腳本

>> flask db init >> flask db migrate -m 'first migrate'

步驟3：檢查生成的腳本（./migrations/versions/XXXX.py)

def upgrade():# ### commands auto generated by Alembic - please adjust! ###with op.batch_alter_table('roles', schema=None) as batch_op:batch_op.add_column(sa.Column('name2', sa.String(length=64), nullable=True))batch_op.create_unique_constraint(batch_op.f('uq_roles_name'), ['name'])batch_op.create_unique_constraint(batch_op.f('uq_roles_name1'), ['name1'])batch_op.create_unique_constraint(batch_op.f('uq_roles_name2'), ['name2'])batch_op.drop_constraint('uq_roles_id', type_='unique')with op.batch_alter_table('users', schema=None) as batch_op:batch_op.create_unique_constraint(batch_op.f('uq_users_name555'), ['name555'])# ### end Alembic commands ###def downgrade():# ### commands auto generated by Alembic - please adjust! ###with op.batch_alter_table('users', schema=None) as batch_op:batch_op.drop_constraint(batch_op.f('uq_users_name555'), type_='unique')with op.batch_alter_table('roles', schema=None) as batch_op:batch_op.create_unique_constraint('uq_roles_id', ['name1'])batch_op.drop_constraint(batch_op.f('uq_roles_name2'), type_='unique')batch_op.drop_constraint(batch_op.f('uq_roles_name1'), type_='unique')batch_op.drop_constraint(batch_op.f('uq_roles_name'), type_='unique')batch_op.drop_column('name2')

自動生成的腳本可能有錯，要人工檢查。

‘uq_roles_id’就是按照naming_convention生成的constraint的name,之前該位置的值為None,這是報錯 Constraint must have a name的關鍵。

步驟4：執行更新操作

>> flask db upgrade

三、參考鏈接

https://stackoverflow.com/questions/45527323/flask-sqlalchemy-upgrade-failing-after-updating-models-need-an-explanation-on-h

http://alembic.zzzcomputing.com/en/latest/ops.html#alembic.operations.Operations.create_foreign_key.params.name

http://alembic.zzzcomputing.com/en/latest/batch.html

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