SqlAlchemy 将新字段添加到类并在表中创建相应的列

Posted 2023-03-23

【中文标题】SqlAlchemy 将新字段添加到类并在表中创建相应的列

【英文标题】：SqlAlchemy add new Field to class and create corresponding column in table

【发布时间】：2010-01-20 17:01:53

【问题描述】：

我想向现有的映射类添加一个字段，我将如何自动更新 sql 表。如果将字段添加到类中，sqlalchemy 是否提供使用新列更新数据库的方法。

【参考方案1】：

有时迁移工作量太大 - 您只想在运行更改的代码时自动添加列。所以这里有一个函数可以做到这一点。

注意事项：它涉及 SQLAlchemy 内部结构，并且在每次 SQLAlchemy 进行重大修订时往往需要进行小的更改。 （这样做可能有更好的方法——我不是 SQLAlchemy 专家）。它也不处理约束。

import logging
import re

import sqlalchemy
from sqlalchemy import MetaData, Table, exceptions
import sqlalchemy.engine.ddl

_new_sa_ddl = sqlalchemy.__version__.startswith('0.7')


def create_and_upgrade(engine, metadata):
    """For each table in metadata, if it is not in the database then create it. 
    If it is in the database then add any missing columns and warn about any columns
    whose spec has changed"""
    db_metadata = MetaData()
    db_metadata.bind = engine

    for model_table in metadata.sorted_tables:
        try:
            db_table = Table(model_table.name, db_metadata, autoload=True)
        except exceptions.NoSuchTableError:
            logging.info('Creating table %s' % model_table.name)
            model_table.create(bind=engine)
        else:
            if _new_sa_ddl:
                ddl_c = engine.dialect.ddl_compiler(engine.dialect, None)
            else:
                # 0.6
                ddl_c = engine.dialect.ddl_compiler(engine.dialect, db_table)
            # else:
                # 0.5
                # ddl_c = engine.dialect.schemagenerator(engine.dialect, engine.contextual_connect())

            logging.debug('Table %s already exists. Checking for missing columns' % model_table.name)

            model_columns = _column_names(model_table)
            db_columns = _column_names(db_table)

            to_create = model_columns - db_columns
            to_remove = db_columns - model_columns
            to_check = db_columns.intersection(model_columns)

            for c in to_create:
                model_column = getattr(model_table.c, c)
                logging.info('Adding column %s.%s' % (model_table.name, model_column.name))
                assert not model_column.constraints, \
                    'Arrrgh! I cannot automatically add columns with constraints to the database'\
                        'Please consider fixing me if you care!'
                model_col_spec = ddl_c.get_column_specification(model_column)
                sql = 'ALTER TABLE %s ADD %s' % (model_table.name, model_col_spec)
                engine.execute(sql)

            # It's difficult to reliably determine if the model has changed 
            # a column definition. E.g. the default precision of columns
            # is None, which means the database decides. Therefore when I look at the model
            # it may give the SQL for the column as INTEGER but when I look at the database
            # I have a definite precision, therefore the returned type is INTEGER(11)

            for c in to_check:
                model_column = model_table.c[c]
                db_column = db_table.c[c]
                x =  model_column == db_column

                logging.debug('Checking column %s.%s' % (model_table.name, model_column.name))
                model_col_spec = ddl_c.get_column_specification(model_column)
                db_col_spec = ddl_c.get_column_specification(db_column)

                model_col_spec = re.sub('[(][\d ,]+[)]', '', model_col_spec)
                db_col_spec = re.sub('[(][\d ,]+[)]', '', db_col_spec)
                db_col_spec = db_col_spec.replace('DECIMAL', 'NUMERIC')
                db_col_spec = db_col_spec.replace('TINYINT', 'BOOL')

                if model_col_spec != db_col_spec:
                    logging.warning('Column %s.%s has specification %r in the model but %r in the database' % 
                                       (model_table.name, model_column.name, model_col_spec, db_col_spec))

                if model_column.constraints or db_column.constraints:
                    # TODO, check constraints
                    logging.debug('Column constraints not checked. I am too dumb')

            for c in to_remove:
                model_column = getattr(db_table.c, c)
                logging.warning('Column %s.%s in the database is not in the model' % (model_table.name, model_column.name))


def _column_names(table):
    # Autoloaded columns return unicode column names - make sure we treat all are equal
    return set((unicode(i.name) for i in table.c)) 

【讨论】：

非常感谢！正是我想要的。

【参考方案2】：

SQLAlchemy 本身不支持架构的自动更新，但是有一个第三方SQLAlchemy Migrate 工具可以自动迁移。看看"Database schema versioning workflow" chapter 看看它是如何工作的。

【讨论】：

我可以要求一个新的链接吗？那个人似乎已经死了。提前致谢。

Alembic 取代了 SQLAlchemy-Migrate 项目，该项目是 SQLAlchemy 的原始迁移工具，现在被视为旧版。

【参考方案3】：

Alembic 是提供数据库迁移的最新软件包。

见sqlalchemy docs regarding migration here。

【参考方案4】：

# database.py has definition for engine.
# from sqlalchemy import create_engine
# engine = create_engine('mysql://......', convert_unicode=True)

from database import engine
from sqlalchemy import DDL

add_column = DDL('ALTER TABLE USERS ADD COLUMN city VARCHAR(60) AFTER email')
engine.execute(add_column)

【参考方案5】：

可以使用 sqlalchemy-migrate 但实际上不使用迁移：

sqlalchemy.MetaData(bind=dbinterface.db.engine)

table = sqlalchemy.schema.Table(table_name, meta_data)

try:
    col = sqlalchemy.Column('column_name', sqlalchemy.String)
    col.create(table)
except Exception as e:
    print "Error adding column: ".format(e)

要与 python 3 一起使用，我需要 sqlalchemy-migrate==0.12.0。

【参考方案6】：

您可以安装“DB Browser (SQLite)”并打开您当前的数据库文件并在您的数据库中简单地添加/编辑表并保存它，然后运行您的应用程序 （保存上述过程后在模型中添加脚本）