使用SQLAlchemy将表连接到两个数据库中

Question

我正在使用两个MySQL数据库.我想在DBAlchemy中加入来自DB 1的表和DB2中的表.

我在sqlalchemy中创建数据访问层时使用的是automap_base,如下所示......

class DBHandleBase(object):

    def __init__(self, connection_string='mysql+pymysql://root:xxxxxxx@localhost/services', pool_recycle=3600):
            self.Base_ = automap_base()
            self.engine_ = create_engine(connection_string,
                                         pool_recycle = pool_recycle)
            self.Base_.prepare(self.engine_, reflect=True)
            self.session_ = Session(self.engine_)

我的桌子类就像

class T1D1_Repo():


    def __init__(self, dbHandle):
        # create a cursor
        self.Table_ = dbHandle.Base_.classes.t1
        self.session_ = dbHandle.session_

我正在这样做,

db1_handle = DB1_Handle()
db2_handle = DB2_Handle()
t1d1_repo = T1D1_Repo(handle)
t1d2_repo = T1D2_Repo(person_handle)

result = t1d1_repo.session_.query(
            t1d1_repo.Table_,
            t1d2_repo.Table_).join(t1d2_repo.Table_, (
                t1d1_repo.Table_.person_id
                == t1d2_repo.Table_.uuid))

我收到这样的错误:

sqlalchemy.exc.ProgrammingError: (pymysql.err.ProgrammingError) (1146, "Table 'db1.t1d2' doesn't exist") [SQL: 'SELECT 

我们在数据库db1中创建了表t1,在数据库db2中创建了表t2.

是否可以在sqlalchemy ORM中的两个数据库表中进行连接？怎么实现呢？

Answer 1

在MySQL中,数据库与模式同义.例如,在Postgresql中,您可以在数据库中的多个模式之间进行查询,但不能在数据库之间进行查询(直接),您可以在MySQL中的多个数据库之间进行查询,因为两者之间没有区别.

有鉴于此,MySQL中多数据库查询的可能解决方案可能是使用单个引擎,会话和Base处理您的模式并将schema关键字参数传递给表,或者反映两个模式以使它们完全合格. .

由于我没有你的数据,我在一个名为sopython和sopython2的测试服务器上制作了两个模式(MySQL数据库):

mysql> create database sopython;
Query OK, 1 row affected (0,00 sec)

mysql> create database sopython2;
Query OK, 1 row affected (0,00 sec)

并在每个中添加了一个表格:

mysql> use sopython
Database changed
mysql> create table foo (foo_id integer not null auto_increment primary key, name text);
Query OK, 0 rows affected (0,05 sec)

mysql> insert into foo (name) values ('heh');
Query OK, 1 row affected (0,01 sec)

mysql> use sopython2
Database changed
mysql> create table bar (bar_id integer not null auto_increment primary key, foo_id integer, foreign key (foo_id) references `sopython`.`foo` (foo_id)) engine=InnoDB;
Query OK, 0 rows affected (0,07 sec)

mysql> insert into bar (foo_id) values (1);
Query OK, 1 row affected (0,01 sec)

在Python中:

In [1]: from sqlalchemy import create_engine

In [2]: from sqlalchemy.orm import sessionmaker

In [3]: from sqlalchemy.ext.automap import automap_base

In [4]: Session = sessionmaker()

In [5]: Base = automap_base()

创建引擎而不指定默认使用的模式(数据库):

In [6]: engine = create_engine('mysql+pymysql://user:pass@:6603/')

In [7]: Base.prepare(engine, reflect=True, schema='sopython')

In [8]: Base.prepare(engine, reflect=True, schema='sopython2')
/home/user/SO/lib/python3.5/site-packages/sqlalchemy/ext/declarative/clsregistry.py:120: SAWarning: This declarative base already contains a class with the same class name and module name as sqlalchemy.ext.automap.foo, and will be replaced in the string-lookup table.
  item.__name__

警告是我不完全理解的,可能是由于两个表之间的外键引用导致重新反映foo,但它似乎没有引起麻烦.

警告是第二次调用重新prepare()创建和替换第一次调用中反映的表的类的结果.避免所有这一切的方法是首先使用元数据反映来自两个模式的表,然后准备:

Base.metadata.reflect(engine, schema='sopython')
Base.metadata.reflect(engine, schema='sopython2')
Base.prepare()

毕竟这可以查询加入foo和bar:

In [9]: Base.metadata.bind = engine

In [10]: session = Session()

In [11]: query = session.query(Base.classes.bar).\
    ...:     join(Base.classes.foo).\
    ...:     filter(Base.classes.foo.name == 'heh')

In [12]: print(query)
SELECT sopython2.bar.bar_id AS sopython2_bar_bar_id, sopython2.bar.foo_id AS sopython2_bar_foo_id 
FROM sopython2.bar INNER JOIN sopython.foo ON sopython.foo.foo_id = sopython2.bar.foo_id 
WHERE sopython.foo.name = %(name_1)s

In [13]: query.all()
Out[13]: [<sqlalchemy.ext.automap.bar at 0x7ff1ed7eee10>]

In [14]: _[0]
Out[14]: <sqlalchemy.ext.automap.bar at 0x7ff1ed7eee10>

In [15]: _.foo
Out[15]: <sqlalchemy.ext.automap.foo at 0x7ff1ed7f09b0>