Re: [sqlalchemy] cross-database joins with MySQL
Michael Bayer wrote:
We have engines set up like:
engine1 = create_engine('mysql://username:passw...@server/db1')
engine2 = create_engine('mysql://username:passw...@server/db2')
..and then have them bound to separate sessions, with separate model classes
mapped to them.
Now, mysql supports cross database joins, eg:
select t1.colwhatever
from db1.table1 as t1,db2.table2 as t2
where t1.something=t2.something
Is it possible to express that in SQLAlchemy, particularly at the ORM layer
with the multiple session/engine/model setup described above?
(I suppose the case to test would be, if ModelA is bound to engine1 and ModelB
is bound to engine2, how would we do:
session.query(ModelA,ModelB,ModelA.something==ModelB.something)
...or something similar, if the above isn't possible?
its not possible across two distinct database connections, no. Only the
database can do joins, and that requires a single connection session to do so.
Right, but how can I create an engine such that it can be used to access
two databases? Is it as simple as setting the __tablename__ as
'db.tablename' rather than just 'tablename'?
Chris
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RE: [sqlalchemy] cross-database joins with MySQL
-Original Message-
From: sqlalchemy@googlegroups.com
[mailto:sqlalch...@googlegroups.com] On Behalf Of Chris Withers
Sent: 29 June 2010 10:28
To: sqlalchemy@googlegroups.com
Subject: Re: [sqlalchemy] cross-database joins with MySQL
Michael Bayer wrote:
We have engines set up like:
engine1 = create_engine('mysql://username:passw...@server/db1')
engine2 = create_engine('mysql://username:passw...@server/db2')
..and then have them bound to separate sessions, with
separate model classes mapped to them.
Now, mysql supports cross database joins, eg:
select t1.colwhatever
from db1.table1 as t1,db2.table2 as t2
where t1.something=t2.something
Is it possible to express that in SQLAlchemy, particularly
at the ORM layer with the multiple session/engine/model setup
described above?
(I suppose the case to test would be, if ModelA is bound
to engine1 and ModelB is bound to engine2, how would we do:
session.query(ModelA,ModelB,ModelA.something==ModelB.something)
...or something similar, if the above isn't possible?
its not possible across two distinct database connections,
no. Only the database can do joins, and that requires a
single connection session to do so.
Right, but how can I create an engine such that it can be
used to access
two databases? Is it as simple as setting the __tablename__ as
'db.tablename' rather than just 'tablename'?
Chris
You want the 'schema' parameter to the Table:
http://www.sqlalchemy.org/docs/metadata.html#specifying-the-schema-name
(Probably need to use __table_args__ if you are using declarative)
Simon
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Re: [sqlalchemy] cross-database joins with MySQL
On Jun 11, 2010, at 12:51 PM, Chris Withers wrote:
Hi All,
We have engines set up like:
engine1 = create_engine('mysql://username:passw...@server/db1')
engine2 = create_engine('mysql://username:passw...@server/db2')
..and then have them bound to separate sessions, with separate model classes
mapped to them.
Now, mysql supports cross database joins, eg:
select t1.colwhatever
from db1.table1 as t1,db2.table2 as t2
where t1.something=t2.something
Is it possible to express that in SQLAlchemy, particularly at the ORM layer
with the multiple session/engine/model setup described above?
(I suppose the case to test would be, if ModelA is bound to engine1 and
ModelB is bound to engine2, how would we do:
session.query(ModelA,ModelB,ModelA.something==ModelB.something)
...or something similar, if the above isn't possible?
its not possible across two distinct database connections, no. Only the
database can do joins, and that requires a single connection session to do so.
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