[sqlalchemy] Many to One vs session
I have a table (T) that has a many-to-one relationship (via foreign key inclusion) to a category table (C). Let's name the relationship category. When I retrieve an existing T record, SA populates the category field with an instance of C. I can change the category to a different value by storing a different instance of C into category. My question is does it matter if the various instances of C are associated with the same session as the T instance? Can the C instances come from a different session, or can the be expunge'd from the session that retrieved them? Can I store an entirely new C instance, and if so, will SA do an insert into the C table? Thanks, Mark -- You received this message because you are subscribed to the Google Groups sqlalchemy group. To post to this group, send email to sqlalch...@googlegroups.com. To unsubscribe from this group, send email to sqlalchemy+unsubscr...@googlegroups.com. For more options, visit this group at http://groups.google.com/group/sqlalchemy?hl=en.
Re: [sqlalchemy] Many to One vs session
On 10/29/2010 01:23 PM, Mark Erbaugh wrote: I have a table (T) that has a many-to-one relationship (via foreign key inclusion) to a category table (C). Let's name the relationship category. When I retrieve an existing T record, SA populates the category field with an instance of C. I can change the category to a different value by storing a different instance of C into category. Technically, SQLAlchemy by default does not populate the category relation until you access it the first time (i.e. it is a lazy load). My question is does it matter if the various instances of C are associated with the same session as the T instance? I'm not sure what you mean here. There is no problem having multiple C instances in the same session as the T instance. SQLAlchemy will complain (hopefully; I haven't tried it) if multiple C instances in the same session share a primary key. Can the C instances come from a different session, or can the be expunge'd from the session that retrieved them? The only restriction is, if the T instance is part of session S, then the C instance that you assign to T.category must be part of S or not be part of any session. It is allowed if you first expunge the C instance from another session and merge it into S before assigning it to T.category. Note that there are only a few use cases for moving instances from one session to another that I am aware of: * Caching query results. You expunge the instances from the original session, keep them around in memory (or serialized to disk, memcache, etc.). Then you can merge them back into subsequent sessions to avoid repeated DB queries. * Passing objects between threads. Sessions are not thread-safe, so if you want to pass objects from thread A to thread B, you have to merge session A's objects into session B before thread B can use them. Can I store an entirely new C instance, and if so, will SA do an insert into the C table? Yes, this is part of the save-update cascade[1] which is enabled by default on a relationship. Generally you only worry about cascade for one-to-many or many-to-many relationships. At least that's my experience. -Conor [1] http://www.sqlalchemy.org/docs/orm/relationships.html#the-relationship-api -- You received this message because you are subscribed to the Google Groups sqlalchemy group. To post to this group, send email to sqlalch...@googlegroups.com. To unsubscribe from this group, send email to sqlalchemy+unsubscr...@googlegroups.com. For more options, visit this group at http://groups.google.com/group/sqlalchemy?hl=en.
Re: [sqlalchemy] Many to One vs session
On Oct 29, 2010, at 9:39 PM, Conor wrote: I have a table (T) that has a many-to-one relationship (via foreign key inclusion) to a category table (C). Let's name the relationship category. When I retrieve an existing T record, SA populates the category field with an instance of C. I can change the category to a different value by storing a different instance of C into category. Technically, SQLAlchemy by default does not populate the category relation until you access it the first time (i.e. it is a lazy load). My question is does it matter if the various instances of C are associated with the same session as the T instance? I'm not sure what you mean here. There is no problem having multiple C instances in the same session as the T instance. SQLAlchemy will complain (hopefully; I haven't tried it) if multiple C instances in the same session share a primary key. Can the C instances come from a different session, or can the be expunge'd from the session that retrieved them? The only restriction is, if the T instance is part of session S, then the C instance that you assign to T.category must be part of S or not be part of any session. It is allowed if you first expunge the C instance from another session and merge it into S before assigning it to T.category. Thanks, that was what I was asking. I think I've tested it with C instances belonging to the same session and to C instances that have been expunged, but I didn't test C instances that were still an active part of a different session. Note that there are only a few use cases for moving instances from one session to another that I am aware of: Caching query results. You expunge the instances from the original session, keep them around in memory (or serialized to disk, memcache, etc.). Then you can merge them back into subsequent sessions to avoid repeated DB queries. Passing objects between threads. Sessions are not thread-safe, so if you want to pass objects from thread A to thread B, you have to merge session A's objects into session B before thread B can use them. Can I store an entirely new C instance, and if so, will SA do an insert into the C table? Yes, this is part of the save-update cascade[1] which is enabled by default on a relationship. Generally you only worry about cascade for one-to-many or many-to-many relationships. At least that's my experience. Mark -- You received this message because you are subscribed to the Google Groups sqlalchemy group. To post to this group, send email to sqlalch...@googlegroups.com. To unsubscribe from this group, send email to sqlalchemy+unsubscr...@googlegroups.com. For more options, visit this group at http://groups.google.com/group/sqlalchemy?hl=en.
