You guys have done some great work, thanks. (How many developers are you?)
On Feb 8, 11:50 am, "Michael Bayer" <mike...@zzzcomputing.com> wrote: > Kent wrote: > > Thank you again for your response and time. > > > I think a different approach may be better for me, but was hoping for > > your input. > > > I think I'd like to let session.merge() work out whether it is going > > to do an add() vs. update properties on the object retrieved > > from .get() > > > Then, after merge(), but before the flush(), I can change anything I > > need/run validations and populate primary keys, etc. > > Then I can call flush(). > > > I think this approach should work for me, but here is my question: > > When merge() returns the merged object, how can I tell whether a > > new_instance was created vs get() having returned an existing object? > > "obj in session.new" > > > > > Can I inspect the object (and its children) to find this out? (Am I > > really wondering if this object is persistent? How do I determine > > this?) Or can I inspect the session to determine this? > > > Can I tell which children are "scheduled" for deletion by inspecting > > the session or merged object somehow? > > "obj in session.deleted" > > > > > Lastly, by the time merge() returns, we have no reference any longer > > to the original data, only the merged data. If, for my validations, I > > wanted to compare the old property to the new property, can you > > recommend an approach to this? I'd rather not hit the database twice, > > since merge is going to look up the data anyway. > > whatever Python state you send into merge() remains unchanged. the "new" > data is what is returned. So you could compare those two things. > Alternatively, whats returned also represents in itself a "diff" of the > data you passed into merge vs. what was already in the DB. So if you > wanted to look at individual attributes on the newly merged objects to see > what changed: > > from sqlalchemy.orm import attributes > history = attributes.get_history(instance, "somekey") > > this returns a tuple of (new, unchanged, deleted). For a scalar, each > member is a one-item tuple. > > > > > I was hoping I might be able to extend Session or override its > > behavior so as to keep a copy of the object returned from .get() > > before it merges the changes. > > > Input and ideas for me? > > > Thanks very much in advance, > > Kent > > > On Feb 6, 5:47 pm, Michael Bayer <mike...@zzzcomputing.com> wrote: > >> On Feb 6, 2010, at 2:16 PM, Kent wrote: > > >> > There might/probably is a better way to achieve what I am trying to > >> > achieve. Let me explain: > > >> > I've got a typical Customer/Order/Orderdetail relation. > > >> > merge() appeals to me because it does most all the same work I'd > >> > otherwise need to do. I get passed to me an "Order" object will the > >> > customer object and the details objects. > > >> > Now, when I run merge, if there are any changes to the customer, these > >> > get automagically updated. If a line is missing, it is automatically > >> > deleted. If a new line exists, it is automatically inserted. > > >> > session.merge() does all this. > > >> > However, the above is a bit too simplified. If the Customer's primary > >> > key is not supplied, I need to create this in a special way (not from > >> > a sequence) and then insert it. In this case, I don't even want merge > >> > to *attempt* to get() from the database first because if, by chance, > >> > the customerid I construct is already in use, I do not want to > >> > accidentally update that record, I *want* the database to complain on > >> > the INSERT that the primary key already exists. > > >> > Same with the order and orderdetails. If an orderdetail is supplied > >> > to me without a primary key, I would like to choose the next > >> > appropriate primary key and INSERT for that particular record, so that > >> > (if something is messed up or a race condition exists with another > >> > instance of the program choosing the same primary key) I get a > >> > database error instead of merge() accidentally updating that other > >> > record. > > >> > I don't want to give up the benefits of merge() by programmatically > >> > figuring this out myself. In other words, if I add() to the session > >> > all the lines I *know* should be INSERTs then I need to work out all > >> > the other magical things merge() is doing for me (like deleting > >> > missing lines). > > >> your primary key generation scheme here requires manual steps - you > >> create a new identifier, then you need to check that it doesnt exist. > >> merge() and add() won't do this for you, so you do this yourself as > >> needed with your given objects, and assign those identifiers to the > >> objects. Then you can proceed to merge() them normally. There is no > >> "trick" that will do this for you. > > >> > I tried seeing what would happen if I just called session.add() for > >> > the cases of an INSERT and then calling session.merge() for the entire > >> > cascading order(customer/details), but when I tried that I got: > >> > New Instance conflicts with persistent instance type error. > > >> > I assume this means: you tried to session.add() and object and now I > >> > just looked up this same instance during the session.merge(). > > >> unless I'm forgetting something (possible), merge() should not be able > >> to produce that error, since it is not placing any new instances in the > >> session unless that identity did not exist. Did you turn off autoflush > >> perhaps ? a flush needs to occur after your add() so that the pending > >> object now becomes persistent (this is why Session does its work in a > >> transaction, in case you're concerned about "writes"). > > > -- > > 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. > > -- 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.
