On Thu, Aug 13, 2015 at 2:03 AM, Jinghui Niu <niujing...@gmail.com> wrote: > About the Session.merge(), the documentation gives several examples of use > cases, I think there might be a slight ambiguity in this one: > >> An application which reads an object structure from a file and wishes to >> save it to the database might parse the file, build up the structure, and >> then use merge() to save it to the database, ensuring that the data within >> the file is used to formulate the primary key of each element of the >> structure. Later, when the file has changed, the same process can be re-run, >> producing a slightly different object structure, which can then be merged in >> again, and the Session will automatically update the database to reflect >> those changes, loading each object from the database by primary key and then >> updating its state with the new state given. > > > > In this case when "the same process re-run, producing a slightly different > object structure," does this newer object carry a primary key? If it's just > been produced by parsing from the file, then I think probably it doesn't > have a primary key. If it doesn't have a primary key, then merge() will > create an entirely new entry in database for it instead of updating the > older one, right? And this would defeat the updating intention, right? > > I think I probably missed some point here. Could someone elaborate it a > little bit? Thanks. >
I think the bit you missed is: >> ensuring that the data within the file is used to formulate >> the primary key of each element of the structure ie. for this to work, the file must contain enough information to construct the primary key for each object. I don't *think* it even makes sense to use Session.merge() with an object that doesn't already have a primary key, as the primary key dictates which existing database rows you are trying to merge with. Hope that helps, Simon -- You received this message because you are subscribed to the Google Groups "sqlalchemy" group. To unsubscribe from this group and stop receiving emails from it, send an email to sqlalchemy+unsubscr...@googlegroups.com. To post to this group, send email to sqlalchemy@googlegroups.com. Visit this group at http://groups.google.com/group/sqlalchemy. For more options, visit https://groups.google.com/d/optout.
