> On Mar 31, 2008, at 12:02 PM, pyplexed wrote: > > > > > The continuing adventures of a newbie SA user... > > > Could anyone tell me if there is a simple way of setting up a query > > which takes an object as an argument and checks for the existence of > > an object in the database? > > by primary key ? assuming as below you don't have any prior > knowledge of the object's properties or type, > sess > .query > (obj.__class__).get(object_mapper(obj).primary_key_from_instance(obj))
Actually, I would know the type, so maybe that makes it simpler. I'm in a situation where I want to store data which is generated externally to my system, and which is polled regularly but individual items change relatively infrequently. It seemed sensible to try to quickly spot the items which are already in the database (i.e. all fields the same), as these require no further action. I would then process any of the remaining items which have changed singe the last data update, and create records for the new versions, and finally insert any completely new items into the database. > > > I know I can construct a query hard coding attributes to filter on, > > but I'm sure there is a better way to do this. > > the above is a little ugly so you could make a function that does it, > given any object (or even make it a classmethod on a base class). > > > Ideally, the query would need to handle attributes containing multiple > > objects expressing many to many relationships. > > Im not entirely sure what this means. You'd like to query for an > object based on its many-to-many association to another object ? This > would look like: > > sess > .query > (DesiredClass > ).filter(DesiredClass.somerelation.contains(some_given_object)).all() Sorry. I'm still learning the language here. As things stand, my code just inserts the new records, including some items in tables associated via many-to-many relationships. I'd like to add the logic described above, and be able to query the database using the same externally derived data object as I am currently saving in the session and database, using it instead (initially) as a query argument to see if the data it contains already exists in the database - i.e. it contains no new information to add, including any data in joined tables.. I wondered if there was a query syntax in SA which would take a full or partially populated data object and return a match or matches on the basis of the populated fields. (Null values would probably have to be ignored, thinking about it). Being a newbie, I'm not sure if I'm asking something obvious, or if I'm asking for the Moon on a stick. :-) I'll take a look at the suggestions you made. Many thanks for them. --~--~---------~--~----~------------~-------~--~----~ You received this message because you are subscribed to the Google Groups "sqlalchemy" group. To post to this group, send email to [email protected] To unsubscribe from this group, send email to [EMAIL PROTECTED] For more options, visit this group at http://groups.google.com/group/sqlalchemy?hl=en -~----------~----~----~----~------~----~------~--~---
