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.
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