It's not so much that I'm querying but that I get a set of id's from the
user and I've got some logic that will often change some of the values.
I wanted to take advantage of SA's orm capabilities as opposed to
issuing selects and updates. It's possible in the logic that I already
have some of the entities retrieved, but it didn't make sense for me to
pass around a map, knowing that SA already keeps one. And that still
doesn't seem quite right--having to manage which objects I've previously
selected. Perhaps I should just issue the updates.
Christiaan Putter wrote:
Hi,
Small extract from the query's _get method:
if not self._populate_existing and not refresh_state and not
self._mapper_zero().always_refresh and lockmode is None:
try:
instance = self.session.identity_map[key]
state = attributes.instance_state(instance)
if state.expired:
try:
state()
except orm_exc.ObjectDeletedError:
self.session._remove_newly_deleted(state)
return None
return instance
except KeyError:
pass
So you have access to the identity map through
session.identity_map[key], where key is:
key = self._only_mapper_zero(get() can only be used against a single
mapped class.).identity_key_from_primary_key(ident)
ident is the primary key for the record you're looking for.
The above will basically return the instance to you if it's already in
the identity map. Though state() will still execute sql to refresh
attributes.
So what you can do is build a loop using something like that going
through all the primary keys you'd like to get directly from the
identity mapper. I'm assuming it's using a pretty good hash so access
should be some constant factor. Then you can use the normal
query.get() to retrieve the ones that failed from the database.
This is really something I would advise against though.
What exactly are you querying for? Why don't you just keep a list of
all the instances you've already loaded?
2009/3/25 Dan F danielfal...@gmail.com:
I understand what get() is supposed to do, but it doesn't clear it up
because it still seems like there should be a way of retrieving a
*set* of records back from the database at once. I only see a couple
choices currently. Either I can use filter() and retrieve every
record in the set (even the ones that are mapped), or I can use get on
each row individually. Since get() checks the identity map, it won't
get records it doesn't need to get, but on the other hand, each record
has to get retrieved in its own call.
Does this make sense, and do I have it right?
Thanks.
On Mar 24, 2:08 am, Christiaan Putter ceput...@googlemail.com wrote:
Hi,
You won't be able to get() multiple objects at the same time.
query(SomeClass).get(pk1, pk2, pk3) takes in a tuple of values
representing the primary key of some record in your table. In this
case the primary key consists of three separate columns (thus a
composite key), though the record they identify will always be unique
within your table. That's sort of the point of it being a primary
key. Read the docs for an explanation of what parameters get()
expects.
Of course you could use filter() and get the same result. I'm not
sure but I guess the actual SQL executed by SA should look exactly the
same, set echo to True and have a look.
The difference being of course that you can use filter to return more
then one record.
I'm not sure how get() works on databases that don't need primary
keys. I'm guessing it won't. Maybe it's in the docs.
Hope that cleared things up.
Regards,
Christian
2009/3/23 Dan F danielfal...@gmail.com:
Hi,
Is there a difference between using query.get(ident) and using
query.filter(MyClass.id.in_())? Specifically with regard to how the
mapper is used? If I'm right in my assumption, get() uses the map to
avoid extra lookups, but I question whether the filter method is doing
the same. If I'm correct, shouldn't there be a way to get() multiple
objects at the same time?
Thanks.
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