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 <[email protected]>:
>
>> 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 <[email protected]> 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 <[email protected]>:
>>>
>>>
>>>
>>>
>>>> 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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