"Only ancestor queries are allowed inside transactions"

[Will]

Hi all,

I got this error when I tried to run a query in a transaction, "Only ancestor queries are allowed inside transactions".

I have a class, C1, whose entities have no ancestors. I want to query a particular entity from it, modify, and put it back, for example,

item = C1.gql("WHERE p1 = :a AND p2 = :b ORDER BY p2", a = None, b = 23).fetch(1)
item.p1 = sth
item.put()

The above code should run in a transaction. What should I do?

Thanks in advance.

Will
           

[Tonny]

what is the type of p2 and sth?

[Nick Johnson (Google)]

Hi Will,

As the error message says, the only queries you can execute inside a transaction are ancestor queries. You need to execute the query outside the transation then, if necessary, check the preconditions still hold inside the transaction.

-Nick Johnson

--
Nick Johnson, Developer Programs Engineer, App Engine
Google Ireland Ltd. :: Registered in Dublin, Ireland, Registration Number: 368047

[Will]

The type of p1, p2 and sth is datetime.

Can you give me some examples of 'ancestor queries'? If possible, I'd like to change the existing ones into 'ancestor queries' and fit the whole into a transaction, because that is exactly what a transaction designed for.

Thanks,

Will

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[Stephen]

On Nov 13, 2:27 am, Will <vocalster....@gmail.com> wrote:
>
> Can you give me some examples of 'ancestor queries'? If possible, I'd like
> to change the existing ones into 'ancestor queries' and fit the whole into a
> transaction, because that is exactly what a transaction designed for.

http://code.google.com/appengine/docs/python/datastore/transactions.html#What_Can_Be_Done_In_a_Transaction

[Will]

Yes, I've read the document. I'm looking for some examples.

As I said, I have a class, C1, whose entities have no ancestors. I want to query a particular entity from it, modify, and put it back, for example,

item = C1.gql("WHERE p1 = :a AND p2 < :b ORDER BY p2", a = None, b = today).fetch(1)
item.p1 = now
item.put()

Both p1 and p2 are datetime. How can I build this into a transaction? Is there a way I can give the entities a 'place holder' ancestor so I can later use an 'ancestor filter' in the query to satisfy the requirement?

Thanks,

Will

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[风笑雪]

Why you need a transaction to update just one entity? Just save it and
you may get an exception when update failed.

BTW, DateTimeProperty has an parameter auto_now, you can use it to
automatic update its time by datastore.

And if you really want a transaction, you need fetch it before start
the transaction:

item = C1.gql("WHERE p1 = :a AND p2 < :b ORDER BY p2", a = None, b =

today).get()
if item:
key = item.key()
def update_time(key)
item = C1.get(key)
if item:
item.p1 = now
item.put()
db.run_in_transaction(update_time, key)

2009/11/14 Will <vocals...@gmail.com>:

[Will]

I want to find that particular entity, change it and save it back. When one request is doing this, I don't want another request pick up the same entity and modify it again. In short, I want the 'seek-change-save' to be serialized, as one reason the GAE transaction is designed for. Finding out the key of the entity first out of a transaction and then using the key inside the transaction defeats the purpose.

Will

[Eli Jones]

you should read the section here:

http://code.google.com/appengine/docs/python/datastore/functions.html#run_in_transaction

where they show an example of iterating a counter in a transaction.

If you don't already know the key for the object... then you sort of need to get it to run your transaction.  In the decrement example, they grab the entity first so that they can send the entity key to the decrement function.

Now, if you want to modify their decrement function to see if it works by getting the key inside of the transaction..  then just try that.. it's only a few lines of code.

[Eli Jones]

Nevermind, I see what you mean.

So you want a lock on the entity once you know it exists (and satisfies your where clause).

..since something might have come along and updated the p1 column from None to Something before your transaction began.. and thus the key would refer to an entity that does not satisfy your where clause anymore.

If you were willing to let whoever got to it first to update the p1 column.. you could have your transaction verify that p1 was still None once it got the entity inside of the transaction.

If p1 had changed.. then just do nothing since another transaction already won the race.

I think that would work just the same.  Because, assume you have two processes looking for an entity where p1 = None and p2 = Today.. and they each really, really want to update p1 to some value.

One of the processes must win in your scenario.. you just don't want the other one to re-update the entity since it has already been updated.. and p1 <> None.  Now, you still need the transaction of course.. but just add a check after grabbing the entity to verify that p1 = None before doing any updates.

[Will]

Thanks Eli. Your solution solves one issue. But ideally, I want the 2nd concurrent request, after waiting, pick up the next item in queue and process, without complicating the code. After all, that's what a transaction is designed for.

Reading another post, it seems one has to create a dummy parent entity for those entities you want to query in a transaction. What I'm looking for is a guide with good examples for querying (otherwise) root entities in a transaction.

Will