I'm also encountering this issue, but it's 100% reproducible for me.

I have a "sessiondata" table and a "videodata" table, videodata contains a FK reference to a sessiondata PK id.

A GET to following URL pattern will always give me a ValueError:

patterns = ['/sessiondata/session_uuid/{sessiondata.session_uuid}/videos[videodata.session_id]'] #All entries from the video table matching session uuid
## e.g. http://127.0.0.1:8000/api/sessiondata/session_uuid/TESTTEST00004000A0003000000000001/videos

  File "...web2py\gluon\dal.py", line 1952, in log_execute
    if not self.connection: raise ValueError(a[0])
ValueError: SELECT  sessiondata.id, sessiondata.session_uuid FROM sessiondata WHERE (sessiondata.id = 4) LIMIT 1 OFFSET 0;

(The result of the above query is actually id=4 session_uuid=TESTTEST00004000A0003000000000001 so the rows being requested do exist and there isn't anything wrong with the SQL.)

It seems that the DAL has somehow lost it's connection (it gets set to None) before completing the join. Other, more simple queries like /sessiondata/session_uuid/{sessiondata.session_uuid}/ evaluate fine.

Web2Py version: 2.9.11-stable+timestamp.2014.09.15.23.35.11
Python version: 2.7.1
OS: Windows 7 x64 & Debian 7 x64
Most importantly (as I suspect this might be related to table locking):

db = DAL('sqlite://storage.sqlite',pool_size=10,check_reserved=['all'])

Like @niphlod , I've spent about 2 days trying to figure out what is happening. This is just a simple FK join, so how can I make it work?

I have absolutely the same issue with 2.9.12-stable+timestamp.2015.01.17.06.11.03
on win7 + mysql.
Before it was fine, now mostly the second select show self.connection = None
File "C:...\gluon\dal\adapters\base.py", line 1304, in log_execute
if not self.connection: raise ValueError(a[0])
ValueError: SELECT category.id, category.name, ....

I create connection like:

db = DAL(config.db_uri)
current.db = db

and use current.db everywhere including modules.

Thanks.

My bad.
Found the issue, I've saved current.db to static class variable.
Updated (not to save)
No issues.

IMHO this has been fixed, and is closely related on how people misuse the DAL object in their apps. Until further notices by @pi-aej for 2.9.12, I'd close this.

ok, closing in absence of further notices

In my case it was caused by usage of DAL object in current on module level (to avoid writing current.db instead of db all the time):

from gluon import current
db = current.db

def func():
   #module level db object usage here

As far as I understand this is what happened:
module is loaded one time, and after module level variable (db) is instantiated, it won't change anymore
thread in pool is destroyed, but db is still referring to it
code crashes

While accessing db in current inside the function retrieves actual value at each call and everything goes fine. Just for others googling the cause of such issue.