On Thursday, February 8, 2018 at 6:23:59 PM UTC-8, Anthony wrote:
>
> Looks like the book is wrong -- according to the code, rows1 & rows2
> simply combines the records from each Rows object -- it does not find the
> intersection.
>
Okay, then my results match that!
> Anyway, given your code, it looks like we would not expect any results in
> the intersection of the two Rows objects, as the conditions of each query
> are mutually exclusive (i.e., "waypoint" either equals X or Y). Of course,
> if X == Y, then the two sets of Rows are identical, but I assume that isn't
> generally the case.
>
>
A segment row will only have one waypoint, but multiple segment rows may
have the same parent run. I chose to make the run table very simple (and
in particular, not to have a list of children), but the segment table
points upward to the run.
If you do really have non-mutually-exclusive results, why not just generate
> the intersection with the SQL query (i.e., query1 & query2)?
>
>
I'm not very sophisticated at joining in.
> Also, note, you queries should use "==", not a single "=".
>
>
erk
> Anthony
>
>
/dps
> On Tuesday, February 6, 2018 at 4:57:29 PM UTC-5, Dave S wrote:
>>
>> I have an application (used for comparing routes) with 2 tables relevant
>> to this question. (Paraphrasing; code is at home,
>> where the router jammed.)
>>
>> db.define_table('run',
>> Field('when', 'date'),
>> Field('description', 'string))
>>
>> db.define_table('segment',
>> Field('waypoint', 'string'),
>> Field('timeofday', 'time'),
>> Field('odometer', 'decimal'),
>> Field('partof', 'reference run'))
>>
>> I wanted to find the runs that had X as a waypoint and also had Y as a
>> waypoint, so I looked at
>> <URL:
>> http://web2py.com/books/default/chapter/29/06/the-database-abstraction-layer#Combining-rows
>> >
>> and wrote:
>>
>> def fromto():
>> # not shown -- X and Y from request.vars
>> q1 = db.segment.waypoint = X
>> q2 = db.segment.waypoint = Y
>> runs1 = db(run.id == segment.partof & q1).select(...,distinct=True)
>> runs2 = db(run.id == segment.partof & q2).select(...,distinct=True)
>> theruns = runs1 & runs2
>> print len(runs1) + len(runs2) + len(theruns)
>> return ("runs" = theruns)
>>
>> For the first waypoints I tried, the print produced counts of 111 10 121
>> which is clearly NOT the intersection. (I have verified that theruns
>> includes runs where Y is not included,
>> and the page shows the Y runs at the end of the X runs (that is,
>> appended).)
>>
>> If I go for union ('|' in line 7), the counts are 111 10 112.
>> (1 Y run did not go through X).
>>
>> I add the "distinct=True" after the first attempt, in case multiple
>> appearances of a row was confusing things. It made no apparent difference.
>>
>> I think this is on 2.14.6; I may be able to test newer web2py's tonight.
>> Am I doing something wrong, or was there a hiccup in intersection?
>>
>> Dave S
>> /dps
>>
>>
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