[WWdevel] webwork: problem values getting set to zero

[P G. L. <gl...@um...>]

Hi all,

In the course of adding the GatewayQuiz module to the HEAD branch in the
CVS, I've found that when I look at StudentProgress all student scores are
being reported as zero (regardless of how many correct problems they have
in the set).

I think what's happening is that when we get NULL values out of the
database they are promoted to empty strings, but when we put an empty
string back into an integer field in the database that gets stored as
zero.

In particular, when a set is assigned to a student the appropriate entries
in the problem_user table are created.  For those the value field is set
to NULL.  When we get() the problems out of the table (with
$db->{problem_user}->get()), the NULL is promoted to "", so that
$userProblem->value() is then the empty string.

As the user problem is processed by Problem.pm (or GatewayQuiz.pm),
$userProblem->value() isn't changed.  Then, when we put the problem back
into the database after grading (with $db->{problem_user}->put()), we're
putting an empty string into the integer value field, which is stored as
zero.

When we then get the merged problem out again, $db->getMergedProblem()
checks for empty fields to decide whether to use the global problem's
value.  However the entry in problem_user now has value == 0, so the
global problem's value (== 1) isn't used.  This means that when
StudentProgress.pm (et al.) calculate the student's score (by taking
$userProblem->status() * $userProblem->value()), the score is zero.

Does that sound right?  If so, the work-around/solution may be to make
$db->{}->put() demote empty strings to NULL.  Comments?

Thanks,
Gavin

-- 
P. Gavin LaRose, Ph.D.  Program Manager (Instructional Tech.)
Math Dept., University of Michigan
gl...@um...                        "There's no use in trying," [Alice]
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http://www.math.lsa.umich.edu/~glarose/  things." "I daresay you haven't had
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