Perl -- DBI selectall_arrayref when querying getting Not Hash Reference - sql-server

Question

I am very new to perl (but from a c# background) and I am trying to move some scripts to a windows box.
Due to some modules not working easily with windows I have changed the way it connects to the DB.
I have an sqlserver DB and I had a loop reading each row in a table, and then within this loop another query was sent to select different info.
I was the error where two statements can't be executed at once within the same connection.
As my connection object is global I couldn't see an easy way round this, so decided to store the first set of data in an array using:
my $query = shift;
my $aryref = $dbh->selectall_arrayref($query) || die "Could not select to array\n";
return($aryref);
(this is in a module file that is called)
I then do a foreach loop (where #$s_study is the $aryref returned above)
foreach my $r_study ( #$s_study ) {
~~~
my $surveyId=$r_study->{surveyid}; <-------error this line
~~~~
};
When I run this I get an error "Not a hash reference". I don't understand?!
Can anyone help!
Bex

Answer 1

You need to provide the { Slice => {} } parameter to selectall_arrayref if you want each row to be stored as a hash:
my $aryref = $dbh->selectall_arrayref($query, { Slice => {} });
By default, it returns a reference to an array containing a reference to an array for each row of data fetched.

Answer 2

$r_study->{surveyid} is a hashref
$r_study->[0] is an arrayref
this is your error.
You should use the second one

Answer 3

If you have a problem with a method, then a good first step is to read the documentation for that method. Here's a link to the documentation for selectall_arrayref. It says:
This utility method combines
"prepare", "execute" and
"fetchall_arrayref" into a single
call. It returns a reference to an
array containing a reference to an
array (or hash, see below) for each
row of data fetched.
So the default behaviour is to return a reference to an array which contains an array reference for each row. That explains your error. You're getting an array reference and you're trying to treat it as a hash reference. I'm not sure that the error could be much clearer.
There is, however, that interesting bit where it says "or hash, see below". Reading on, we find:
You may often want to fetch an array
of rows where each row is stored as a
hash. That can be done simple using:
my $emps = $dbh->selectall_arrayref(
"SELECT ename FROM emp ORDER BY ename",
{ Slice => {} }
);
foreach my $emp ( #$emps ) {
print "Employee: $emp->{ename}\n";
}
So you have two options. Either switch your code to use an array ref rather than a hash ref. Or add the "{ Slice => {} }" option to the call, which will return a hash ref.
The documentation is clear. It's well worth reading it.

Answer 4

When you encounter something like "Not a hash reference" or "Not an array reference" or similar you can always take Data::Dumper to just dump out your variable and you will quickly see what data you are dealing with: arrays of arrayrefs, hashes of something etc.
And concerning reading the data, this { Slice => {} } is most valuable addition.