Error "attribute parameter is not in hash ref" when using Perl DBI selectall_hashref - mysql

Question

I'm trying to get my first select to work using selectall_hashref from the Perl DBI module. I've opened a connection to the database (MySQL) successfully. I'm getting an error when I execute the following:
$dbh->selectall_hashref('SELECT id FROM users WHERE login=?',undef,"myusername");
DBI::st=HASH(0x1505a60)->_prepare(...): attribute parameter 'myusername' is not a hash ref at /usr/lib/x86_64-linux-gnu/perl5/5.20/DBD/mysql.pm line 238.
My table should be able to support this query, it has an id column and login column for each user.
The examples I've found for selectall_hashref show the ? substitution parameter being passed as the third parameter. The DBI documentation says that the second and third arguments should be %attr and #bind_values but doesn't give much documentation about them or show working examples.
What is causing the error, and more importantly how do you actually use the %attr and #bind_values correctly?

Answer 1

If you want to store everything as an arrayref where each row is a hashref (which is what your comment seems to indicate), you can use the selectall_arrayref() method with the Slice attribute:
$dbh->selectall_arrayref('SELECT id FROM users WHERE login=?', {Slice => {}}, 'myusername');
It's a little weird, but here's how it works:
If $slice is a hash reference, fetchall_arrayref fetches each row as
a hash reference. If the $slice hash is empty then the keys in the
hashes have whatever name lettercase is returned by default. (See
"FetchHashKeyName" attribute.) If the $slice hash is not empty,
then it is used as a slice to select individual columns by name. The
values of the hash should be set to 1. The key names of the returned
hashes match the letter case of the names in the parameter hash,
regardless of the "FetchHashKeyName" attribute.
It's a good idea to set the FetchHashKeyName attribute on the database handle to make your hash key names consistent; I happen to like NAME_lc in my applications.

Answer 2

The methods expects key column as the second parameter and attributes ref is passed as third one. In the result it builds a hash with the specified column as a key. What you probably want, is selectall_arrayref:
$ dbh->selectall_arrayref('SELECT id FROM users WHERE login=?',undef,"myusername");