bartender1382 has asked for the wisdom of the Perl Monks concerning the following question:

I am trying to transfer data from MySQL, over to an app in Swift.

I use the following code when I have to send over just ONE record:

This creates a hash of every field in my table

    
sub reader {
  foreach my $rows (@$data) {
    my %GenericData;
    @GenericData{ @rawFields } = @$rows; # NOTE added
    push @rawRecords, \%GenericData;
  }
return \@rawRecords;

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Then I encode the JSON like so, so I can send it over as HTTP DATA

my $rows = reader($mysqlFilename);

...in reader
my $json = encode_json($hashref, {
  lastname     => JSON_TYPE_STRING,
  firstname    => JSON_TYPE_STRING,
  age       => JSON_TYPE_INT}
);
...and I return $json which is the single record.

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However, I am now pulling more than one record, and I wondered if it's possible to export multiple records to JSON?

Replies are listed 'Best First'.
Re: Is there a way to make a JSON out of multiple records from MySQL?
by Corion (Patriarch) on May 11, 2022 at 06:16 UTC

    JSON also understands arrays, so you can encode the array as JSON:

    my $json = encode_json(\@rawRecords);

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    If you want to pass the type hints, you have to pass the type hints as an array as well:

    my $typehint_person =  {
  lastname     => JSON_TYPE_STRING,
  firstname    => JSON_TYPE_STRING,
  age       => JSON_TYPE_INT};
my $typehint_persons = [ ($typehint_person) x @rawRecords ];
my $json = encode_json(\@rawRecords, $typehint_persons);

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      > my $typehint_persons = [ ($typehint_person) x @rawRecords ];

      For arrays of the same type, there's a function where you don't need to know the number:

      my $typehint_persons = json_type_arrayof($typehint_person);

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      map{substr$_->[0],$_->[1]||0,1}[\*||{},3],[[]],[ref qr-1,-,-1],[{}],[sub{}^*ARGV,3]
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      Thank you, it was the "type hints" part that was throwing me off. I will have to read about the "x" operator many times before I figure out how this works.
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      Actually have a lot of debugging to do. As I am getting a lot of the following errors, AND in addition to what appears to be a valid JSON output

      For each record, I get:

      Use of uninitialized value in subroutine entry..

      AND a complete JSON output

      If I lacked clarity, in my call to retrieve the records I am using:

      my $data = $dbHandle->fetchall_arrayref();

      Then I push each hash reference onto an array 

          foreach my $rows (@$data) {
            my %GenericData;
            @GenericData{ @rawFields } = @$rows; 

            push @rawRecords, \%GenericData;
        }
     return \@rawRecords;

     ##################
     #  which is returned to the calling function as $data #

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      Then I pass the reference $data to:

      
makeJSONToExport($data);

sub makeJSONToExport {
    my ($hashref) = @_;

    my $typehint_raw =  {
            age        => JSON_TYPE_INT,
            lastname      => JSON_TYPE_STRING,
            firstname          => JSON_TYPE_STRING};

    my $typehint_raws = [ ($typehint_raw) x  @$hashref];
    my $json = encode_json($hashref, $typehint_raws);
    # my $json = encode_json($hashref, );
    return $json;
}

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      While I'm glad I'm getting a legit JSON, I am concerned about the error msg

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        Use of uninitialized value in subroutine entry..

        You should see at least a line number associated with this warning (not error) message, and it should be fairly easy to zero in on the argument the subroutine is expecting and not getting.

        my $typehint_raws = [ ($typehint_raw) x  @$hashref];

        If @$hashref is indeed a hash reference, I don't see how the code runs beyond that statement. 

        Win8 Strawberry 5.8.9.5 (32)  Wed 05/11/2022 21:07:27
C:\@Work\Perl\monks
>perl

use strict;
use warnings;

my $hr = { qw(a b  c d) };

print "got here \n";
print @$hr, "\n";
# print %$hr, "\n";  # works
print "and here \n";

^Z
got here
Not an ARRAY reference at - line 8.

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        Give a man a fish:  <%-{-{-{-<

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