How can I merge two XML files into one? - ruby

Question

I need to merge two XML files. I saw this question before, but that poster wanted to simply concatenate the two files. I want to merge based on a specific child element, in this case, id.
I have two XML files that have the following structure:
File #1:
<document>
<row>
<id>1</id>
<data_field1>aaaa</data_field1>
<data_field2>bbbb</data_field2>
</row>
</document>
File #2:
<document>
<row>
<id>1</id>
<data_field3>cccc</data_field3>
</row>
</document>
And I want them to be merged into File #3:
<document>
<row>
<id>1</id>
<data_field1>aaaa</data_field1>
<data_field2>bbbb</data_field2>
<data_field3>cccc</data_field3>
</row>
</document>
Where it uses the id element to join each XML entry.

Answer 1

The code below will do this, using XML::Twig
It will work with more than 2 docs, and work even if not all id's are present in both docs. It will load both files in memory though, if you want to be able to work with documents too big to fit in memory, the code will be a bit more complex. The rows will be in the same order as in the first document, then in the second one (for those that only appear in the second one).
Since it is written as a test, you can make the test case more complex, or add more tests, which would probably be a good idea.
#!/usr/bin/perl
use strict;
use warnings;
use Test::More;
use XML::Twig;
# normally you would read the documents from file,
# but it's easier to write a self-contained test
my $d1='
<document>
<row>
<id>1</id>
<data_field1>aaaa</data_field1>
<data_field2>bbbb</data_field2>
</row>
</document>
';
my $d2='
<document>
<row>
<id>1</id>
<data_field3>cccc</data_field3>
</row>
</document>
';
my $merged=
'<document>
<row>
<id>1</id>
<data_field1>aaaa</data_field1>
<data_field2>bbbb</data_field2>
<data_field3>cccc</data_field3>
</row>
</document>
';
$merged=~ s{\n}{}g; # remove \n's,
# if you want the result indented, look at the pretty_print option
is( merged( $d1, $d2), $merged, 'one test to rule them all');
done_testing();
sub merged
{
my #docs= map { XML::Twig->new->parse( $_) } #_;
my $merged= XML::Twig->new->parse( '<document></document>');
my %row_id; # hash id => row_element
foreach my $doc (#docs)
{ foreach my $row ($doc->root->children( 'row'))
{ my $eid= $row->first_child( 'id');
my $id= $eid->text;
# if the row hasn't been created in the merged doc, do it
if( ! $row_id{$id})
{ $row_id{$id}= $merged->root->insert_new_elt( last_child => 'row');
$row_id{$id}->insert_new_elt( last_child => id => $id);
}
# move the data fields to the end of the row
foreach my $data_field ($eid->next_siblings)
{ $data_field->move( last_child => $row_id{$id}); }
}
}
return $merged->sprint;
}