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Date: Sun, 2 Jan 2011 11:52:23 +0000
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Subject: Document versioning with multiple documents
From: Joe Freeman <joe.freeman@bitroot.com>
To: user@couchdb.apache.org
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I'm trying to implement document versioning using a variation of the
method described under 'Multiple Documents' here:
http://blog.couchone.com/post/632718824/simple-document-versioning-with-couchdb

(I'd like to have been able to use the 'attachPrevRev' approach, but
unfortunately that doesn't seem to allow me to setup a view that lists
all the revisions for a document - i.e., I can't access the older
revisions (attachments) from the view server.)

I have documents like this:

{_id: '1', type: 'page'}
{_id: '2', type: 'page_revision', page: '1', created_at: [2011, 1, 1,
11, 28, 0], content: 'First revision of first page.'}
{_id: '3', type: 'page'}
{_id: '4', type: 'page_revision', page: '3', created_at: [2011, 1, 1,
11, 29, 0], content: 'First revision of second page.'}
{_id: '5', type: 'page_revision', page: '3', created_at: [2011, 1, 1,
11, 30, 0], content: 'Second revision of second page.'}

My view then has a map function:

function(doc) {
  if (doc.type == 'page_revision') {
    emit([doc.page, doc.created_at], doc);
  }
}

Which emits the following keys:

[1, [2011, 1, 1, 11, 28, 0]] // First revision of first page.
[3, [2011, 1, 1, 11, 29, 0]] // First revision of second page.
[3, [2011, 1, 1, 11, 30, 0]] // Second revision of second page.

So I add a reduce function to get the latest revision for each page
(and use group_level=1):

function(keys, values) {
  return values[values.length - 1];
}

When I try to run this, I get a 'reduce_overflow_error' ("Reduce
output must shrink more rapidly"), and I think I understand why.

However, if I change the reduce function to this:

function(keys, values) {
  return values[values.length - 1];
/*aaaaaaaaaaaaaaaaaaaaaaa
aaaaaaaaaaaaaaaaaaaaaaaaa
aaaaaaaaaaaaaaaaaaaaaaaaa
aaaaaaaaaaaaaaaaaaaaaaaaa
aaaaaaaaaaaaaaaaaaaaaa*/
}

(i.e., pad out the content of the function with a comment)

Then I don't get the overflow error, and I get the result I was expecting.

So this raises two questions:

1) Does the fact that I initially get the overflow error suggest that
there's something wrong with my approach?

2) Why don't I get the error in the second case?

And hence, can anyone suggest how I should be solving this problem?

Thanks :)