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Date: Fri, 13 Jan 2012 12:29:55 +0100
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 <CAMWZAj=yxTBedhbbLW+qQj82kab21EFd8A+M4qGpMQEHs9jSYg@mail.gmail.com>
Subject: Re: Lookup views
From: Marcello Nuccio <marcello.nuccio@gmail.com>
To: user@couchdb.apache.org
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2012/1/13  <matthew-m.rose@ubs.com>:
[...]
> Unfortunately, sometimes the enrichment changes. For example each trade has
> a fruit field and we have a mapping from fruit to fruit type. Today apples,
> oranges and pears are classified as round fruit whereas bananas are long
> fruit so we can put a fruit type field on the document as we load it in but
> tomorrow that classification might change and pears might become lumpy fruit
> instead of round fruit. We could go back and update every pear trade in the
> database but we'd rather not have fruit type on the document at all and
> maintain a separate mapping "table".
>
> We thought that perhaps the map function could reference another database
> that was keyed on fruit and had documents containing the fruit type but
> don't seem to be able to perform this lookup in the mapping. We could get
> the document itself on the result but not extract just one field to augment
> the original documents. We also considered generating the view's design
> dynamically out of a mapping table and replicating the mapping into explicit
> logic in the javascript but this seemed like an abuse.

If update of all documents is not an option for you, the easiest
solution that I see is hinted in your last phrase: "We also considered
generating the view's design dynamically out of a mapping table and
replicating the mapping into explicit logic in the javascript but this
seemed like an abuse."

In map functions, you can load CommonJS[1] modules. You can put the
mapping table in a module, and update only that module when the data
changes.

[1]: http://wiki.apache.org/couchdb/CommonJS_Modules

HTH,
  Marcello