[Koha] catalogin, dewey, and marc-to-koha mapping

[Jeffrey LePage]

We have a small library using dewey.  I'm trying to figure out how to get dewey decimal numbers into Koha without error.


BACKGROUND: MARC tags
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When we do a z39.50 search we often don't get the dewey decimal number.  According to http://www.loc.gov/marc/bibliographic/bdsummary.html there are the following dewey related MARC fields:
082  	Dewey Decimal Classification Number
083 	Additional Dewey Decimal Classification Number
090 - Shelf Location (AM) [OBSOLETE, 1977] [USMARC only]
090 - Local Call Number [OBSOLETE, 1982]
091 - Microfilm Shelf Location (AM) [OBSOLETE, 1977] [USMARC only]
852  	Location

I have also found fields like this at some libraries:
092    _a428.2  (tag 92, subfield a = dewey decimal number)
       _bOCO    (tag 92, subfiedl b = dewey decimal number)

Koha has this mapping by default:
tag 952, subfield o = items.itemcallnumber (full call number)

BACKGROUND: our initial import
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Originally we imported records from Athena.  Athena uses tag 952, subfields k,h,i,m (k=Call number prefix, h=Classification part, i=Item part,m=Call number suffix).  We mapped (Koha tag 952, subfield o) to (Athena tag 852, concatenation of subfields k,h,i,m).  We used this script:http://contribs.koha.org/revision_view.php?rid=15.  (Note: I created this script and uploaed it to contribs.koha.org.  If the script doesn't map the fields correctly I need to fix it before my mistakes cause problems for others.)


Question: How do I do the mapping?
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Now it looks like I need to 
1) map some dewey related MARC tag/subfield (which one?) to a items.itemcallnumber
2) make that newly mapped MARC tag/subfield mandatory

When I do this, what happens to my previous records which had the default MARC mapping ([tag 952, subfield o]= items.itemcallnumber) ?


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