[OPEN-ILS-DEV] TCN value question related to Keeping bib / auth / MFHD / record identifiers in sync post

[Dan Scott]

On Fri, 2009-11-20 at 08:46 -0500, Jason Stephenson wrote:
> Quoting Alan Rykhus <alan.rykhus at mnsu.edu>:
> 
> > Hello,
> >
> > I've read the 4 posts related to the Dan Scott's post on keeping the 001
> > field in sync with the EG record_entry id and it brings up a question
> > for me.
> 
> I read those posts, also, and was thinking of replying, but saw your  
> question and thought my reply would fit nicely in your thread.

Hey, great to hear from other people on the subject! I have a quick
response to the source of the tcn_source / tcn_value columns, at least
outside of bulk imports, below...

> 
> >
> > What exactly is the real purpose of the tcn_source and tcn_value columns
> > in the biblio.record_entry table?
> 
> My understanding is that the tcn_value column is meant to hold the tcn  
> value from the MARC 001 field and tcn_source is the id if the record  
> source from the config.bib_source table. I could be mistaken but  
> that's what I discerned from reading the code.

Well, the database schema just defines biblio.record_entry.tcn_source as
a non-null TEXT column with a default value of "AUTOGEN" - no further
constraints. So it's not tied to the config.bib_source table.

In the _find_tcn_info() subroutine in
OpenILS/Application/Cat/BibCommon.pm, the tcn_value is derived from 001,
and the tcn_source is derived from 003. 

If there's no 003, then the tcn_source is set to "System Local". 

If the tcn_value that was found already exists, then the find_free_tcn()
subroutine is invoked that attempts to derive the tcn_value / tcn_source
from the following subfields in order, stopping if a tcn_value is found:

  * 901a / 901b
  * 039a / 039b (or "System Local" if no tcn_source was retrieved)

The system then checks to see if the tcn_value already exists in the
system; if so, or if a tcn_value wasn't found in one of these subfields,
then it tries again, processing the following tcn_value / tcn_source in
order and checking to see if the tcn_value already exists after each
pair; if it exists, then it moves on to the next match possibility:

  * 020a / "ISBN"
  * 022a / "ISSN"
  * 010 / "LCCN"
  * 035a / "System Legacy"
  * return undef

The tcn_source is set to the text string (such as "ISBN") if the
corresponding tcn_value is found.

Hopefully that helps determine where the TCN value and source are
derived from, at least in the current trunk code, outside of bulk
imports.