Allow local fields, subfields and indicators

[fjorba]

Marc21 makes room for a local installation to define its own local field with the convention that they contain a 9, either as a tag (typically 9XX tags, but also like 59X), indicators or subfields.

Invenio should make it easy to follow this convention without resorting to tortuous modifications of internal syntax rules.

[tiborsimko]

Yes, this will be possible by composing various JSON schemas together, see my reply in another issue. For a concrete example, @egabancho is working on defining the JSON schemas for CDS records. These have many local fields and subfields defined. @egabancho can you please post here a small example if you already have one?

[egabancho]

There are many ways that you can do it already, this is how I am do it for CDS so far (keep in mind this is work in progress).

First I define my own "model" (in my case it contains all the marc21 rules by default)

from dojson.overdo import Overdo
from dojson.contrib.marc21 import marc21

class CDSMarc21(Overdo):

    """Translation Index for CDS specific MARC21."""

    def __init__(self):
        """Constructor.

        Initializes the list of rules with the default ones from doJSON.
        """
        super(CDSMarc21, self).__init__()
        self.rules.extend(marc21.rules)

cds_marc21 = CDSMarc21()

And then I add new fields to it

from dojson import utils
from ..model import cds_marc21

@cds_marc21.over('subject_indicator', '^690C_')
@utils.for_each_value
def subject_indicator(self, key, value):
    """Subject Indicator."""
    return value.get('a')

This is also somehow related with https://github.com/inveniosoftware/invenio-records/pull/3, it allows to use different processors, i.e. cds_marc21 instead of marc21.