def _process_pub_dbxref(self, limit):
        """
        Xrefs for publications (ie FBrf = PMID)
        :param limit:
        :return:

        """

        if self.test_mode:
            graph = self.testgraph
        else:
            graph = self.graph
        model = Model(graph)
        raw = '/'.join((self.rawdir, 'pub_dbxref'))
        LOG.info("processing pub_dbxref")

        line_counter = 0

        with open(raw, 'r') as f:
            filereader = csv.reader(f, delimiter='\t', quotechar='\"')
            f.readline()  # read the header row; skip
            for line in filereader:
                (pub_dbxref_id, pub_id, dbxref_id, is_current) = line
                # 49648	43222	395730	t

                pub_key = pub_id
                pub_id = self.idhash['publication'][pub_key]

                if self.test_mode and int(pub_key) not in self.test_keys['pub']:
                    continue

                # get any dbxrefs for pubs, including pmids and dois
                dbxref_key = dbxref_id
                if str(dbxref_key) in self.dbxrefs:
                    dbxrefs = self.dbxrefs[str(dbxref_key)]
                    # pub_dbs = [75, 51, 76, 95, 126]
                    pmid_ids = [50, 77, 275, 286, 347]
                    # flybase_ids = [4]  # TODO unused
                    isbn = [75, 51]
                    for d in dbxrefs:
                        dbxref_id = None
                        if int(d) in pmid_ids:
                            if re.match(r'^PMID', dbxrefs[d]):
                                dbxref_id = dbxrefs[d].strip()
                            else:
                                dbxref_id = 'PMID:'+dbxrefs[d].strip()
                            model.makeLeader(dbxref_id)
                        elif int(d) in isbn:
                            dbxref_id = 'ISBN:'+dbxrefs[d].strip()
                        elif int(d) == 161:
                            dbxref_id = 'DOI:'+dbxrefs[d].strip()
                        # elif int(d) == 4:
                        #     dbxref_id = 'FlyBase:'+dbxrefs[d].strip()

                        if dbxref_id is not None:
                            reference = Reference(
                                graph, dbxref_id, self.globaltt['publication'])
                            reference.addRefToGraph()
                            model.addSameIndividual(pub_id, dbxref_id)
                            line_counter += 1

                if not self.test_mode and limit is not None and line_counter > limit:
                    break

        return
Xrefs for publications (ie FBrf = PMID)
        :param limit:
        :return: