def exon_by_id(self, exon_id):
        """Construct an Exon object from its ID by looking up the exon"s
        properties in the given Database.
        """
        if exon_id not in self._exons:
            field_names = [
                "seqname",
                "start",
                "end",
                "strand",
                "gene_name",
                "gene_id",
            ]

            contig, start, end, strand, gene_name, gene_id = self.db.query_one(
                select_column_names=field_names,
                filter_column="exon_id",
                filter_value=exon_id,
                feature="exon",
                distinct=True)

            self._exons[exon_id] = Exon(
                exon_id=exon_id,
                contig=contig,
                start=start,
                end=end,
                strand=strand,
                gene_name=gene_name,
                gene_id=gene_id)

        return self._exons[exon_id]
Construct an Exon object from its ID by looking up the exon"s
        properties in the given Database.