Fetches the metadata for the table and merges it in

def fetch_metadata(self):
        """Fetches the metadata for the table and merges it in"""
        try:
            table = self.get_sqla_table_object()
        except Exception as e:
            logging.exception(e)
            raise Exception(_(
                "Table [{}] doesn't seem to exist in the specified database, "
                "couldn't fetch column information").format(self.table_name))

        M = SqlMetric  # noqa
        metrics = []
        any_date_col = None
        db_engine_spec = self.database.db_engine_spec
        db_dialect = self.database.get_dialect()
        dbcols = (
            db.session.query(TableColumn)
            .filter(TableColumn.table == self)
            .filter(or_(TableColumn.column_name == col.name
                        for col in table.columns)))
        dbcols = {dbcol.column_name: dbcol for dbcol in dbcols}

        for col in table.columns:
            try:
                datatype = col.type.compile(dialect=db_dialect).upper()
            except Exception as e:
                datatype = 'UNKNOWN'
                logging.error(
                    'Unrecognized data type in {}.{}'.format(table, col.name))
                logging.exception(e)
            dbcol = dbcols.get(col.name, None)
            if not dbcol:
                dbcol = TableColumn(column_name=col.name, type=datatype)
                dbcol.sum = dbcol.is_num
                dbcol.avg = dbcol.is_num
                dbcol.is_dttm = dbcol.is_time
                db_engine_spec.alter_new_orm_column(dbcol)
            else:
                dbcol.type = datatype
            dbcol.groupby = True
            dbcol.filterable = True
            self.columns.append(dbcol)
            if not any_date_col and dbcol.is_time:
                any_date_col = col.name

        metrics.append(M(
            metric_name='count',
            verbose_name='COUNT(*)',
            metric_type='count',
            expression='COUNT(*)',
        ))
        if not self.main_dttm_col:
            self.main_dttm_col = any_date_col
        self.add_missing_metrics(metrics)
        db.session.merge(self)
        db.session.commit()