Merges the ds config from druid_config into one stored in the db.

def sync_to_db_from_config(
            cls,
            druid_config,
            user,
            cluster,
            refresh=True):
        """Merges the ds config from druid_config into one stored in the db."""
        session = db.session
        datasource = (
            session.query(cls)
            .filter_by(datasource_name=druid_config['name'])
            .first()
        )
        # Create a new datasource.
        if not datasource:
            datasource = cls(
                datasource_name=druid_config['name'],
                cluster=cluster,
                owners=[user],
                changed_by_fk=user.id,
                created_by_fk=user.id,
            )
            session.add(datasource)
        elif not refresh:
            return

        dimensions = druid_config['dimensions']
        col_objs = (
            session.query(DruidColumn)
            .filter(DruidColumn.datasource_id == datasource.id)
            .filter(DruidColumn.column_name.in_(dimensions))
        )
        col_objs = {col.column_name: col for col in col_objs}
        for dim in dimensions:
            col_obj = col_objs.get(dim, None)
            if not col_obj:
                col_obj = DruidColumn(
                    datasource_id=datasource.id,
                    column_name=dim,
                    groupby=True,
                    filterable=True,
                    # TODO: fetch type from Hive.
                    type='STRING',
                    datasource=datasource,
                )
                session.add(col_obj)
        # Import Druid metrics
        metric_objs = (
            session.query(DruidMetric)
            .filter(DruidMetric.datasource_id == datasource.id)
            .filter(DruidMetric.metric_name.in_(
                spec['name'] for spec in druid_config['metrics_spec']
            ))
        )
        metric_objs = {metric.metric_name: metric for metric in metric_objs}
        for metric_spec in druid_config['metrics_spec']:
            metric_name = metric_spec['name']
            metric_type = metric_spec['type']
            metric_json = json.dumps(metric_spec)

            if metric_type == 'count':
                metric_type = 'longSum'
                metric_json = json.dumps({
                    'type': 'longSum',
                    'name': metric_name,
                    'fieldName': metric_name,
                })

            metric_obj = metric_objs.get(metric_name, None)
            if not metric_obj:
                metric_obj = DruidMetric(
                    metric_name=metric_name,
                    metric_type=metric_type,
                    verbose_name='%s(%s)' % (metric_type, metric_name),
                    datasource=datasource,
                    json=metric_json,
                    description=(
                        'Imported from the airolap config dir for %s' %
                        druid_config['name']),
                )
                session.add(metric_obj)
        session.commit()