How to query for only 1 field with Spring Data Cassandra?

Question

I am using Spring Data Cassandra 1.0.0. I have managed to persist and read back my entity. However, now I want to do a query that only returns 1 field of the entity.

This is what I have tried:

public Optional<DateTime> getTimeOfOldestIntervalTrafficDataMessage( MessageSource messageSource, IntegrationPeriod integrationPeriod, TrafficDataType trafficDataType )
{
    Select select = QueryBuilder.select( "message_time" ).from( "messages", "data_message" );
    select.where( QueryBuilder.eq( "message_source_id", messageSource.getId().getId() ) )
            .and( QueryBuilder.eq( "data_type", trafficDataType.name() ) )
            .and( QueryBuilder.eq( "integration_period", integrationPeriod.name() ) );

    List<Date> result = cassandraOperations.select( select, Date.class );
    if (result.size() > 0)
    {
        return Optional.of( new DateTime( FluentIterable.from( result ).toSortedSet( Ordering.natural() ).first() ) );
    }
    else
    {
        return Optional.absent();
    }
}

cassandraOperations is a CassandraTemplate instance.

So I only select message_time from the data_message table. This is a timestamp column, so I pass in a Date.class to the select() method, but this gives the following exception:

org.springframework.data.mapping.model.MappingException: No mapping metadata found for java.util.Date
    at org.springframework.data.cassandra.convert.MappingCassandraConverter.readRow(MappingCassandraConverter.java:111)
    at org.springframework.data.cassandra.convert.MappingCassandraConverter.read(MappingCassandraConverter.java:202)
    at org.springframework.data.cassandra.core.CassandraConverterRowCallback.doWith(CassandraConverterRowCallback.java:47)
    at org.springframework.data.cassandra.core.CassandraTemplate.select(CassandraTemplate.java:455)
    at org.springframework.data.cassandra.core.CassandraTemplate.select(CassandraTemplate.java:253)
    at com.mycomp.app.infrastructure.cassandra.CassandraMessageRepository.getTimeOfOldestIntervalTrafficDataMessage(CassandraMessageRepository.java:130)

How should I query for just 1 field?

PS: If you would know a more performant way to do this query, I would be happy to hear it also.

Answer 1

Found out how to do it:

public Optional<DateTime> getTimeOfOldestIntervalTrafficDataMessage( MessageSource messageSource, IntegrationPeriod integrationPeriod, TrafficDataType trafficDataType )
    {
        Select select = QueryBuilder.select( "message_time" ).from( "messages", "data_message" );
        select.where( QueryBuilder.eq( "message_source_id", messageSource.getId().getId() ) )
                .and( QueryBuilder.eq( "traffic_data_type", trafficDataType.name() ) )
                .and( QueryBuilder.eq( "integration_period", integrationPeriod.name() ) );
        select.orderBy( QueryBuilder.asc( "message_time" ) );
        select.limit( 1 );

        Date date = m_cassandraOperations.queryForObject( select, Date.class );
        if( date != null )
        {
            Optional<DateTime> result = Optional.of( new DateTime( date ) );
            logger.debug( "oldest interval: {} ({}/{}/{})", result, messageSource.getName(), trafficDataType, integrationPeriod );
            return result;
        }
        else
        {
            return Optional.absent();
        }
    }

queryForObject() function is the way to go. I also used ordering and limit to make Cassandra do the work of finding what I need.