How to store Java 8 Instant to MongoDB as date type using Spring MongoTemplate?

I have a Java class that has a member variable Instant

:

public class SomeRecord {
    private String someId;

    private Instant someInstant;

    // getters and setters
}

      

        
        
        
      

    

I am using MongoTemplate to update a field someInstant

in a database:

public SomeRecord updateSomeRecordBySomeId(final String someId, Object someInstant) {
        Query query = new Query();
        query.addCriteria(Criteria.where("someId").is(someId));

        Update update = new Update();
        update.set("someInstant", someInstant);

        return operations.findAndModify(query, update, new FindAndModifyOptions().returnNew(true), SomeRecord.class);
}

      

        
        
        
      

    

This works great if I call the method like:

updateSomeRecordBySomeId("SOME-ID", Instant.now());

storing the field in the DB as a type Date

: "someInstant" : ISODate("2017-07-11T07:26:44.269Z")

Now this method can also be called as follows: updateSomeRecordBySomeId("SOME-ID", "2017-07-11T07:26:44.269Z");

In this case, I am getting an exception like:

org.springframework.core.convert.ConverterNotFoundException: There is no converter that can be converted from type [java.lang.String] to type [java.time.Instant]

which makes sense. (It updates the field in the DB like String

though. "someInstant" : "2017-07-11T07:26:44.269Z"

)

MongoConfig.java:

@Configuration
@ComponentScan(basePackages = {"dao package path here"})
public class MongoConfig {
    @Autowired
    private MongoDbFactory mongoDbFactory;

    @Bean
    public MongoTemplate mongoTemplate() {
        MappingMongoConverter converter = new MappingMongoConverter(new DefaultDbRefResolver(mongoDbFactory),
                new MongoMappingContext());

        converter.setCustomConversions(new CustomConversions(Collections.singletonList(new StringToInstantConverter())));

        return new MongoTemplate(mongoDbFactory, converter);
    }
}

      

        
        
        
      

    

StringToInstantConverter.java:

public class StringToInstantConverter implements Converter<String, Instant> {
    @Override
    public Instant convert(String utcString) {
        // TODO: Make it generic for any time-zone
        return Instant.parse(utcString);
    }
}

      

        
        
        
      

    

After adding the above converter, I don't get any more ConverterNotFoundException

, but the field is someInstant

saved as a simple string:"someInstant" : "2017-07-11T07:26:44.269Z"

And that's my question. I know the converter is identified, so I no longer get the exception. But why doesn't the converter convert String

to Instant

? Why is the field saved as normal String

? Is the inverter installed correctly? How do I write a converter for this case?

Note:

• I've simplified the code to focus on the real problem. The method does not actually receive a field someInstant
  
  as a parameter. Therefore, you won't write an overloaded method here. Also any type of validation instanceOf
  
  within a method will not work for a real scenario. So the focus is on why the conversion isn't happening?

• The actual data store for us is DocumentDB, but we are using DocumentDB with the MongoDB API (as Spring Data does not support DocumentDB) for our database operations.