I am attempting to change the data type of an attribute in one of my DDB tables, but because this data is read from and written to, altering the data type of the attribute causes subsequent read failures when reading old records, which look like this:

could not unconvert attribute
DynamoDBMappingException: expected M in value {N: 1000,}

My question is about how I can change the data type of an attribute in my table, and architect the change such that I can still read the Double value that exists in previous records. Here is the class in question:

@DynamoDBTable(tableName = "Sections")
@Data
@EqualsAndHashCode(callSuper = false)
@NoArgsConstructor
@AllArgsConstructor
@Builder
public class SectionRecord {
    private Map<String, Double> sectionTarget; //previous definition: private Double sectionTarget;
    
    public void setSectionTarget(Double sectionTarget, String key) {
       if (this.sectionTarget == null) {
          this.sectionTarget = new HashMap<Double, String>();
       }
       this.sectionTarget.put(key, sectionTarget);
    }
    
    public void getSectionTarget(String key) {
       return this.sectionTarget.get(key);
    }

}

And eventually, I try to read a record like this:

mapper.load(SectionRecord.class, id);

Which is presumably where the issue comes from – I’m trying to read a Double (which exists in the ddb currently) as a map (the changes I’ve made to the attribute).

I’d love to hear some guidance on how best to architect such a change such that these backwards compatibility issues could be mitigated.