org.hl7.fhir.core/org.hl7.fhir.convertors

Resource Conversion

IMPORTANT

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The conversion code in this module is maintained as part of the development of the standard, but always under considerable time pressure. Only part of the code is rigorously tested as detailed here. Implementers should regard this code as a 'scaffold' for actual reliable conversions.

ALWAYS TEST ANY CONVERSION ROUTINES BEFORE USING THEM IN PRODUCTION!

Ideally, this should be via unit tests in your code, or better yet unit tests contributed to FHIR.

A note regarding syntax

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Within the code, we use a set naming convention to organize the classes used for conversion between the various versions of FHIR.

Version	Code
dstu2	10
dstu2016may	14
dstu3	30
r4	40
r5	50

The files themselves follow the naming convention:

(NAME) + (VERSION CODE) + _ + (VERSION CODE)

Where NAME is the proper name of the resource or datatype being converted, and the two VERSION CODE indicate the two versions of FHIR that the code will convert the given resource or datatype between.

So, in the repository, you may come across a file name Account30_40. This would indicate that the code in this file is related to the conversion of the Account resource between versions dstu3 and r4

N.B. This information is only for code navigation purposes. It is important that when converting between versions you use the provided conversion factory classes as your entry point.

Using the conversion library

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The majority of use cases for conversion will involve using the provided VersionConvertorFactory_V1_V2 classes to convert to and from the various versions of FHIR.

They provide two statically accessed base methods for converting resources:

public static (V1 Resource) convertResource((V2 Resource) src)

public static (V2 Resource) convertResource((V1 Resource) src)

as well as two statically accessed base methods for converting types:

public static (V1 Type) convertType((V2 Type) src)

public static (V2 Type) convertType((V1 Type) src)

It's important to note that these methods convert from the base Resource of one version to the base Resource of another version, or from the base Type of one version to the base Type of another version (or DataType in the case of r5), so the result will need to be cast to the correct class.

Example:

    // Converting a r5 StructureDefinition to dstu3.
    org.hl7.fhir.r5.model.StructureDefinition r5_structure_def = new StructureDefinition();
    org.hl7.fhir.dstu3.model.StructureDefinition dstu3_converted_structure_def 
        = (StructureDefinition) VersionConvertorFactory_30_50.convertResource(r5_structure_def);

It gets complicated...

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As the specification has evolved over time, the versions of FHIR have built on top of one another, adding new fields within existing resources, changing the name of existing resources, or adding entirely new resources altogether. As a result of this conversions are inherently lossy operations.

A quick example of this would be ValueSet Expression extension type. This exists in the r4 version of the specification, but no such type exists in dstu2.

If we were to convert a R4 resource, such as a questionnaire, that contained an extension of this type from r4 -> dstu2, without any special intervention, the extension would be ignored, and the data would be lost in the conversion process.

This is where advisors come in.

Using conversion advisors

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When you call the base conversion factory methods convertType(...) or convertResource(...), the library does a predefined conversion, using the standard conversion (which could be a lossy one, or one that makes assumptions).

These defaults/assumptions are defined in the convertor advisor classes. Each pair of versions has a BaseAdvisor, which is used by default when you call the factory methods. For example, here is the advisor class which handles conversions between dstu2 and r5:

public class BaseAdvisor_10_50 extends BaseAdvisor50<org.hl7.fhir.dstu2.model.Extension> {
  final List<String> conformanceIgnoredUrls = Collections.singletonList("http://hl7.org/fhir/3.0/StructureDefinition/extension-CapabilityStatement.acceptUnknown");
  private final List<Class<?>> ignoredExtensionTypes = new ArrayList<>(Collections.singletonList(Expression.class));

  public BaseAdvisor_10_50() {
  }

  public BaseAdvisor_10_50(Boolean failFast) {
    this.failFast = failFast;
  }

  public boolean ignoreExtension(@Nonnull String path,
                                 @Nonnull String url) {
    List<String> paths = Arrays.asList(path.split(","));
    return (paths.get(paths.size() - 1).equals("Conformance")) && (conformanceIgnoredUrls.contains(url));
  }

  public boolean ignoreType(@Nonnull String path,
                            @Nonnull DataType type) {
    return ignoredExtensionTypes.contains(type.getClass());
  }
}

You can see in the above, that when converting extensions, we check if the given conversion is for a Conformance resource, and if we are converting an Extension within that Conformance with a set url, we ignore it.

As mentioned above, we provide a stock set of implied conversion rules that are used by default. However, there may be cases where you need to add specific behavior to your conversion. Within the BaseAdvisor class, there exist a number of overrideable methods that can be used to modify the outcome of any given conversion:

  public void handleCodeSystem(@Nonnull CodeSystem tgtcs, @Nonnull ValueSet source)

  public boolean ignoreEntry(@Nonnull Bundle.BundleEntryComponent src, @Nonnull FhirPublication targetVersion)

  public CodeSystem getCodeSystem(@Nonnull ValueSet src) throws FHIRException

  public boolean ignoreExtension(@Nonnull String path, @Nonnull Extension ext) throws FHIRException

  public boolean ignoreExtension(@Nonnull String path, @Nonnull T ext) throws FHIRException

  public boolean ignoreExtension(@Nonnull String path, @Nonnull String url) throws FHIRException

  public boolean ignoreType(@Nonnull String path, @Nonnull DataType type) throws FHIRException

  public boolean ignoreType(@Nonnull String path, @Nonnull Object type) throws FHIRException

  public boolean useAdvisorForExtension(@Nonnull String path, @Nonnull Extension ext) throws FHIRException

  public boolean useAdvisorForExtension(@Nonnull String path, @Nonnull T ext) throws FHIRException

  public void handleExtension(@Nonnull String path, @Nonnull Extension src, @Nonnull T tgt) throws FHIRException

  public void handleExtension(@Nonnull String path, @Nonnull T src, @Nonnull Extension tgt) throws FHIRException

Through overriding these methods and implementing your own custom advisor, you can customize the output of any given conversion operation to suit your specific usecase.

For example, above we briefly mentioned ValueSet Expression extension type. This exists in the r4 version of the specification, but no such type exists in dstu2.

Our stock advisor just ignores this extension when converting from r5 to dstu2. However, if we wanted, we could create our own conversion advisor, as follows:

public class ExpressionAdvisor50 extends BaseAdvisor_10_50 {

  public boolean useAdvisorForExtension(@Nonnull String path, @Nonnull org.hl7.fhir.r5.model.Extension ext) {
    return ext.hasValue() && ext.getValue() instanceof org.hl7.fhir.r5.model.Expression;
  }

  public void handleExtension(@Nonnull String path, 
                              @Nonnull org.hl7.fhir.r5.model.Extension src, 
                              @Nonnull org.hl7.fhir.dstu2.model.Extension tgt) {
    if (src.getValue() instanceof org.hl7.fhir.r5.model.Expression) {
      StringType type = new StringType();
      if (src.getValue() == null) {
        throw new NullPointerException("null cannot be cast to non-null type org.hl7.fhir.r5.model.Expression");
      } else {
        type.setValueAsString(((Expression) src.getValue()).getExpression());
        tgt.setValue(type);
        if (src.hasUrlElement()) {
          tgt.setUrlElement(Uri10_50.convertUri(src.getUrlElement()));
        }
      }
    } else {
      throw new FHIRException("Unknown extension type passed in to custom convertor method.");
    }
  }
}

Here, we first check to see if the extension is of type org.hl7.fhir.r5.model.Expression and has a value set in the useAdvisorForExtension method, then in the handleExtension method, we manually create a StringType extension, and copy the value from the r5 Expression into it. This results in no data being lost, and a the new custom conversion behavior for our particular usecase.

Once you've created your new advisor, they can be provided as an argument when calling the conversion factory classes.

public static (V1 Resource) convertResource((V2 Resource) src, <T extends BaseAdvisor> advisor)

public static (V2 Resource) convertResource((V1 Resource) src, <T extends BaseAdvisor> advisor)

Development notes

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Reliable conversion code

The FHIR project maintains and tests conversions on the following resources, from old versions to R5:

• CodeSystem
• ValueSet
• ConceptMap
• StructureDefinition
• StructureMap
• ImplementationGuide
• CapabilityStatement
• OperationDefinition
• NamingSystem

These can be relied on and are subject to extensive testing.

Test cases

Some conversions have test cases for particular resources and particular version combinations. Where test cases exist, they will continue to be maintained and expected to pass.

Contributing test cases is highly encouraged! To contribute, create a PRs to the core library, or even better, to the FHIR test cases library.