org.hl7.fhir.core/org.hl7.fhir.convertors/CONVERTORS.md

# Resource Conversion

##### Let's talk about converting resources between the various versions of FHIR...

### A note regarding syntax

-----

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](http://hl7.org/fhir/STU3/account.html)
and [r4](http://hl7.org/fhir/R4/account.html)

**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

-----
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:

```java
    // 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...

-----

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](https://www.hl7.org/fhir/extension-valueset-expression.html)
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

-----
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:

```java
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:

```java
  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](https://www.hl7.org/fhir/extension-valueset-expression.html)
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:

```java
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)`