Convert identifier to String when writing entity to SecretDocument

We now properly convert the identifier value into String before writing it to SecretDocument.

Closes gh-777

spring-vault-core/src/main/java/org/springframework/vault/repository/convert/MappingVaultConverter.java

@@ -409,21 +409,21 @@ protected void writeInternal(Object obj, SecretDocumentAccessor sink, @Nullable
 
 	protected void writeInternal(Object obj, SecretDocumentAccessor sink, VaultPersistentEntity<?> entity) {
 
-		PersistentPropertyAccessor accessor = entity.getPropertyAccessor(obj);
+		PersistentPropertyAccessor<?> accessor = entity.getPropertyAccessor(obj);
 
 		VaultPersistentProperty idProperty = entity.getIdProperty();
 		if (idProperty != null && !sink.hasValue(idProperty)) {
 
 			Object value = accessor.getProperty(idProperty);
 
 			if (value != null) {
-				sink.put(idProperty, value);
+				sink.put(idProperty, value instanceof String ? value : conversionService.convert(value, String.class));
 			}
 		}
 		writeProperties(entity, accessor, sink, idProperty);
 	}
 
-	private void writeProperties(VaultPersistentEntity<?> entity, PersistentPropertyAccessor accessor,
+	private void writeProperties(VaultPersistentEntity<?> entity, PersistentPropertyAccessor<?> accessor,
 			SecretDocumentAccessor sink, @Nullable VaultPersistentProperty idProperty) {
 
 		// Write the properties

spring-vault-core/src/test/java/org/springframework/vault/repository/convert/MappingVaultConverterUnitTests.java

@@ -21,11 +21,13 @@
 import java.util.List;
 import java.util.Map;
 import java.util.Objects;
+import java.util.UUID;
 
 import org.junit.jupiter.api.BeforeEach;
 import org.junit.jupiter.api.Test;
 
 import org.springframework.core.convert.converter.Converter;
+import org.springframework.data.annotation.Id;
 import org.springframework.data.annotation.Version;
 import org.springframework.vault.repository.mapping.VaultMappingContext;
 
@@ -310,6 +312,19 @@ void shouldWriteEntityWithListOfEntities() {
 		assertThat((List<Map<String, Object>>) sink.get("nested")).contains(walter, skyler);
 	}
 
+	@Test
+	void shouldConvertIdentifier() {
+
+		WithUuidId entity = new WithUuidId(UUID.randomUUID(), "foo");
+
+		SecretDocument sink = new SecretDocument();
+
+		this.converter.write(entity, sink);
+
+		assertThat(sink.getId()).isEqualTo(entity.id.toString());
+		assertThat(sink.getBody()).containsEntry("name", "foo");
+	}
+
 	static class SimpleEntity {
 
 		String id;
@@ -561,6 +576,20 @@ public String getName() {
 
 	}
 
+	static class WithUuidId {
+
+		@Id
+		private final UUID id;
+
+		private final String name;
+
+		public WithUuidId(UUID id, String name) {
+			this.id = id;
+			this.name = name;
+		}
+
+	}
+
 	enum DocumentToPersonConverter implements Converter<SecretDocument, Person> {
 
 		INSTANCE;