Add UNSPECIFIED to BigDecimalRepresentation.

Closes: #5054

spring-data-mongodb/src/main/java/org/springframework/data/mongodb/core/convert/MongoCustomConversions.java

@@ -175,7 +175,7 @@ public class MongoCustomConversions extends org.springframework.data.convert.Cus
 				LocalDateTime.class);
 
 		private boolean useNativeDriverJavaTimeCodecs = false;
-		private @Nullable BigDecimalRepresentation bigDecimals;
+		private BigDecimalRepresentation bigDecimals = BigDecimalRepresentation.UNSPECIFIED;
 		private final List<Object> customConverters = new ArrayList<>();
 
 		private final PropertyValueConversions internalValueConversion = PropertyValueConversions.simple(it -> {});
@@ -476,7 +476,13 @@ public class MongoCustomConversions extends org.springframework.data.convert.Cus
 		/**
 		 * Store numbers using {@link org.bson.types.Decimal128} (default). Requires MongoDB Server 3.4 or later.
 		 */
-		DECIMAL128
+		DECIMAL128,
+
+		/**
+		 * Pass on values to the MongoDB Java Driver without any prior conversion.
+		 * @since 5.0
+		 */
+		UNSPECIFIED
 
 	}
 

spring-data-mongodb/src/test/java/org/springframework/data/mongodb/core/convert/MappingMongoConverterUnitTests.java

@@ -2239,6 +2239,7 @@ class MappingMongoConverterUnitTests {
 	static Stream<Arguments> representations() {
 
 		return Stream.of(Arguments.argumentSet("None (default)", new Object[] { null }), //
+				Arguments.argumentSet("UNSPECIFIED", BigDecimalRepresentation.UNSPECIFIED), //
 				Arguments.argumentSet("STRING", BigDecimalRepresentation.STRING), //
 				Arguments.argumentSet("DECIMAL128", BigDecimalRepresentation.DECIMAL128));
 	}
@@ -2260,6 +2261,23 @@ class MappingMongoConverterUnitTests {
 		assertThat(target.get("bigDecimal")).isEqualTo(source.bigDecimal);
 	}
 
+	@Test // GH-5037, GH-5054
+	void shouldWriteBigNumbersAsIsWhenUsingUnspecified() {
+
+		converter = createConverter(BigDecimalRepresentation.UNSPECIFIED);
+
+		WithoutExplicitTargetTypes source = new WithoutExplicitTargetTypes();
+		source.bigInteger = BigInteger.TWO;
+		source.bigDecimal = new BigDecimal("123.456");
+
+		org.bson.Document target = new org.bson.Document();
+
+		converter.write(source, target);
+
+		assertThat(target.get("bigInteger")).isEqualTo(source.bigInteger);
+		assertThat(target.get("bigDecimal")).isEqualTo(source.bigDecimal);
+	}
+
 	@Test // GH-5037
 	void shouldReadTypedBigNumbersFromDecimal128() {