Fix regression in value to String mapping.

Previous versions allow arbitrary values to be mapped to an string property by calling the ObjectToString converter. This behaviour got lost and is not reestablished.

Closes #4371
Original pull request #4373

Author: christophstrobl

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

@@ -2315,8 +2315,10 @@ public <S extends Object> S convert(Object source, TypeInformation<? extends S>
 			if (source instanceof Collection) {
 
 				Class<?> rawType = typeHint.getType();
-				if (!Object.class.equals(rawType)) {
+				if (!Object.class.equals(rawType) && !String.class.equals(rawType)) {
+
 					if (!rawType.isArray() && !ClassUtils.isAssignable(Iterable.class, rawType)) {
+
 						throw new MappingException(
 								String.format(INCOMPATIBLE_TYPES, source, source.getClass(), rawType, getPath()));
 					}
@@ -2345,11 +2347,6 @@ public <S extends Object> S convert(Object source, TypeInformation<? extends S>
 				return (S) dbRefConverter.convert(context, (DBRef) source, typeHint);
 			}
 
-			if (source instanceof Collection) {
-				throw new MappingException(
-						String.format(INCOMPATIBLE_TYPES, source, BasicDBList.class, typeHint.getType(), getPath()));
-			}
-
 			if (BsonUtils.supportsBson(source)) {
 				return (S) documentConverter.convert(context, BsonUtils.asBson(source), typeHint);
 			}

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

@@ -2831,6 +2831,18 @@ public org.bson.Document write(@Nullable String domainValue, MongoConversionCont
 		assertThat(converter.read(Cyclic.class, source).cycle.value).isEqualTo("v2");
 	}
 
+	@Test // GH-4371
+	void shouldConvertTypesToStringTargetType() {
+		
+		org.bson.Document source = org.bson.Document.parse("""
+				{
+				  city : ["Gotham", "Metropolis"]
+				}
+				""");
+
+		assertThat(converter.read(Address.class, source).city).isEqualTo("Gotham,Metropolis");
+	}
+
 	static class GenericType<T> {
 		T content;
 	}