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

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

@@ -2329,8 +2329,10 @@ public class MappingMongoConverter extends AbstractMongoConverter implements App
 			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()));
 					}
@@ -2359,11 +2361,6 @@ public class MappingMongoConverter extends AbstractMongoConverter implements App
 				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

@@ -2858,6 +2858,14 @@ class MappingMongoConverterUnitTests {
 		assertThat(converter.read(Cyclic.class, source).cycle.value).isEqualTo("v2");
 	}
 
+	@Test // GH-4371
+	void shouldConvertTypesToStringTargetType() {
+
+		org.bson.Document source = org.bson.Document.parse("{\n" + "  city : [\"Gotham\", \"Metropolis\"]\n" + "}\n");
+
+		assertThat(converter.read(Address.class, source).city).isEqualTo("Gotham,Metropolis");
+	}
+
 	static class GenericType<T> {
 		T content;
 	}