diff --git a/spring-data-mongodb/src/main/java/org/springframework/data/mongodb/repository/support/SpringDataMongodbSerializer.java b/spring-data-mongodb/src/main/java/org/springframework/data/mongodb/repository/support/SpringDataMongodbSerializer.java
index d2f0dfef1..2ab007824 100644
--- a/spring-data-mongodb/src/main/java/org/springframework/data/mongodb/repository/support/SpringDataMongodbSerializer.java
+++ b/spring-data-mongodb/src/main/java/org/springframework/data/mongodb/repository/support/SpringDataMongodbSerializer.java
@@ -1,5 +1,5 @@
 /*
- * Copyright 2011-2016 the original author or authors.
+ * Copyright 2011-2018 the original author or authors.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -20,6 +20,7 @@ import java.util.HashSet;
 import java.util.Set;
 import java.util.regex.Pattern;
 
+import com.mongodb.util.JSON;
 import org.springframework.data.mapping.context.MappingContext;
 import org.springframework.data.mongodb.core.convert.MongoConverter;
 import org.springframework.data.mongodb.core.convert.QueryMapper;
@@ -40,7 +41,7 @@ import com.querydsl.mongodb.MongodbSerializer;
 
 /**
  * Custom {@link MongodbSerializer} to take mapping information into account when building keys for constraints.
- * 
+ *
  * @author Oliver Gierke
  * @author Christoph Strobl
  */
@@ -64,7 +65,7 @@ class SpringDataMongodbSerializer extends MongodbSerializer {
 
 	/**
 	 * Creates a new {@link SpringDataMongodbSerializer} for the given {@link MappingContext}.
-	 * 
+	 *
 	 * @param mappingContext must not be {@literal null}.
 	 */
 	public SpringDataMongodbSerializer(MongoConverter converter) {
@@ -115,12 +116,29 @@ class SpringDataMongodbSerializer extends MongodbSerializer {
 	@Override
 	protected DBObject asDBObject(String key, Object value) {
 
-		if (ID_KEY.equals(key)) {
-			return mapper.getMappedObject(super.asDBObject(key, value), null);
+		if (key.endsWith(ID_KEY)) {
+			return convertId(key, value);
 		}
 		return super.asDBObject(key, value instanceof Pattern ? value : converter.convertToMongoType(value));
 	}
 
+	/**
+	 * Convert a given, already known to be an {@literal id} or even a nested document id, value into the according id
+	 * representation following the conversion rules of {@link QueryMapper#convertId(Object)}.
+	 *
+	 * @param key the property path to the given value.
+	 * @param idValue the raw {@literal id} value.
+	 * @return the {@literal id} representation in the required format.
+	 */
+	private DBObject convertId(String key, Object idValue) {
+
+		Object convertedId = mapper.convertId(idValue);
+
+		DBObject mappedIdValue = mapper.getMappedObject(super.asDBObject(key, convertedId),
+				null);
+		return (DBObject) JSON.parse(JSON.serialize(mappedIdValue));
+	}
+
 	/*
 	 * (non-Javadoc)
 	 * @see com.querydsl.mongodb.MongodbSerializer#isReference(com.querydsl.core.types.Path)
diff --git a/spring-data-mongodb/src/test/java/org/springframework/data/mongodb/repository/support/QuerydslRepositorySupportTests.java b/spring-data-mongodb/src/test/java/org/springframework/data/mongodb/repository/support/QuerydslRepositorySupportTests.java
index e4b6ba694..d96efbc67 100644
--- a/spring-data-mongodb/src/test/java/org/springframework/data/mongodb/repository/support/QuerydslRepositorySupportTests.java
+++ b/spring-data-mongodb/src/test/java/org/springframework/data/mongodb/repository/support/QuerydslRepositorySupportTests.java
@@ -18,13 +18,18 @@ package org.springframework.data.mongodb.repository.support;
 import static org.hamcrest.CoreMatchers.*;
 import static org.junit.Assert.*;
 
+import lombok.Data;
+
 import java.util.Arrays;
 
+import org.bson.types.ObjectId;
 import org.junit.Before;
 import org.junit.Test;
 import org.junit.runner.RunWith;
 import org.springframework.beans.factory.annotation.Autowired;
+import org.springframework.data.annotation.Id;
 import org.springframework.data.mongodb.core.MongoOperations;
+import org.springframework.data.mongodb.core.mapping.Document;
 import org.springframework.data.mongodb.core.query.Query;
 import org.springframework.data.mongodb.repository.Person;
 import org.springframework.data.mongodb.repository.QPerson;
@@ -49,7 +54,9 @@ public class QuerydslRepositorySupportTests {
 	@Before
 	public void setUp() {
 
+		operations.remove(new Query(), Outer.class);
 		operations.remove(new Query(), Person.class);
+
 		person = new Person("Dave", "Matthews");
 		operations.save(person);
 
@@ -97,4 +104,54 @@ public class QuerydslRepositorySupportTests {
 		assertThat(queryUsingIdField.fetchOne(), equalTo(person));
 		assertThat(queryUsingIdField.fetchOne(), equalTo(queryUsingRefObject.fetchOne()));
 	}
+
+	@Test // DATAMONGO-1998
+	public void shouldLeaveStringIdThatIsNoValidObjectIdAsItIs() {
+
+		Outer outer = new Outer();
+		outer.id = "outer-1";
+		outer.inner = new Inner();
+		outer.inner.id = "inner-1";
+		outer.inner.value = "go climb a rock";
+
+		operations.save(outer);
+
+		QQuerydslRepositorySupportTests_Outer o = QQuerydslRepositorySupportTests_Outer.outer;
+		SpringDataMongodbQuery<Outer> query = repoSupport.from(o).where(o.inner.id.eq(outer.inner.id));
+
+		assertThat(query.fetchOne(), equalTo(outer));
+	}
+
+	@Test // DATAMONGO-1998
+	public void shouldConvertStringIdThatIsAValidObjectIdIntoTheSuch() {
+
+		Outer outer = new Outer();
+		outer.id = new ObjectId().toHexString();
+		outer.inner = new Inner();
+		outer.inner.id = new ObjectId().toHexString();
+		outer.inner.value = "eat sleep workout repeat";
+
+		operations.save(outer);
+
+		QQuerydslRepositorySupportTests_Outer o = QQuerydslRepositorySupportTests_Outer.outer;
+		SpringDataMongodbQuery<Outer> query = repoSupport.from(o).where(o.inner.id.eq(outer.inner.id));
+
+		assertThat(query.fetchOne(), equalTo(outer));
+	}
+
+	@Data
+	@Document
+	public static class Outer {
+
+		@Id String id;
+		Inner inner;
+	}
+
+	@Data
+	public static class Inner {
+
+		@Id String id;
+		String value;
+
+	}
 }