DATAMONGO-2241 - Retain MongoConverter using MongoTemplate.withSession(…).

We now reuse the MongoConverter instance that is associated with a MongoTemplate when using withSession(…) instead of creating a new converter instance.
In consequence, we're reusing PersistentEntity instances, EntityInstantiators and generated accessor classes.

Original Pull Request: #734

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

@@ -33,6 +33,7 @@ import org.bson.codecs.Codec;
 import org.bson.conversions.Bson;
 import org.slf4j.Logger;
 import org.slf4j.LoggerFactory;
+
 import org.springframework.beans.BeansException;
 import org.springframework.context.ApplicationContext;
 import org.springframework.context.ApplicationContextAware;
@@ -259,8 +260,7 @@ public class MongoTemplate implements MongoOperations, ApplicationContextAware,
 		this.mongoDbFactory = dbFactory;
 		this.exceptionTranslator = that.exceptionTranslator;
 		this.sessionSynchronization = that.sessionSynchronization;
-		this.mongoConverter = that.mongoConverter instanceof MappingMongoConverter ? getDefaultMongoConverter(dbFactory)
+		this.mongoConverter = that.mongoConverter;
-				: that.mongoConverter;
 		this.queryMapper = that.queryMapper;
 		this.updateMapper = that.updateMapper;
 		this.schemaMapper = that.schemaMapper;

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

@@ -19,7 +19,6 @@ import static org.assertj.core.api.Assertions.*;
 import static org.springframework.data.mongodb.core.query.Criteria.*;
 import static org.springframework.data.mongodb.core.query.Query.*;
 
-import com.mongodb.MongoClient;
 import lombok.AllArgsConstructor;
 import lombok.Data;
 
@@ -30,7 +29,9 @@ import org.junit.ClassRule;
 import org.junit.Rule;
 import org.junit.Test;
 import org.junit.rules.TestRule;
+
 import org.springframework.data.annotation.Id;
+import org.springframework.data.mongodb.core.convert.MongoConverter;
 import org.springframework.data.mongodb.core.query.Query;
 import org.springframework.data.mongodb.test.util.MongoTestUtils;
 import org.springframework.data.mongodb.test.util.MongoVersion;
@@ -39,6 +40,7 @@ import org.springframework.data.mongodb.test.util.ReplicaSet;
 import org.springframework.data.util.Version;
 
 import com.mongodb.ClientSessionOptions;
+import com.mongodb.MongoClient;
 import com.mongodb.client.ClientSession;
 
 /**
@@ -89,6 +91,18 @@ public class ClientSessionTests {
 		session.close();
 	}
 
+	@Test // DATAMONGO-2241
+	public void shouldReuseConfiguredConverter() {
+
+		ClientSession session = client.startSession(ClientSessionOptions.builder().causallyConsistent(true).build());
+
+		MongoConverter converter = template.getConverter();
+		MongoConverter sessionTemplateConverter = template.withSession(() -> session)
+				.execute(MongoOperations::getConverter);
+
+		assertThat(sessionTemplateConverter).isSameAs(converter);
+	}
+
 	@Test // DATAMONGO-1920
 	@MongoVersion(asOf = "3.7.3")
 	public void withCommittedTransaction() {