MongoRepository的findById方法查不出_id值的解决方案.md

搭一个新项目&＃xff0c;从mongo数据库中查询数据&＃xff0c;我直接使用的spring-data-mongodb模块。直接创建Repository接口&＃xff0c;继承MongoRepository

public interface ResourceRepository extends MongoRepository {
}


使用MongoRepository的findById方法&＃xff0c;查询"_id"为某个值&＃xff0c;程序中查询不出来&＃xff0c;但是数据库中有值。并且使用db.getCollection(‘resource’).find({"_id":})验证过。
然后查询的时候debug了下&＃xff0c;走到了MongoTemplate的findById方法中&＃xff0c;它将_id替换为了id&＃xff1a;

public T findById(Object id, Class entityClass, String collectionName) {Assert.notNull(id, "Id must not be null!");Assert.notNull(entityClass, "EntityClass must not be null!");Assert.notNull(collectionName, "CollectionName must not be null!");MongoPersistentEntity persistentEntity &＃61; mappingContext.getPersistentEntity(entityClass);String idKey &＃61; ID_FIELD;//_idif (persistentEntity !&＃61; null) {if (persistentEntity.getIdProperty() !&＃61; null) {idKey &＃61; persistentEntity.getIdProperty().getName();//赋值为id}}return doFindOne(collectionName, new Document(idKey, id), new Document(), entityClass);}


idKey &＃61; persistentEntity.getIdProperty().getName();这句代码把“_id”的key换为了“id”&＃xff0c;这样如果mongo集合中只有“_id”字段&＃xff0c;就查询不出来了。

解决方案

继承MongoTemplate类&＃xff0c;重写它的findById方法&＃xff0c;我Debug后MongoTemplate执行的是public MongoTemplate(MongoDbFactory mongoDbFactory, &＃64;Nullable MongoConverter mongoConverter) 的构造方法&＃xff1a;
我们也创建类似的构造方法&＃xff1a;

&＃64;Component
public class PMongoTemplate extends MongoTemplate{private static final String ID_FIELD &＃61; "_id";&＃64;Autowiredpublic PMongoTemplate(MongoDbFactory mongoDbFactory, &＃64;Nullable MongoConverter mongoConverter) {super(mongoDbFactory,mongoConverter);}&＃64;Overridepublic T findById(Object id, Class entityClass, String collectionName) {Assert.notNull(id, "Id must not be null!");Assert.notNull(entityClass, "EntityClass must not be null!");Assert.notNull(collectionName, "CollectionName must not be null!");String idKey &＃61; ID_FIELD;return doFindOne(collectionName, new Document(idKey, id), new Document(), entityClass);}
}


启动之后报如下错误&＃xff0c;没有创建Bean ‘mongoTemplate’&＃xff1a;

因为我们自定了一个MongoTemplate类型的Bean&＃xff0c;所以Spring在初始化的时候&＃xff0c;不不再创建同一类型的了。所以我们之前创建的ResourceRepository等接口都无法使用了&＃xff0c;这些接口继承MongoRepository&＃xff0c;启动后创建对应的SimpleMongoRepository&＃xff0c;SimpleMongoRepository间接依赖了mongoTemplate。

这样走不通&＃xff0c;那我们就用MongoTemplate自带的方法&＃xff1a;

protected T doFindOne(String collectionName, Document query, Document fields, Class entityClass) {MongoPersistentEntity entity &＃61; mappingContext.getPersistentEntity(entityClass);Document mappedQuery &＃61; queryMapper.getMappedObject(query, entity);Document mappedFields &＃61; queryMapper.getMappedObject(fields, entity);if (LOGGER.isDebugEnabled()) {LOGGER.debug("findOne using query: {} fields: {} for class: {} in collection: {}", serializeToJsonSafely(query),mappedFields, entityClass, collectionName);}return executeFindOneInternal(new FindOneCallback(mappedQuery, mappedFields),new ReadDocumentCallback(this.mongoConverter, entityClass, collectionName), collectionName);}&＃64;Nullable&＃64;Overridepublic T findOne(Query query, Class entityClass, String collectionName) {Assert.notNull(query, "Query must not be null!");Assert.notNull(entityClass, "EntityClass must not be null!");Assert.notNull(collectionName, "CollectionName must not be null!");if (ObjectUtils.isEmpty(query.getSortObject()) && !query.getCollation().isPresent()) {return doFindOne(collectionName, query.getQueryObject(), query.getFieldsObject(), entityClass);} else {query.limit(1);List results &＃61; find(query, entityClass, collectionName);return results.isEmpty() ? null : results.get(0);}}


doFindOne是protected修饰的我们无法直接调用&＃xff0c;findOne可以调用&＃xff0c;我们只需要创建Query对象&＃xff0c;将条件传递进去即可&＃xff0c;下面是简单的实现&＃xff1a;

&＃64;Component
public class MongoQLService {&＃64;AutowiredMongoTemplate mongoTemplate;public T findById(Object id,Class entityClass,String collectionName) {Query query &＃61; new Query();query.addCriteria( new Criteria("_id").is(id));return mongoTemplate.findOne(query, entityClass, collectionName);}
}


关于Query的构建&＃xff0c;参考Spring Data MongoDB 基本文档查询