当MongoDB主键为String时，mongoTemplate无法根据id查询的问题

MongoDB推荐使用ObjectId作为主键，但国内的开发都知道，事情往往不如人所愿，当我们真的出现了“_id”主键的类型为String时，且还必须想用mongoTemplate.findOne或findList时，直接使用该方法会导致查询结果为空。

因为mongoTemplate会在查询的时候将主键转换为ObjectId。

实际上这一步转换是MappingMongoConverter中做的，源码如下：

	/**
	 * Converts the given raw id value into either {@link ObjectId} or {@link String}.
	 *
	 * @param id can be {@literal null}.
	 * @param targetType must not be {@literal null}.
	 * @return {@literal null} if source {@literal id} is already {@literal null}.
	 * @since 2.2
	 */
@Nullable
	default Object convertId(@Nullable Object id, Class targetType) {

		if (id == null || ClassUtils.isAssignableValue(targetType, id)) {
			return id;
		}

		if (ClassUtils.isAssignable(ObjectId.class, targetType)) {

			if (id instanceof String) {

				if (ObjectId.isValid(id.toString())) {
					return new ObjectId(id.toString());
				}

				// avoid ConversionException as convertToMongoType will return String anyways.
				return id;
			}
		}

		try {
			return getConversionService().canConvert(id.getClass(), targetType)
					? getConversionService().convert(id, targetType)
					: convertToMongoType(id, (TypeInformation) null);
		} catch (ConversionException o_O) {
			return convertToMongoType(id,(TypeInformation)  null);
		}
	}

注意示例版本为spring-data-mongodb:3.3.5，一些更古早的版本，如2.1.9，需要重写其他部分，如果有人需要请留言，我会抽时间补充，但鉴于之前的版本有比较重大的漏洞，所以推荐升级版本。

 if (ObjectId.isValid(id.toString())) { return new ObjectId(id.toString()); }

这段就是mongoTemplate将String转换为ObjectId的步骤。

所以我们要做的就是重写这部分，并且替换掉原有的：

1、新建一个Converter

public class MyMongoConverter extends MappingMongoConverter {
    public MyMongoConverter(DbRefResolver dbRefResolver, MappingContext, MongoPersistentProperty> mappingContext) {
        super(dbRefResolver, mappingContext);
    }

    @Override
    public Object convertId(@Nullable Object id, Class targetType) {
        if (id == null || ClassUtils.isAssignableValue(targetType, id)) {
            return id;
        }

        if (ClassUtils.isAssignable(ObjectId.class, targetType)) {
            if (id instanceof String) {
                return id;
            }
        }

        try {
            return getConversionService().canConvert(id.getClass(), targetType)
                    ? getConversionService().convert(id, targetType)
                    : convertToMongoType(id, (TypeInformation) null);
        } catch (ConversionException o_O) {
            return convertToMongoType(id,(TypeInformation)  null);
        }
    }
}

2、在构建MongoTemplate时用我们自己写的替换

MongoTemolate mongoTemplate = new MongoTemplate(mongoDbFactory(), getDefaultMongoConverter(mongoDbFactory()))

protected static MongoConverter getDefaultMongoConverter(MongoDatabaseFactory factory) {

        DbRefResolver dbRefResolver = new DefaultDbRefResolver(factory);
        MongoCustomConversions conversions = new MongoCustomConversions(Collections.emptyList());

        MongoMappingContext mappingContext = new MongoMappingContext();
        mappingContext.setSimpleTypeHolder(conversions.getSimpleTypeHolder());
        mappingContext.afterPropertiesSet();

        MyMongoConverter converter = new MyMongoConverter(dbRefResolver, mappingContext);

        converter.setCustomConversions(conversions);
        converter.afterPropertiesSet();
        converter.setTypeMapper(new DefaultMongoTypeMapper(null));

        return converter;
    }
    /**
     * @return {@link MongoDatabaseFactory}
     */
    protected MongoDatabaseFactory mongoDbFactory() {
        ServerAddress serverAddress = new ServerAddress(host, port);

        MongoClientSettings.Builder setting = MongoClientSettings.builder()
                .applyToClusterSettings(builder -> builder.hosts(Collections.singletonList(serverAddress)))
                .retryWrites(false);
        if (ObjectUtil.isNotEmpty(username)) {
            MongoCredential credential = MongoCredential.createCredential(username, authenticationDatabase, password.toCharArray());
            setting.credential(credential);
        }
        MongoClient mongoClient = MongoClients.create(setting.build());
        return new SimpleMongoClientDatabaseFactory(mongoClient, database);
    }

host,port,username,authenticationDatabase,password,database

分别为MongoDB的地址，端口号，用户名，认证数据库，密码，数据库名