Spring Data JPA中的Inner Join查询和基于接口的Projection

我正在使用运行MySql DB的Spring Data JPA进行Spring MVC项目，其中我有四个实体对象：旅行，费用，货币和基金 .

这是我的数据库架构的直观表示：

在ExpenseRepository接口中，我扩展了JpaRepository接口 .

现在我正在尝试运行本机SQL查询，我将传递expenseId，我将从Expense表中获取费用和金额，并从货币表中获取currency_name . （你可以看到我必须做两个内连接来获取currency_name . ）

最后，我创建了另一个接口ExpenseOutput，将三列合并为一个单独的非实体接口（根据Spring Data JPA文档中提到的基于接口的投影映射） .

以下是代码：

package com.binarycraftbd.ksktravelbackend.Repo

import com.binarycraftbd.ksktravelbackend.JoinQueries.ExpenseData
import com.binarycraftbd.ksktravelbackend.Model.Expense
import org.springframework.data.jpa.repository.JpaRepository
import org.springframework.data.jpa.repository.Query

interface ExpenseRepo : JpaRepository<Expense, Int> {


    @Query("select expense, amount from expense, currencyName from currency inner join fund on expense.fund_id=fund.id inner join currency on fund.currency_id=currency.id where expense.id=?1", nativeQuery = true)
    fun getCurrencyByExpId(expId:Int): ExpenseOutput

    interface ExpenseOutput{
        fun getExpense():String
        fun getAmount(): String
        fun getCurrencyname(): String
    }
}

但是，当我通过RestController函数运行代码时，我收到以下错误：

This application has no explicit mapping for /error, so you are seeing this as a fallback.

Sat Sep 22 20:51:25 BDT 2018
There was an unexpected error (type=Internal Server Error, status=500).
could not extract ResultSet; SQL [n/a]; nested exception is org.hibernate.exception.SQLGrammarException: could not extract ResultSet

我也在这里给出了实体类：

旅游类

@Entity
@Table(name = "travel")
class Travel(
        @Id
        @GeneratedValue(strategy = GenerationType.SEQUENCE)
        val id: Int=0,
        val travelName: String="",

        @OneToMany(mappedBy = "travel")
        @JsonIgnore
        val funds: List<Fund> ?= null,

        @OneToMany(mappedBy = "travel")
        @JsonIgnore
        val expenses: List<Expense>?=null

)

货币类

@Entity
@Table(name = "currency")
class Currency(

        @Id
        @GeneratedValue(strategy = GenerationType.SEQUENCE)
        val id: Int=0,
        val currencyName: String="",

        @OneToMany(mappedBy = "currency")
        @JsonIgnore
        val funds: List<Fund>?=null
)

基金类

@Entity
class Fund(
        @Id
        @GeneratedValue(strategy = GenerationType.SEQUENCE)
        val id:Int=0,
        val fundName:String="",

        @OneToMany(mappedBy = "fund")
        @JsonIgnore
        val expenses: List<Expense>?= null,

        @ManyToOne
        val travel: Travel?=null,

        @ManyToOne
        val currency:Currency?=null
)

费用等级

@Entity
class Expense(


        @Id
        @GeneratedValue(strategy = GenerationType.SEQUENCE)
        val id:Int=0,
        val date:String="",
        val time:String="",
        val expense:String="",
        val amount:String="",
        val category:String="",

        @ManyToOne
        val travel: Travel?=null,

        @ManyToOne
        val fund: Fund?=null
)

如何解决这个问题呢？在ExpenseRepository中编写查询代码是不正确的吗？或者Sql查询有问题吗？皮斯帮助!!