Java Spring Boot JPA关系映射和联接查询

开始学习Spring Boot，JPA。我发现很难理解JPA关系的概念，我尝试连接两个表，但未能达到预期的结果。有人能帮助我获得预期的结果吗

低于要求

我们有两张如下的表格

产品主模型

产品分类模型

ProductMasterRepository

调用getProductCat方法时，将结果获取为

productMasterModel值[productMasterModel[productId=1011， productName=Pencil，productCategoryId=10]，ProductMasterModel [productId=1012，productName=Mobile，productCategoryId=11]]

因为ProductMasterModel没有productType变量，所以它没有显示productType

我需要下面的结果连接两个表，请帮助我实现这一点

[[productName=Pencil，productType=Stational]，[productName= 移动设备，产品类型=电子设备]]

---

您需要在这两个表之间建立ont-to-one关系。 看看这个：

---

是的，一对一的关系应该有效。 应该在POJO中进行更改

import javax.persistence.Column;
import javax.persistence.Entity;
import javax.persistence.GeneratedValue;
import javax.persistence.Id;
import javax.persistence.Table;

@Entity
@Table(name = "product_master")
public class ProductMasterModel {
    @Id
    @GeneratedValue
    @Column(name = "product_id")
    private int productId;

    @Column(name = "product_name")
    private String productName;

    @Column(name = "product_category_id")
    private int productCategoryId;

    @OneToOne(mappedBy= product_master, fetch = FetchType.LAZY)
    public ProductCatagoryMasterModel productCatagory;    

    public int getProductId() {
        return productId;
    }

    public void setProductId(int productId) {
        this.productId = productId;
    }

    public String getProductName() {
        return productName;
    }

    public void setProductName(String productName) {
        this.productName = productName;
    }

    public int getProductCategoryId() {
        return productCategoryId;
    }

    public void setProductCategoryId(int productCategoryId) {
        this.productCategoryId = productCategoryId;
    }

    public ProductMasterModel() {

    }

    public ProductMasterModel(String productName, int productCategoryId) {
        super();
        this.productName = productName;
        this.productCategoryId = productCategoryId;

    }
}

下一个地址是您的类别模型

import javax.persistence.Column;
import javax.persistence.Entity;
import javax.persistence.GeneratedValue;
import javax.persistence.Id;
import javax.persistence.Table;

@Entity
@Table (name="product_catagory")
public class ProductCatagoryMasterModel {

    @Id
    @GeneratedValue
    @Column(name="product_catogory_id")
    private int productCategoryId;

    @Column(name="product_type")
    private String productType;

    @OneToOne(cascade = CascadeType.ALL)
    @JoinColumn(name = "product_master", referencedColumnName = "product_id")
    private ProductMasterModel productMaster;

    public int getProductCategoryId() {
        return productCategoryId;
    }

    public void setProductCategoryId(int productCategoryId) {
        this.productCategoryId = productCategoryId;
    }

    public String getProductType() {
        return productType;
    }

    public void setProductType(String productType) {
        this.productType = productType;
    }

    public ProductCatagoryMasterModel() {
        super();
    }

    public ProductCatagoryMasterModel(String productType) {
        super();
        this.productType = productType;

    }   
}

我们也需要刀

import org.springframework.data.jpa.repository.Query;
import org.springframework.data.repository.CrudRepository;
import org.springframework.stereotype.Repository;
import com.lollipop.model.ProductMasterModel;

@Repository
public interface ProductMasterRepository extends CrudRepository<ProductMasterModel, Integer> {

}

产品服务

import java.util.List;
import javax.transaction.Transactional;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.stereotype.Service;
import com.lollipop.model.ProductMasterModel;
import com.lollipop.repository.ProductMasterRepository;

@Service
@Transactional
public class ProductService {

    @Autowired
    private ProductMasterRepository productMasterRepository;

    public List<ProductMasterModel > getAllProducts() {
    return productMasterRepository.findAll();
    }

    public Optional<ProductMasterModel > getProductById(int productId) {
    if (!productMasterRepository.existsById(productId)) {
        throw new ResourceNotFoundException("Product with id " + productId+ " not found");
    }
    return productMasterRepository.findById(productId);
}
    }
}

---

在我的模型中添加了OnetoOne关系，但下面列出了例外情况：我们在这里建立的列引用是否错误？。。为了更加清晰，我有product_category表，其中包含product_category_idPK列、product_type列和product_master表，其中product_idPK列、product_name列、，product\u category\u idFK到product\u category表product\u category\u id列异常：原因：org.hibernate.AnnotationException:未知映射项位于：com.lollipop.model.ProductMasterModel.productCatagory，引用的属性未知：com.lollipop.model.ProductCatagoryMasterModel.product_master位于org.hibernate.cfg.OneToOneSecondPass.doSecondPass.doSecondPass.java:168~[hibernate-core-5.3.10.Final.jar:5.3.10.Final]]这就是我们在您的案例中应该如何处理一对一关系的示例，但是我可以看到我错过了放置映射by=product\u master和product\u category谢谢。。我解决了得到的异常，并成功地创建了关系，但仍然不确定如何显示预期结果。在ProductService类productMasterRepository.findAll中，正在从product_主表返回数据，但我需要结果作为product_名称和product_类型，它位于product_类别表上。你能帮我接电话吗this@user7294900你能帮个忙吗

import javax.persistence.Column;
import javax.persistence.Entity;
import javax.persistence.GeneratedValue;
import javax.persistence.Id;
import javax.persistence.Table;

@Entity
@Table(name = "product_master")
public class ProductMasterModel {
    @Id
    @GeneratedValue
    @Column(name = "product_id")
    private int productId;

    @Column(name = "product_name")
    private String productName;

    @Column(name = "product_category_id")
    private int productCategoryId;

    @OneToOne(mappedBy= product_master, fetch = FetchType.LAZY)
    public ProductCatagoryMasterModel productCatagory;    

    public int getProductId() {
        return productId;
    }

    public void setProductId(int productId) {
        this.productId = productId;
    }

    public String getProductName() {
        return productName;
    }

    public void setProductName(String productName) {
        this.productName = productName;
    }

    public int getProductCategoryId() {
        return productCategoryId;
    }

    public void setProductCategoryId(int productCategoryId) {
        this.productCategoryId = productCategoryId;
    }

    public ProductMasterModel() {

    }

    public ProductMasterModel(String productName, int productCategoryId) {
        super();
        this.productName = productName;
        this.productCategoryId = productCategoryId;

    }
}

import javax.persistence.Column;
import javax.persistence.Entity;
import javax.persistence.GeneratedValue;
import javax.persistence.Id;
import javax.persistence.Table;

@Entity
@Table (name="product_catagory")
public class ProductCatagoryMasterModel {

    @Id
    @GeneratedValue
    @Column(name="product_catogory_id")
    private int productCategoryId;

    @Column(name="product_type")
    private String productType;

    @OneToOne(cascade = CascadeType.ALL)
    @JoinColumn(name = "product_master", referencedColumnName = "product_id")
    private ProductMasterModel productMaster;

    public int getProductCategoryId() {
        return productCategoryId;
    }

    public void setProductCategoryId(int productCategoryId) {
        this.productCategoryId = productCategoryId;
    }

    public String getProductType() {
        return productType;
    }

    public void setProductType(String productType) {
        this.productType = productType;
    }

    public ProductCatagoryMasterModel() {
        super();
    }

    public ProductCatagoryMasterModel(String productType) {
        super();
        this.productType = productType;

    }   
}

import org.springframework.data.jpa.repository.Query;
import org.springframework.data.repository.CrudRepository;
import org.springframework.stereotype.Repository;
import com.lollipop.model.ProductMasterModel;

@Repository
public interface ProductMasterRepository extends CrudRepository<ProductMasterModel, Integer> {

import java.util.List;
import javax.transaction.Transactional;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.stereotype.Service;
import com.lollipop.model.ProductMasterModel;
import com.lollipop.repository.ProductMasterRepository;

@Service
@Transactional
public class ProductService {

    @Autowired
    private ProductMasterRepository productMasterRepository;

    public List<ProductMasterModel > getAllProducts() {
    return productMasterRepository.findAll();
    }

    public Optional<ProductMasterModel > getProductById(int productId) {
    if (!productMasterRepository.existsById(productId)) {
        throw new ResourceNotFoundException("Product with id " + productId+ " not found");
    }
    return productMasterRepository.findById(productId);
}
    }
}