jpa 多表关联查询

单向manyToOne

manyToOne many方

@AllArgsConstructor
@NoArgsConstructor
@Entity
@Builder
@Getter
@Setter
@Table(name = "product"
)
public class Product{@Id@GeneratedValue(strategy = GenerationType.AUTO)private Integer id;@Column(name = "productCode")private String productCode;/*** 商品名*/@Column(name = "productName")private String productName;/*** 商品价格*/@Column(name = "productPrice")private BigDecimal productPrice;/*** 商品数量*/@Column(name = "productStock")private Integer productStock;@Column(name = "foreignCode")private String foreignCode;@ManyToOne(fetch = FetchType.LAZY)@JoinColumn(name = "foreignCode",referencedColumnName = "code",insertable=false,updatable=false,foreignKey = @ForeignKey(name = "none", value =ConstraintMode.NO_CONSTRAINT))private JoinTable joinTable;
}

注：

  @Column(name = "foreignCode")private String foreignCode;

当前many方关联的one方字段，对应下方@JoinColumn 中的name = “foreignCode”

@JoinColumn(name = "foreignCode",referencedColumnName = "code",insertable=false,updatable=false,foreignKey = @ForeignKey(name = "none", value =ConstraintMode.NO_CONSTRAINT))

name：当前表格关联one方的字段
referencedColumnName：one方字段
insertable=false,updatable=false 不自动更新新增

one方

@AllArgsConstructor
@NoArgsConstructor
@Data
@Entity
@Builder
@Table(name = "join_table"
)
public class JoinTable implements Serializable {@Id@GeneratedValue(strategy = GenerationType.AUTO)private Integer id;// 所对应@Column(name = "code")private String code;@Column(name = "joinInfo")private String joinInfo;
}

注：

@Column(name = "code")
private String code;

与many方对应的字段，
many方 referencedColumnName = “code”

查询sql语句

select * from product left join join_table on product.foreignCode = join_table.code where join_table.joinInfo = ?

实现

List<Product> list = productRepository.findAll(new Specification<Product>() {@Overridepublic Predicate toPredicate(Root<Product> root, CriteriaQuery<?> criteriaQuery, CriteriaBuilder criteriaBuilder) {List<Predicate> list = new ArrayList<>();Join<Product, JoinTable> join = root.join("joinTable", JoinType.LEFT);list.add(criteriaBuilder.equal(join.get("joinInfo"), "123"));Predicate[] pre = new Predicate[list.size()];criteriaQuery.where(list.toArray(pre));return criteriaBuilder.and(list.toArray(pre));}});

注
1.

 Join<Product, JoinTable> join = root.join("joinTable", JoinType.LEFT);

“joinTable” ：many方关联的实体类名

 @ManyToOne(fetch = FetchType.LAZY)@JoinColumn(name = "foreignCode",referencedColumnName = "code",insertable=false,updatable=false,foreignKey = @ForeignKey(name = "none", value =ConstraintMode.NO_CONSTRAINT))private JoinTable joinTable;

list.add(criteriaBuilder.equal(join.get("joinInfo"), "123"));

“joinInfo”：one方需要查询字段名 sql为 where join_table.joinInfo = ?

@Column(name = "joinInfo")
private String joinInfo;

双向manyToOne oneToMany

many方

public class Product{@Id@GeneratedValue(strategy = GenerationType.AUTO)private Integer id;@Column(name = "productCode")private String productCode;/*** 商品名*/@Column(name = "productName")private String productName;/*** 商品价格*/@Column(name = "productPrice")private BigDecimal productPrice;/*** 商品数量*/@Column(name = "productStock")private Integer productStock;@Column(name = "foreignCode")private String foreignCode;//    @JsonIgnore@ManyToOne(fetch = FetchType.LAZY)@JoinColumn(name = "foreignCode",referencedColumnName = "code",insertable=false,updatable=false,foreignKey = @ForeignKey(name = "none", value =ConstraintMode.NO_CONSTRAINT))private JoinTable joinTable;public BigDecimal getProductPrice() {return productPrice;}public Integer getId() {return id;}public Integer getProductStock() {return productStock;}public JoinTable getJoinTable() {joinTable.setProductList(null);return joinTable;}public String getForeignCode() {return foreignCode;}public String getProductCode() {return productCode;}public String getProductName() {return productName;}public void setForeignCode(String foreignCode) {this.foreignCode = foreignCode;}public void setId(Integer id) {this.id = id;}public void setJoinTable(JoinTable joinTable) {this.joinTable = joinTable;}public void setProductCode(String productCode) {this.productCode = productCode;}public void setProductName(String productName) {this.productName = productName;}public void setProductPrice(BigDecimal productPrice) {this.productPrice = productPrice;}public void setProductStock(Integer productStock) {this.productStock = productStock;}
}

one方

public class JoinTable implements Serializable {@Id@GeneratedValue(strategy = GenerationType.AUTO)private Integer id;// 所对应@Column(name = "code")private String code;@Column(name = "joinInfo")private String joinInfo;@OneToMany(fetch = FetchType.LAZY,mappedBy = "joinTable")List<Product> productList;@Overridepublic String toString() {return "";}
}

注
@OneToMany(fetch = FetchType.LAZY,mappedBy = "joinTable")

“joinTable” ： many方对应的字段名，仅双向关联有用

@ManyToOne(fetch = FetchType.LAZY)
@JoinColumn(name = "foreignCode",referencedColumnName = "code",insertable=false,updatable=false,foreignKey = @ForeignKey(name = "none", value =ConstraintMode.NO_CONSTRAINT))
private JoinTable joinTable;

问题，双向依赖会导致堆栈溢出

解决：

注解 @JsonIgnore（fastjson情况）
注解到不希望展示的字段

2.重写get方法
举例：

   public JoinTable getJoinTable() {joinTable.setProductList(null);return joinTable;}

3.使用dto接收

举例：
查询时，返回

 List<Product> list = productRepository.findAll();List<ProductVo> result = new ArrayList<>();for(int i=0;i<list.size();i++){ProductVo productVo = new ProductVo();BeanUtils.copyProperties(list.get(i),productVo,"joinTable");
//            productVo.setForeignCode(list.get(i).getForeignCode());
//            productVo.setProductCode(list.get(i).getProductCode());
//            productVo.setProductName(list.get(i).getProductName());
//            productVo.setProductPrice(list.get(i).getProductPrice());
//            productVo.setProductStock(list.get(i).getProductStock());result.add(productVo);}return result;

ProductVo 去除会依赖的内容

public class ProductVo {private String productCode;/*** 商品名*/private String productName;/*** 商品价格*/private BigDecimal productPrice;/*** 商品数量*/private Integer productStock;private String foreignCode;
}