使用EntityGraph解决JPA下N+1问题

JPA解决N+1问题（指查询对象A，结果把A中的子对象B、C等也查询出来），我们可以使用EntityGraph。

@NamedEntityGraph(
        name = "aWithBWithC",
        attributeNodes = {
                @NamedAttributeNode(value = "bs", subgraph = "bWithC"),
                @NamedAttributeNode("c"),},
        subgraphs = {
                @NamedSubgraph(name = "bWithC",
                        attributeNodes = {@NamedAttributeNode("cs")})
        }
)
@Entity
@Table(name = "a", catalog = "funny", schema = "testing")
public class A {
    @OneToMany//默认懒加载
    @JoinTable( //if you have a table a_b for this relationship. Not related to the topic of entityGraph though. 
            name = "a_b", 
            joinColumns = @JoinColumn(name = "a_id"),
            inverseJoinColumns = @JoinColumn(name = "