@Result(property = "parent", column = "parent_id", javaType = Parent.class, one = @One(select = "com.mybatis.parent.findById")),

Caused by: org.mybatis.spring.MyBatisSystemException: nested exception is org.apache.ibatis.exceptions.PersistenceException:
### Error querying database.  Cause: java.lang.IllegalArgumentException:
Mapped Statements collection does not contain value for com.mybatis.parent.findById
### The error may exist in com/mybatis/child/ChildMapper.java (best guess)
### The error may involve com.mybatis.child.ChildMapper.findById
### The error occurred while handling results
### SQL: select c.id, c.name, c.age, c.parent_id from child c where c.id = ?
### Cause: java.lang.IllegalArgumentException: Mapped Statements collection does not contain value for com.mybatis.parent.findById

public class Child {

    private Long id;

    private String name;

    private Integer age;

    private Parent parent;

    public Child() {
    }

//Getters and Setters

@Mapper
public interface ChildMapper {

@Select("select c.id, c.name, c.age, c.parent_id from child c where c.id = #{id}")
@Results(value = {
        @Result(property = "parent", column = "parent_id", javaType = Parent.class, one = @One(select = "findParentById")),
})
Child findById(Long id);

@Select("select p.id, p.name, p.age from parent p where p.id = #{id}")
Parent findParentById(Long id);
}

@Repository
public class ChildRepository {

    private ChildMapper childMapper;

    public ChildRepository(ChildMapper childMapper) {
        this.childMapper = childMapper;
    }

    public Child findByid(Long id){
            return this.childMapper.findById(id);
    }
}

public class Parent {

    private Long id;

    private String name;

    private Integer age;

    public Parent() {
    }
    //Getters and Setters

@Mapper
public interface ParentMapper {
    @Select("select p.id, p.name, p.age from parent p where p.id = #{id}")
    Parent findById(Long id);
}

@Repository
public class ParentRepository {

    private ParentMapper parentMapper;

    public ParentRepository(ParentMapper parentMapper) {
        this.parentMapper = parentMapper;
    }

    public Parent findByid(Long id){
        return this.parentMapper.findById(id);
    }
}

@SpringBootApplication
public class ParentChildApplication  implements CommandLineRunner{

    public static void main(String[] args) {
        SpringApplication.run(ParentChildApplication.class, args);
    }

    @Autowired
    ChildRepository childRepository;

    @Autowired
    ParentRepository parentRepository;

    @Override>
    public void run(String... strings) throws Exception {

        Child child = this.childRepository.findByid(2L);

        Parent parent = this.parentRepository.findByid(1L);

    }
}