Hibernate - @ManyToOne Annotation

@ManytoOne annotation in Hibernate is used to create a many-to-one relationship between two entities. The @ManyToOne annotation indicates that the many instances of one entity can be associated with only one instance of another entity. When we annotate a field of the method with @ManyToOne annotation the Hibernate will create the many-to-one relation between these two entities. This annotation is used to map a single-valued annotation where multiple instances of one entity are associated with a single instance of another entity. 

Examples of @ManyToOne Annotation

Example 1:

// on the below line creating an entity for section
@Entity
public class Section {
    // on the below line creating an id for the section
    // which is the generated value.
    @Id
      @GeneratedValue
      private long id;
    // on the below line creating a field for section name.
    private String sectionName;
    // on the below line creating a field for students name
    // array list present in that section.
    private ArrayList<String> studentsNameList;
    // on the below line creating a field for school which
    // we are annotating with many to one annotation.
    @ManyToOne 
      private School school;
}
// on the below line creating an entity for School.
@Entity
public class School {
    // on the below line creating an id for school.
    @Id 
      @GeneratedValue
      private long id;
    // on the below line creating a field for the school
    // name.
    private String schoolName;
}

Code Explanation:

In the above example we are creating two entities for School and Section and these entities are linked with each other through many-to-one annotation. We are creating different fields in section entities such as id for the section which is annotated with @Id and @GeneratedValue to generate it automatically. Then we are creating a field for section name, students name array list which is an array of student’s names present in that section. Lastly, we are creating a file for the section which is annotated with @ManyToOne. @ManyToOne annotation indicates that the section entity is having a many-to-one relationship with the School entity. The @ManyToOne annotation is used to annotate the school field in the Section entity which indicates that the section belongs to a single school. 

Example 2:

// on the below line creating an entity for Employee
@Entity
public class Employee {
    // on the below line creating an id for an employee
    // which is generated value.
    @Id
      @GeneratedValue
      private long id;
    // on the below line creating a field for employee name.
    private String name;
    // on the below line creating a field for employee age
    private int age;
    // on the below line creating a field for employee
    // gender.
    private String gender;
    // on the below line creating a field for the department
    // which we are annotating with many to one annotation.
    @ManyToOne
      private Department department;
}
// on the below line creating an entity for the Department.
@Entity
public class Department {
    // on the below line creating an id for the department.
    @Id 
      @GeneratedValue
      private long id;
    // on the below line creating a field for department
    // name.
    private String name;
}

Code Explanation:

In the above example, we are creating two entities, one for the Employee and another for the Department. The employee entity consists of different fields such as employee name, age, gender, and id which are annotated with @Id and @GeneratedValue to generate it automatically. Then we are creating one more field for the department which we are annotating with @ManyToOne which indicates the current employee is associated In many to one relationship with the Department entity. Then we are creating an entity for the Department which has fields such as id which is auto-generated as we are adding @GeneratedValue annotation to it. Then we are creating a field for the name of the department.