Spring Data Many To One Mapping

The many to one mapping is from the one-to-many relationship. It is from the child side of the one-to-many relationship. Check out the one to many relationship here.

Many to one relationship mean that many instances of some entity can have a relationship with only one instance of another entity. For example, many cars can belong to only one person or user.

Many to One Bidirectional

package com.lovemesomecoding.car;

import java.io.Serializable;

import javax.persistence.Column;
import javax.persistence.Entity;
import javax.persistence.FetchType;
import javax.persistence.GeneratedValue;
import javax.persistence.GenerationType;
import javax.persistence.Id;
import javax.persistence.JoinColumn;
import javax.persistence.ManyToOne;
import javax.persistence.Table;

import com.fasterxml.jackson.annotation.JsonIgnoreProperties;
import com.fasterxml.jackson.annotation.JsonInclude;
import com.fasterxml.jackson.annotation.JsonInclude.Include;
import com.lovemesomecoding.user.User;

@JsonInclude(value = Include.NON_NULL)
@Entity
@Table(name = "car")
public class Car implements Serializable {

	private static final long serialVersionUID = 1L;

	@Id
	@GeneratedValue(strategy = GenerationType.IDENTITY)
	@Column(name = "id", nullable = false, updatable = false, unique = true)
	private Long id;
	
	@Column(name = "name")
	private String name;
	
	@Column(name = "brand")
	private String brand;
	
	@Column(name = "model")
	private String model;
	
	@Column(name = "price")
	private Double price;
	
	@JsonIgnoreProperties(value= {"cars"})
	@ManyToOne(fetch = FetchType.EAGER)
        @JoinColumn(name = "user_id", nullable=false)
	private User user;
	
	public Car() {
		super();
		// TODO Auto-generated constructor stub
	}
        // setters and getters

package com.lovemesomecoding.user;

import java.io.Serializable;
import java.util.HashSet;
import java.util.Set;

import javax.persistence.CascadeType;
import javax.persistence.Column;
import javax.persistence.Entity;
import javax.persistence.FetchType;
import javax.persistence.GeneratedValue;
import javax.persistence.GenerationType;
import javax.persistence.Id;
import javax.persistence.JoinColumn;
import javax.persistence.JoinTable;
import javax.persistence.ManyToMany;
import javax.persistence.OneToMany;
import javax.persistence.OneToOne;
import javax.persistence.Table;

import org.apache.commons.lang3.builder.EqualsBuilder;
import org.apache.commons.lang3.builder.HashCodeBuilder;
import org.apache.commons.lang3.builder.ToStringBuilder;

import com.fasterxml.jackson.annotation.JsonIgnoreProperties;
import com.fasterxml.jackson.annotation.JsonInclude;
import com.fasterxml.jackson.annotation.JsonInclude.Include;
import com.lovemesomecoding.car.Car;

@JsonInclude(value = Include.NON_NULL)
@Entity
@Table(name = "user")
public class User implements Serializable {

	private static final long serialVersionUID = 1L;

	@Id
	@GeneratedValue(strategy = GenerationType.IDENTITY)
	@Column(name = "id", nullable = false, updatable = false, unique = true)
	private Long id;
	
	@Column(name = "uid", unique = true, nullable=false, updatable=false)
	private String uid;

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

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

	@Column(name = "age")
	private int age;
	
	@JsonIgnoreProperties(value= {"user"})
	@OneToMany(mappedBy = "user", cascade=CascadeType.ALL, fetch=FetchType.EAGER)
	private Set<Car> cars;

	public User() {
		super();
		// TODO Auto-generated constructor stub
	}
        // getters and setters

Many to One Unidirectional
It’s very useful to map an entity to another entity which does not map back. This is a perfect solution in a situation where you are not using the child class very often. For example, you have a PaymentMethod class which you want to access whose user belongs to at runtime. When you load the PaymentMethod class from the database you also want the owner(user) of the payment method details loaded and available. But when you load the user details from the database, you don’t want to load his/her payment method details. You might just want to load the payment method details when you need to use them.

package com.lovemesomecoding.paymentmethod;

import java.io.Serializable;

import javax.persistence.Column;
import javax.persistence.Entity;
import javax.persistence.FetchType;
import javax.persistence.GeneratedValue;
import javax.persistence.GenerationType;
import javax.persistence.Id;
import javax.persistence.JoinColumn;
import javax.persistence.ManyToOne;
import javax.persistence.Table;

import com.fasterxml.jackson.annotation.JsonIgnoreProperties;
import com.fasterxml.jackson.annotation.JsonInclude;
import com.fasterxml.jackson.annotation.JsonInclude.Include;
import com.lovemesomecoding.user.User;

@JsonInclude(value = Include.NON_NULL)
@Entity
@Table(name = "payment_method")
public class PaymentMethod implements Serializable {

	private static final long serialVersionUID = 1L;

	@Id
	@GeneratedValue(strategy = GenerationType.IDENTITY)
	@Column(name = "id", nullable = false, updatable = false, unique = true)
	private Long id;
	
	@Column(name = "name")
	private String name;
	
	@Column(name = "brand")
	private String brand;
	
	@Column(name = "last4")
	private String last4;
	
	@ManyToOne(fetch = FetchType.EAGER)
        @JoinColumn(name = "user_id", nullable=false)
	private User user;

	public PaymentMethod() {
		super();
		// TODO Auto-generated constructor stub
	}
	// getters and setters

Source code github

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