EJB3 JPA Entity Many-To-One Relationship

A Many-To-One relationship is whereby one object class is “part of” or being consisted in many other object classes. Where can you discover many-To-One relationship? Usually it appears the moment when you perform a website registration whereby you have to select your country details from a drop down box when the website ask for your profile and address details, this is when many user records shares the same record of country and states.


Today, let us take a look at how to store property information in the real industry. Regardless of whatever property the real estate firm sells, properties will still be categorized as apartments, condominiums, link houses, bungalows, land, SOHO, etc. Therefore, every property info will consists of a singular property type.

The below are the database table diagram and the SQL codes to generate the table structure in MySQL.

 SQL | 
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CREATE TABLE APP_PROPERTY_TYPES
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(
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    APP_PT_PK INT NOT NULL AUTO_INCREMENT,
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    PROPERTY_TYPE VARCHAR(255) NOT NULL,
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    PRIMARY KEY(APP_PT_PK),
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    UNIQUE(PROPERTY_TYPE)
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);
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INSERT INTO APP_PROPERTY_TYPES(PROPERTY_TYPE) VALUES ('Apartment');
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INSERT INTO APP_PROPERTY_TYPES(PROPERTY_TYPE) VALUES ('Condominium');
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INSERT INTO APP_PROPERTY_TYPES(PROPERTY_TYPE) VALUES ('Terrace House');
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INSERT INTO APP_PROPERTY_TYPES(PROPERTY_TYPE) VALUES ('Semi-D House');
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INSERT INTO APP_PROPERTY_TYPES(PROPERTY_TYPE) VALUES ('Bungalow');
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INSERT INTO APP_PROPERTY_TYPES(PROPERTY_TYPE) VALUES ('Residential Land');
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INSERT INTO APP_PROPERTY_TYPES(PROPERTY_TYPE) VALUES ('Industrial Land');
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INSERT INTO APP_PROPERTY_TYPES(PROPERTY_TYPE) VALUES ('Commercial Land');
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CREATE TABLE APP_PROPERTIES
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(
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    APP_PROPERTIES_PK BIGINT NOT NULL AUTO_INCREMENT,
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    PROPERTY_ADDRESS_STR VARCHAR(255) NOT NULL,
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    PROPERTY_TYPE_PK INT NOT NULL,
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    PROPERTY_DESC TEXT,
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    OWNER_FULLNAME VARCHAR(255) NOT NULL,
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    OWNER_CONTACT VARCHAR(255) NOT NULL,
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    SELLING_PRICE DECIMAL(12, 2) NOT NULL,
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    PRIMARY KEY(APP_PROPERTIES_PK),
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    FOREIGN KEY(PROPERTY_TYPE_PK) REFERENCES APP_PROPERTY_TYPES(APP_PT_PK) ON UPDATE CASCADE ON DELETE RESTRICT
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);

The APP_PROPERTIES table will contain the records of different types that are available for sale. For the property type info, it just links to the APP_PROPERTY_TYPE table through the foreign key field of APP_PROPERTIES.PROPERTY_TYPE_PK. APP_PROPERTY_TYPE table will consists static records of property types, which will be used by many APP_PROPERTIES records.

As for the Java classes, the below are the codes for both PropertyEntity class which encapsulates the structure in the APP_PROPERTIES table and PropertyTypeEntity, which encapsulates the structure in APP_PROPERTY_TYPES table.

Codes for PropertyEntity class:

 Java(TM) 2 Platform Standard Edition 5.0 | 
 copy code |
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001
package com.developerscrappad.propmng.entity;
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import java.io.Serializable;
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import javax.persistence.Basic;
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import javax.persistence.Column;
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import javax.persistence.Entity;
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import javax.persistence.GeneratedValue;
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import javax.persistence.GenerationType;
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import javax.persistence.Id;
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import javax.persistence.JoinColumn;
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import javax.persistence.ManyToOne;
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import javax.persistence.Table;
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@Entity
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@Table(name = "APP_PROPERTIES")
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public class PropertyEntity implements Serializable {
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    private static final long serialVersionUID = 6397311185164388570L;
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    @Id
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    @GeneratedValue(strategy = GenerationType.IDENTITY)
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    @Column(name = "APP_PROPERTIES_PK")
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    private Long appPropertiesPk;
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    @Basic(optional = false)
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    @Column(name = "PROPERTY_ADDRESS_STR")
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    private String propertyAddressStr;
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    @Column(name = "PROPERTY_DESC")
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    private String propertyDesc;
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    @Column(name = "OWNER_FULLNAME")
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    private String ownerFullname;
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    @Column(name = "OWNER_CONTACT")
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    private String ownerContact;
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    @Column(name = "SELLING_PRICE")
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    private Double sellingPrice;
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    @JoinColumn(name = "PROPERTY_TYPE_PK", referencedColumnName = "APP_PT_PK")
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    @ManyToOne(optional = false)
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    private PropertyTypeEntity propertyType;
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    public PropertyEntity() {
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    }
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    public PropertyEntity(Long appPropertiesPk) {
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        this.appPropertiesPk = appPropertiesPk;
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    }
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    public PropertyEntity(Long appPropertiesPk, String propertyAddressStr, String ownerFullname, String ownerContact, Double sellingPrice, PropertyTypeEntity propertyType) {
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        this.appPropertiesPk = appPropertiesPk;
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        this.propertyAddressStr = propertyAddressStr;
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        this.ownerFullname = ownerFullname;
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        this.ownerContact = ownerContact;
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        this.sellingPrice = sellingPrice;
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        this.propertyType = propertyType;
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    }
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    public Long getAppPropertiesPk() {
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        return appPropertiesPk;
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    }
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    public void setAppPropertiesPk(Long appPropertiesPk) {
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        this.appPropertiesPk = appPropertiesPk;
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    }
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    public String getPropertyAddressStr() {
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        return propertyAddressStr;
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    }
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    public void setPropertyAddressStr(String propertyAddressStr) {
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        this.propertyAddressStr = propertyAddressStr;
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    }
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    public String getPropertyDesc() {
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        return propertyDesc;
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    }
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    public void setPropertyDesc(String propertyDesc) {
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        this.propertyDesc = propertyDesc;
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    }
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    public String getOwnerFullname() {
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        return ownerFullname;
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    }
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    public void setOwnerFullname(String ownerFullname) {
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        this.ownerFullname = ownerFullname;
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    }
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    public String getOwnerContact() {
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        return ownerContact;
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    }
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    public void setOwnerContact(String ownerContact) {
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        this.ownerContact = ownerContact;
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    }
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    public void setSellingPrice(Double sellingPrice) {
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        this.sellingPrice = sellingPrice;
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    }
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    public Double getSellingPrice() {
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        return sellingPrice;
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    }
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    public PropertyTypeEntity getPropertyType() {
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        return propertyType;
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    }
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    public void setPropertyType(PropertyTypeEntity propertyType) {
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        this.propertyType = propertyType;
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    }
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    @Override
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    public int hashCode() {
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        int hash = 0;
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        hash += (appPropertiesPk != null ? appPropertiesPk.hashCode() : 0);
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        return hash;
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    }
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    @Override
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    public boolean equals(Object object) {
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        // TODO: Warning - this method won't work in the case the id fields are not set
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        if (!(object instanceof PropertyEntity)) {
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            return false;
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        }
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        PropertyEntity other = (PropertyEntity) object;
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        if ((this.appPropertiesPk == null && other.appPropertiesPk != null) || (this.appPropertiesPk != null && !this.appPropertiesPk.equals(other.appPropertiesPk))) {
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            return false;
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        }
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        return true;
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    }
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    @Override
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    public String toString() {
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        return "com.developerscrappad.propmng.entity.PropertiesEntity[ appPropertiesPk=" + appPropertiesPk + " ]";
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    }
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}

Codes for PropertyTypeEntity class:

 Java(TM) 2 Platform Standard Edition 5.0 | 
 copy code |
?
 
01
package com.developerscrappad.propmng.entity;
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import java.io.Serializable;
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import javax.persistence.Column;
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import javax.persistence.Entity;
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import javax.persistence.GeneratedValue;
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import javax.persistence.GenerationType;
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import javax.persistence.Id;
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import javax.persistence.Table;
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@Entity
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@Table(name = "APP_PROPERTY_TYPES")
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public class PropertyTypeEntity implements Serializable {
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    private static final long serialVersionUID = 2223674512837985849L;
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    @Id
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    @GeneratedValue(strategy = GenerationType.IDENTITY)
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    @Column(name = "APP_PT_PK")
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    private Integer appPtPk;
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    @Column(name = "PROPERTY_TYPE")
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    private String propertyType;
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    public PropertyTypeEntity() {
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    }
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    public PropertyTypeEntity(Integer appPtPk) {
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        this.appPtPk = appPtPk;
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    }
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    public PropertyTypeEntity(Integer appPtPk, String propertyType) {
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        this.appPtPk = appPtPk;
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        this.propertyType = propertyType;
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    }
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    public Integer getAppPtPk() {
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        return appPtPk;
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    }
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    public void setAppPtPk(Integer appPtPk) {
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        this.appPtPk = appPtPk;
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    }
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    public String getPropertyType() {
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        return propertyType;
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    }
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    public void setPropertyType(String propertyType) {
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        this.propertyType = propertyType;
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    }
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    @Override
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    public int hashCode() {
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        int hash = 0;
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        hash += (appPtPk != null ? appPtPk.hashCode() : 0);
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        return hash;
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    }
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    @Override
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    public boolean equals(Object object) {
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        // TODO: Warning - this method won't work in the case the id fields are not set
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        if (!(object instanceof PropertyTypeEntity)) {
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            return false;
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        }
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        PropertyTypeEntity other = (PropertyTypeEntity) object;
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        if ((this.appPtPk == null && other.appPtPk != null) || (this.appPtPk != null && !this.appPtPk.equals(other.appPtPk))) {
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            return false;
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        }
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        return true;
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    }
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    @Override
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    public String toString() {
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        return "com.developerscrappad.propmng.entity.PropertyTypeEntity[ appPtPk=" + appPtPk + " ]";
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    }
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}

In order to have a PropertyTypeEntity object being part of the PropertyEntity object, we mark the propertyType property in PropertyEntity class as @ManyToOne. Of course additionally, to instruct the app server how to properly tie the relationship is by having an annotation of @JoinColumn as well.

@JoinColumn(name = "PROPERTY_TYPE_PK", referencedColumnName = "APP_PT_PK")
@ManyToOne(optional = false)

In the @JoinColumn annotation, the name attribute always refers to the column of the foreign key in the table that the class represents. In this case, it is APP_PROPERTIES.PROPERTY_TYPE_PK. As for the referenceColumn attribute, the value should be the target reference column of the primary key field of the table that APP_PROPERTIES.PROPERTY_TYPE_PK references to. In this case, it is APP_PROPERTY_TYPES.APP_PT_PK.

Simple isn’t it? This sums up how we can code a Many-To-One relationship in our Java Entity classes.

• EJB3 JPA Entity One-To-One Relationship
• EJB3 JPA Entity One-To-Many Relationship
• EJB3 JPA Entity Many-To-Many Relationship