Hibernate is an ambitious project that aims to be a complete solution to the problem of managing persistent data in java. Even with such an arduous task before them, the hibernate team tries very hard to expose a simple API for developers like us. Still, the complexity behind the API shows its ugly face time and again and I believe it is unavoidable as long as the mismatch between Object and Relational world exists.<\/p>\n
That said, although I have worked with hibernate for many years and have been its advocate in all my organizations, I keep facing newer issues and keep finding newer ways to work with it efficiently and effectively. Recently, when I was working for nboomi.com<\/a>, I faced an issue when mapping a OneToMany<\/em> relationship to the sub-classes of “Single Table Inheritance”<\/em> strategy. After a frustrating couple of hours of debugging I finally landed on the correct solution. So, I thought other developers who will travel this path could get benefited and started writing this blog post.<\/p>\n Let me explain the issue I faced with an example. Assume you have a normal User<\/em> Entity with the typical id<\/em>, version<\/em> and loginId<\/em> properties. Assume this User<\/em> can have many AboutUs<\/em> sections and many Service<\/em> sections. You don’t need to be an architect to model them as OneToMany<\/em> relationships from User<\/em>. So, I modelled UserAboutSection<\/em> and UserServiceSection<\/em> entities and created a OneToMany<\/em> relationship between User<\/em> and these entities. Looking at the commonality between these two, I decided to factor out the common fields into a superclass called UserSection<\/em>. Now, both UserAboutSection<\/em> and UserServiceSection<\/em> extends UserSection<\/em>. I chose to map this Inheritance hierarchy using “Single Table Inheritance” strategy to keep it simple and since most of the fields were common and only a few were specific. <\/p>\n The User<\/em> entity is given below. Notice the List<UserAboutSection><\/em> and a List<UserServiceSection><\/em> mapped using OneToMany<\/em> relationship.<\/p>\n The getters, setters, adders, imports and static imports are omitted for brevity.<\/p>\n Here goes the UserSection<\/em> entity that acts as the base class in this “Single Table Inheritance” strategy. Hibernate uses the @DiscriminatorColumn annotation to distinguish between sub-classes. <\/p>\n Here goes the UserAboutSection<\/em> entity that derives from the UserSection<\/em> entity. Hibernate uses the @DiscriminatorValue annotation to decide if a row in the database belongs to an instance of this class.<\/p>\n Here goes the UserServiceSection<\/em> entity that derives from the UserSection<\/em> entity. Hibernate uses the @DiscriminatorValue annotation to decide if a row in the database belongs to an instance of this class.<\/p>\n Pretty straightforward… huh! When you try to retrieve an instance of User<\/em> along with its aboutUs<\/em> and services<\/em> collections eagerly (or lazily – doesn’t matter), what do you expect? <\/p>\n I expected an instance of User<\/em> with the aboutUs<\/em> collection filled with only UserAboutSection<\/em> instances and the services<\/em> collection filled with only UserServiceSection<\/em> instances corresponding to only the rows they represent in the database. And I believe this expectation is valid, because that is what the mapping looks like and hibernate also has all the information it needs to make this work. <\/p>\n But I got something different. Both the aboutUs<\/em> and services<\/em> collections had all the UserSection<\/em> rows that belong to this User<\/em>. I mean, aboutUs<\/em> collection had all the UserSection<\/em> instances including UserAboutSection<\/em> and UserServiceSection<\/em> instances. This was surprising because hibernate has all the information it needs to populate the right instances. <\/p>\n After quite a bit of debugging, googling and RTFM-ing I landed upon @ForceDiscriminator<\/em> annotation. This annotation has to be applied to the base class in the Inheritance hierarchy for “Single Table Inheritance” strategy. In my case, I had to apply it to UserSection<\/em> entity. The UserSection<\/em> entity after applying this annotation is given below… <\/p>\n Once I ask hibernate to Force Discriminiator, it is happy and populates the aboutUs and services collections with its respective instances. <\/p>\n Ok, Problem Solved! But why did I have to tell hibernate to Force Discriminator. Shouldn’t that be the default behaviour. Is it a bug in hibernate or is it a feature? Am I missing something? If any one of you hibernate fans have walked this path and know the answer, please feel free to drop in a comment. I sincerely hope this post will be a valuable time-saver for other hibernate developers who step on this Bug\/Feature. <\/p>\n","protected":false},"excerpt":{"rendered":" Hibernate is an ambitious project that aims to be a complete solution to the problem of managing persistent data in java. Even with such an arduous task before them, the hibernate team tries very hard to expose a simple API for developers like us. Still, the complexity behind the API shows its ugly face time and again and I believe it is unavoidable as long as the mismatch between Object and Relational world exists. That said, although I have worked with hibernate for many years and have been its advocate in all my organizations, I keep facing newer issues and... \r\n@Entity @Table(name = \"user\")\r\npublic class User extends BaseEntity {\r\n\r\n @Column(name = \"login_id\")\r\n private String loginId;\r\n\r\n @Column(name = \"password\")\r\n private String password;\r\n\r\n ...\r\n\r\n @OneToMany(cascade = {CascadeType.ALL})\r\n @JoinColumn(name = \"user_id\", nullable = false)\r\n @IndexColumn(name = \"user_section_position\")\r\n List<UserAboutSection> aboutUs;\r\n\r\n @OneToMany(cascade = {CascadeType.ALL})\r\n @JoinColumn(name = \"user_id\", nullable = false)\r\n @IndexColumn(name = \"user_section_position\")\r\n List<UserServiceSection> services;\r\n\r\n ... Getters, Setters and Adders\r\n}\r\n<\/pre>\n\r\n@Entity @Table(name = \"user_section\")\r\n@Inheritance(strategy= InheritanceType.SINGLE_TABLE)\r\n@DiscriminatorColumn(name=\"user_section_type\", discriminatorType = STRING) \r\npublic class UserSection extends BaseEntity {\r\n \r\n @Column(name = \"title\") \r\n protected String title;\r\n\r\n @Column(name = \"description\") \r\n protected String description;\r\n\r\n ... Getters and Setters\r\n}\r\n<\/pre>\n\r\n@Entity @DiscriminatorValue(\"ABOUT\")\r\npublic class UserAboutSection extends UserSection {\r\n @ManyToOne \r\n @JoinColumn(name=\"user_id\",updatable=false,insertable=false,nullable=false)\r\n protected User user;\r\n\r\n ... Other Properties specific to UserAboutSection\r\n}\r\n<\/pre>\n\r\n@Entity @DiscriminatorValue(\"SERVICE\")\r\npublic class UserServiceSection extends UserSection {\r\n @ManyToOne \r\n @JoinColumn(name=\"user_id\",updatable=false,insertable=false,nullable=false)\r\n protected User user;\r\n\r\n ... Other Properties specific to UserServiceSection\r\n}\r\n<\/pre>\n\r\n@Entity @Table(name = \"user_section\")\r\n@Inheritance(strategy= InheritanceType.SINGLE_TABLE)\r\n@DiscriminatorColumn(name=\"user_section_type\", discriminatorType = STRING) \r\n@ForceDiscriminator\r\npublic class UserSection extends BaseEntity {\r\n \r\n @Column(name = \"title\") \r\n protected String title;\r\n\r\n @Column(name = \"description\") \r\n protected String description;\r\n\r\n ... Getters and Setters\r\n}\r\n<\/pre>\n
Continue reading...<\/a>","protected":false},"author":1,"featured_media":0,"comment_status":"closed","ping_status":"closed","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[7,3,10],"tags":[],"class_list":["post-100","post","type-post","status-publish","format-standard","hentry","category-hibernate","category-java","category-serverside"],"aioseo_notices":[],"_links":{"self":[{"href":"https:\/\/www.gmarwaha.com\/blog\/wp-json\/wp\/v2\/posts\/100","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/www.gmarwaha.com\/blog\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/www.gmarwaha.com\/blog\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/www.gmarwaha.com\/blog\/wp-json\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"https:\/\/www.gmarwaha.com\/blog\/wp-json\/wp\/v2\/comments?post=100"}],"version-history":[{"count":27,"href":"https:\/\/www.gmarwaha.com\/blog\/wp-json\/wp\/v2\/posts\/100\/revisions"}],"predecessor-version":[{"id":807,"href":"https:\/\/www.gmarwaha.com\/blog\/wp-json\/wp\/v2\/posts\/100\/revisions\/807"}],"wp:attachment":[{"href":"https:\/\/www.gmarwaha.com\/blog\/wp-json\/wp\/v2\/media?parent=100"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/www.gmarwaha.com\/blog\/wp-json\/wp\/v2\/categories?post=100"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/www.gmarwaha.com\/blog\/wp-json\/wp\/v2\/tags?post=100"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}