Understanding Entity Relationship Mapping, Part 2: Annotating Entity Relationships

In Understanding Entity Relationship Mapping, Part 1: The basic concepts I glazed over some of the basic concepts that would be encountered when dealing with establishing relationships between Entities. This post would take a look at how to use Annotations to specify these relationships.

The relationships that would be annotated are the four types that you have in relational data model:

1. One-To-One annotated with @OneToOne
2. Many-To-One annotated with @ManyToOne
3. One-To-Many annotated with @OneToMany
4. Many-To-Many annotated with @ManyToMany

If concepts like Cardinality, Direction of Relationship, Cascades, Owning side and Inverse side of Entity relationship is new to you, then it is would be good to first read Understanding Entity Relationship Mapping, Part 1: The basic concepts

The relationship would be classified into two: Single Valued Mappings and Collection Valued Mappings. It is worth stating that the examples were written with Hibernate's implementation of JPA in mind, but would work with any other standard implementation of JPA 2, as no Hibernate specific annotation is included.

So let us begin.

Understanding Entity Relationship Mapping, Part 1: The basic concepts

It is possible to create a working functionality of a system using tools/technologies that is not fully understood, as long as some minimal knowledge is present: what things are, where to find them, and how to wire them together in other to get them to work. And thank heavens, with the help of search engines, you can easily scour the web for solutions to anomalies that pops up, due to the lack of complete grasp of the tool being used. But without a more in-depth knowledge, or at least a grasp of the basic fundamental concepts, it would become difficult to accomplish more complex or customized tasks, troubleshoot issues and general extend beyond the basic functionality.

This was how I was treating mapping entity relationships in the ORM domain, via Hibernate's implementation of JPA, for a time now. I had the basic knowledge of relational database modelling, how relationships are constructed using primary keys and foreign keys in SQL, how data can be fetched from tables that are related using joins: basic knowledge that has its roots solely in the relational way of looking at databases. So I was able to annotate up entities and set up relationships. Get stuff persisted and maybe updated. But I kept running into minor errors now and then, configuration issues, minor oversights etc.

One of the areas these annoyances kept popping up was in how to describe relationships between entities. After I have had enough of head banging against the wall, I decided to just take a step back to understand, at least, a little bit more, the tool I am using and what I was dealing with here.

This blog post is the outcome of that stepping back. It has two parts to it. This post, which describes some basic terminologies that you would come across while working with most JPA implementations and the second: Understanding Entity Relationship Mapping, Part 2: Annotating Entity Relationships, which shows how entity relationships are annotated.

So here we go: