Category: Join table jpa many to many

The canonical reference for building a production grade API with Spring. In this tutorial, we'll see multiple ways to deal with many-to-many relationships using JPA. To present the ideas, we'll use a model of students, courses, and various relationships between them.

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For the sake of simplicity, in the code examples, we'll only show the attributes and JPA configuration that's related to the many-to-many relationships. A relationship is a connection between two types of entities.

join table jpa many to many

In case of a many-to-many relationship, both sides can relate to multiple instances of the other side. Note, that it's possible for entity types to be in a relationship with themselves. For example, when we model family trees: every node is a person, so if we talk about the parent-child relationship, both participants will be a person. However, it doesn't make such a difference whether we talk about a relationship between single or multiple entity types.

Since it's easier to think about relationships between two different entity types, we'll use that to illustrate our cases. For example, when the students mark the courses they like: a student can like many courses, and many students can like the same course:. Since both sides should be able to reference the other, we need to create a separate table to hold the foreign keys :. Such a table is called a join table. Note, that in a join table, the combination of the foreign keys will be its composite primary key.

Modeling a many-to-many relationship with POJOs is easy. We should include a Collection in both classeswhich contains the elements of the others. After that, we need to mark the class with Entityand the primary key with Id to make them proper JPA entities. Also, we should configure the relationship type. Hence we mark the collections with ManyToMany annotations:. The owner side is where we configure the relationship, which for this example we'll pick the Student class.

We can do this with the JoinTable annotation in the Student class. The joinColumn attribute will connect to the owner side of the relationship, and the inverseJoinColumn to the other side:. However, the strategy JPA uses won't always match the naming conventions we use. Hence the possibility to configure table and column names. On the target side, we only have to provide the name of the field, which maps the relationship.

Note, that since a many-to-many relationship doesn't have an owner side in the databasewe could configure the join table in the Course class and reference it from the Student class.October 18, Atta.

Many-to-many relationships are one of the most commonly used association mappings. They require an additional database table to hold the primary keys of both relationship tables. If you are starting a new project, just use Spring Initializr web tool to bootstrap a new Spring Boot application with the above-mentioned dependencies. For the MySQL database, you need to explicitly define the database connection properties in a properties file.

join table jpa many to many

Spring Boot will automatically configure a DataSource bean based on the above properties. Make sure to change the spring. The hibernate property spring. A typical example of such a many-to-many relationship is the relationship between students and courses. A student can enroll in multiple courses and a course can also have multiple students, thus forming a many-to-many relationship.

To model the above relationship in the database, you need to create three tablesone each for both students and courses, and another one for holding relationship keys, as shown below in the Entity-Relationship ER diagram:. We need to create two entity classes, Student and Courseto map the above many-to-many relationship. You don't need to create a separate entity class for the join table.

The Table annotation is used to specify the name of the database table that should be mapped to this entity.

The id attributes are annotated with both Id and GeneratedValue annotations. The former annotation indicates that they are the primary keys of the entities. The latter annotation defines the primary key generation strategy. It uses the mappedBy attribute to indicate the entity that owns the bidirectional relationship. In a bidirectional relationship, the ManyToMany annotation is defined in both entities but only one entity can own the relationship.

We've picked the Student class as an owner of the relationship in the above example. The JoinTable annotation defines the join table between two entities on the owner's side of the relationship. If the JoinTable annotation is left out, the default values of the annotation elements apply. The name of the join table is supposed to be the table names of the associated primary tables concatenated together owning side first using an underscore.

The next step is to define the repository interfaces for storing and accessing the data from the database. We also defined a few derived query methods like findByFeeLessThan to retrieve all courses that have a fee less than the given value and more.

That's it. Now is the time to create the main application class for our Spring Boot console application to test our many-to-many relationship mapping:.

Next, run the application to see the output. If you are using Gradle, execute the following command to start the application:.

That's all folks. Don't forget to subscribe for updates if you want to be the first to know when new tutorials are available. Follow me on Twitter and LinkedIn. You can also subscribe to RSS Feed.

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I started this blog as a place to share everything I have learned in the last decade. I write about modern JavaScript, Node. The newsletter is sent every week and includes early access to clear, concise, and easy-to-follow tutorials, and other stuff I think you'd enjoy!When normalizing a database or adding tables to an existing database, we need to be able to relate tables to each other.

Handling a one-to-one relationship or a one-or-many relationship can be done by adding the primary key of one table into the other table as a foreign key. We capture details about students who attend classes, among other things.

The rules are:. This would mean we have one column for storing multiple values, which is very hard for maintenance and querying. A joining table is a table that sits between the two other tables of a many-to-many relationship.

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Its purpose is to store a record for each of the combinations of these other two tables. Now, the name of the table is important.

join table jpa many to many

I think this is valid and will get the job done, but having a more descriptive name is helpful, as it tells you more about what the table is. It stores two columns: one for each of the primary keys from the other table. This stores separate records for each combination of student and class. Our student and class tables remain the same:. Having our data structure in this way makes it easier to add more relationships between tables and to update our students and classes without impacting the relationships between them.

Thanks for visiting DZone today. Edit Profile. Sign Out View Profile. Over a million developers have joined DZone. Handling a one-to-one relationship or one-or-many relationship can be done pretty simply.

But many-to-many relationships are a bit different. Like Many-to-many relationship refers to the relationship between two tables A and B in which one row of A may be linked with many rows of B, and vice versa, one row of B may be linked to many rows of A.

In this example, the book and publisher tables have a many-to-many relationship. One book may be published by many publishers and one publisher may publish many books. Column maps the entity's field with the table's column.

If Column is omitted, the field name of the entity will be used as a column name by default. ManyToMany defines a many-to-many relationship between 2 entities. JoinTable defines the join table of 2 associated entities. If the JoinTable annotation is missing, the default values of the annotation elements apply. The name of the join table is assumed to be the table names of the associated primary tables concatenated together owning side first using an underscore. All we need for this example is extends JpaRepository.

Thanks to CascadeType. ALLthe associated entity Publisher will be saved at the same time with Book without the need of calling its save function explicitly.

JPA Tutorial - JPA ManyToMany Join Table Example

You can find the source code on Github. Twitter Facebook. Giau Ngo's Picture. He loves coding, blogging, and traveling.A ManyToMany relationship in Java is where the source object has an attribute that stores a collection of target objects and if those target objects had the inverse relationship back to the source object it would also be a ManyToMany relationship.

All relationships in Java and JPA are unidirectional, in that if a source object references a target object there is no guarantee that the target object also has a relationship to the source object. This is different than a relational database, in which relationships are defined through foreign keys and querying such that the inverse query always exists. The main difference between a OneToMany and a ManyToMany relationship in JPA is that a ManyToMany always makes use of a intermediate relational join table to store the relationship, where as a OneToMany can either use a join table, or a foreign key in target object's table referencing the source object table's primary key.

The JoinTable defines a foreign key to the source object's primary key joinColumnsand a foreign key to the target object's primary key inverseJoinColumns. Normally the primary key of the JoinTable is the combination of both foreign keys. Although a ManyToMany relationship is always bi-directional on the database, the object model can choose if it will be mapped in both directions, and in which direction it will be mapped in.

If you choose to map the relationship in both directions, then one direction must be defined as the owner and the other must use the mappedBy attribute to define its mapping. This also avoids having to duplicate the JoinTable information in both places. If the mappedBy is not used, then the persistence provider will assume there are two independent relationships, and you will end up getting duplicate rows inserted into the join table.

If you have a conceptual bi-directional relationship, but have two different join tables in the database, then you must not use the mappedByas you need to maintain two independent tables. As with all bi-directional relationships it is your object model's and application's responsibility to maintain the relationship in both direction.

There is no magic in JPA, if you add or remove to one side of the collection, you must also add or remove from the other side, see object corruption. A frequent problem is that two classes have a ManyToMany relationship, but the relational join table has additional data.

In this case the best solution is to create a class that models the join table. So a ProjectAssociation class would be created.

Java Persistence/ManyToMany

Some JPA providers also provide additional support for mapping to join tables with additional data. Unfortunately mapping this type of model becomes more complicated in JPA because it requires a composite primary key. The association object's Id is composed of the Employee and Project ids.

The JPA 1. To make your life simpler, I would recommend adding a generated Id attribute to the association class. This will give the object a simpler Id and not require duplicating the Employee and Project ids.

This same pattern can be used no matter what the additional data in the join table is. Another usage is if you have a Map relationship between two objects, with a third unrelated object or data representing the Map key.

The JPA spec requires that the Map key be an attribute of the Map value, so the association object pattern can be used to model the relationship. If the additional data in the join table is only required on the database and not used in Java, such as auditing information, it may also be possible to use database triggers to automatically set the data. From Wikibooks, open books for an open world.

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Hibernate Tip: Many-to-Many Association with additional Attributes

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join table jpa many to many

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JPA and Hibernate Many To Many Mapping without Joined Entity in Spring Boot

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