Managing relationships

When JPA is used, the entity sub-generator can create relationships between entities.

Presentation

Relationships only work when JPA is used. If you choose to use Cassandra or MongoDB, they won’t be available.

A relationship works between two entities, and JHipster will generate the code for:

Managing this relationship with JPA in the generated entities
Creating the correct Liquibase changelog, in order for the relationship to exist in the database
Generating the AngularJS front-end so you can manage this relationship graphically in the user interface

As we use JPA, the usual one-to-many, many-to-one, many-to-many and one-to-one relationships are available:

A bi-directionnal one-to-many relationship
A uni-directional many-to-one relationship
A uni-directional one-to-many relationship
Two one-to-many relationships on the same two entities
A many-to-many relationship
A one-to-one relationship

Tip: the User entity

Please note that the User entity, which is handled by JHipster, is specific. You can do many-to-one relationships to this entity (a Car can have a many-to-one relationship to a User). This will generate a specific query in your new entity repository, so you can filter your entity on the current security user, which is a common requirement.

A bi-directionnal one-to-many relationship

Let’s start with two entities, a Owner and a Car. A owner can have many cars, and a car can have only one owner.

So this is a simple one-to-many relationship (one owner has many cars) on one side, and a many-to-one relationship (many cars have one owner) on the other side:

Owner (1) <-----> (*) Car

As the Car table will need to have a foreign key on the Owner table, we need to create the Owner first. Here are the relevant JHipster questions for the Owner:

yo jhipster:entity Owner
...
Generating relationships with other entities
? Do you want to add a relationship to another entity? Yes
? What is the name of the other entity? Car
? What is the name of the relationship? car
? What is the type of the relationship? one-to-many
? What is the name of this relationship in the other entity? owner

Please note that we selected the default options concerning the names of the relationships.

Now we can generate the Car:

yo jhipster:entity Car
...
Generating relationships with other entities
? Do you want to add a relationship to another entity? Yes
? What is the name of the other entity? Owner
? What is the name of the relationship? owner
? What is the type of the relationship? many-to-one
? When you display this relationship with AngularJS, which field from 'Owner' do you want to use? id

That’s it, you now have a one-to-many relationship between those two entities!

A uni-directional many-to-one relationship

In the previous example we had a bi-directionnal relationship: from a Car instance you could find its owner, and from a Owner instance you could get all of its cars.

A many-to-one uni-directional relationship means that the cars know their owner, but not the opposite.

Owner (1) <----- (*) Car

You would do that relationship for two reasons:

From a business point of view, you only use your entities in this way. So you don’t want to have an API that allows developers to do something which doesn’t make sense.
You have a small performance gain when using the Owner entity (as it won’t have to manage the collection of cars).

In that case, you would still create the Owner first, this time with no relationship:

yo jhipster:entity Owner
...
Generating relationships with other entities
? Do you want to add a relationship to another entity? No

And then the Car entity, as in the previous example:

yo jhipster:entity Car
...
Generating relationships with other entities
? Do you want to add a relationship to another entity? Yes
? What is the name of the other entity? Owner
? What is the name of the relationship? owner
? What is the type of the relationship? many-to-one
? When you display this relationship with AngularJS, which field from 'Owner' do you want to use? id

This will work as in the previous example, but you won’t be able to add or remove cars from the Owner entity.

A uni-directional one-to-many relationship

A many-to-one uni-directional relationship means that the Owner instance can get its collection of cars, but not the opposite. It is the opposite from the previous example.

Owner (1) -----> (*) Car

This type of relationship is not provided by default in JHipster at the moment, see #1569 for more information.

You have two solutions for this:

Do a bi-directionnal mapping, and use it without modification: this is our recommended approach, as it is much simpler
Do a bi-directional mapping, and then modify it to transform it into a uni-directional mapping:
- Remove the “mappedBy” attribute on your @OneToMany annotation
- Generate the required join table: you can do a mvn liquibase:diff to generate that table, see the documentation about using Liquibase diff

Two one-to-many relationships on the same two entities

For this example, a Person can be the owner of many cars, and he can also be the driver of many cars:

Person (1) <---owns-----> (*) Car
Person (1) <---drives---> (*) Car

For this we need to use the relationship names, which we have left with their default values in the previous examples.

Generate the Person entity, which has tow one-to-many relationships to the Car entity:

yo jhipster:entity Person
...
Generating relationships with other entities
? Do you want to add a relationship to another entity? Yes
? What is the name of the other entity? Car
? What is the name of the relationship? ownedCar
? What is the type of the relationship? one-to-many
? What is the name of this relationship in the other entity? owner
...
Generating relationships with other entities
? Do you want to add a relationship to another entity? Yes
? What is the name of the other entity? Car
? What is the name of the relationship? drivedCar
? What is the type of the relationship? one-to-many
? What is the name of this relationship in the other entity? driver

Generate the Car entity, which use the same relationship name has was configured in the Person entity:

yo jhipster:entity Car
...
Generating relationships with other entities
? Do you want to add a relationship to another entity? Yes
? What is the name of the other entity? Person
? What is the name of the relationship? owner
? What is the type of the relationship? many-to-one
? When you display this relationship with AngularJS, which field from 'Person' do you want to use? id
...
Generating relationships with other entities
? Do you want to add a relationship to another entity? Yes
? What is the name of the other entity? Person
? What is the name of the relationship? driver
? What is the type of the relationship? many-to-one
? When you display this relationship with AngularJS, which field from 'Person' do you want to use? id

A Car can now have a driver and a owner, which are both Person entities.

A many-to-many relationship

A Driver can drive many cars, but a Car can also have many drivers.

Driver (*) <-----> (*) Car

At the database level, this means we will have a join table between the Driver and the Car tables.

For JPA, one of those two entities will need to manage the relationship: in our case, that would be the Car entity, which will be responsible to add or remove drivers.

First generate the non-owning side of the relationship, the Driver, with a many-to-many relationship:

yo jhipster:entity Driver
...
Generating relationships with other entities
? Do you want to add a relationship to another entity? Yes
? What is the name of the other entity? Car
? What is the name of the relationship? car
? What is the type of the relationship? many-to-many
? Is this entity the owner of the relationship? No
? What is the name of this relationship in the other entity? driver

Then generate the Car, with the owning side of the many-to-many relationship:

yo jhipster:entity Car
...
Generating relationships with other entities
? Do you want to add a relationship to another entity? Yes
? What is the name of the other entity? Driver
? What is the name of the relationship? driver
? What is the type of the relationship? many-to-many
? Is this entity the owner of the relationship? Yes
? When you display this relationship with AngularJS, which field from 'Driver' do you want to use? id

In the user interface, you will be able to add/remove drivers from the Car management page.

A one-to-one relationship

Following our example, a one-to-one relationship would mean that a Driver can drive only one Car, and a Car can only have one Driver.

Driver (1) <-----> (1) Car

This is the only relationship for which JHipster doesn’t provide an AngularJS front-end, as we don’t think it really makes sense: if there is a one-to-one relationship, you probably want to display both entities on the same page anyway.

First create the non-owning side of the relationship, in our case the Driver:

yo jhipster:entity Driver
...
Generating relationships with other entities
? Do you want to add a relationship to another entity? Yes
? What is the name of the other entity? Car
? What is the name of the relationship? car
? What is the type of the relationship? one-to-one
? Is this entity the owner of the relationship? No
? What is the name of this relationship in the other entity? driver

Then generate the Car, which owns the relationship:

yo jhipster:entity Car
...
Generating relationships with other entities
? Do you want to add a relationship to another entity? Yes
? What is the name of the other entity? Driver
? What is the name of the relationship? driver
? What is the type of the relationship? one-to-one
? Is this entity the owner of the relationship? Yes