RelMongo (Relational Mongo) allows to use relations between mongodb collections in a JPA way
RelMongo is built in top of the Spring Data MongoDB framework.

RelMongo provides :

• @EnableRelationalMongo to enable RelMongo engine
• @OneToMany annotation to address 1..N relations
• @OneToOne annotation to address 1..1 relations
• Two fetching methods ( LAZY and EAGER)
• Cascading operations
• Bidirectional mapping using the mappedBy attribute

To get more details please see the release notes.

Take a tour in the RelMongo wiki

• Maven

   <dependency>
      <groupId>io.github.kaiso.relmongo</groupId>
      <artifactId>relmongo</artifactId>
      <version>x.y.z</version>
   </dependency>

See compatibility on wiki

RelMongo is very simple to use.
given two concepts with "one to *" relation

  __________________                         __________________
 |    Person        |                       |    Car           |
 |__________________| 1                  *  |__________________|
 |  name (string)   |---------------------->|   ....           |
 |  cars (list )    |                       |                  |
 |                  |                       |                  |
 |__________________|                       |__________________|

on your Person mongo entity simply add the following annotations from RelMongo :

    @OneToMany(fetch=FetchType.EAGER, cascade = CascadeType.PERSIST)
    @JoinProperty(name="cars")
    private List<Car> cars;

and on your Spring App config class simply add @EnableRelationalMongo annotation:

    ... Other Annotations
    @EnableRelationalMongo
    public Class AppConfig
    

test your code :

        Car car = new Car();
        car.setColor(Color.BLUE);
        String manufacturer = "BMW";
        car.setManufacturer(manufacturer);
        Person person = new Person();
        person.setName("person");
        person.setEmail("[email protected]");
        person.setCars(Arrays.asList(new Car[] {car}));
        repository.save(person);
        Optional<Person> retreivedPerson = repository.findById(person.getId().toString());
        assertFalse(retreivedPerson.get().getCars().isEmpty());
        assertTrue(retreivedPerson.get().getCars().get(0).getColor().equals(Color.BLUE));
        

database layout when executing this test :

• cars collection :

{
    _id : ObjectId(5afaff0e2557db3a140d0f85),
    manufacturer : BMW,
    color : BLUE
}

• persons collection

  {
    _id : ObjectId(5afaff0e2557db3a140d0f86),
    name : person,
    email : person@mail.com,
    cars : [ 
        {
            _id : ObjectId(5afaff0e2557db3a140d0f85)
            _relmongo_target: cars
        }
    ]
}

• Based on Spring framework and derivatives
• Simple to use
• Ready to use on existing database with few changes and in many cases with no changes
• Bidirectional mapping

• RelMongo may be an alternative for DBREF which allow to use $lookup querries in mongodb while it is not possible with DBREF.
• MongoDB is a document oriented database and is not suitable for relations, if you are using relations massively you may have a design or technical choice problems.
• RelMongo does not garantee integrity in the database since it is not implemented by MongoDB

© Copyright 2018 Kais OMRI.

Licensed under the Apache License, Version 2.0 (the "License"); you may not use this file except in compliance with the License. You may obtain a copy of the License at

  http://www.apache.org/licenses/LICENSE-2.0

Unless required by applicable law or agreed to in writing, software distributed under the License is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the License for the specific language governing permissions and limitations under the License.