Auto Incrementing Field On Spring Data MongoDB

MongoDB came with all sophisticated ObjectId generation feature, but often you just jumped the ship from relational database, and you still want an easy to read / communicate numeric identifier field which automatically increments every time new record is inserted.

One neat suggestion from MongoDB tutorial is to use a counter collection with a ‘counter name’ as its id, and a ‘seq’ field to store the last used number:

db.counters.insert({
  _id: "users",
  seq: 0
})

Then use findAndModify function to increment and obtain the next value.

db.counters.findAndModify({
	query: { _id: name },
	update: { $inc: { seq: 1 } },
	new: true
})

When developing using Spring Data MongoDB, this neat trick can be written as a simple service. Here I used the collection name as the counter name so it’s easy to guess / remember.

import static org.springframework.data.mongodb.core.query.Query.*;
import static org.springframework.data.mongodb.core.query.Criteria.*;
import static org.springframework.data.mongodb.core.FindAndModifyOptions.*;

@Service
public class CounterService {
  @Autowired private MongoOperations mongo;
  
  public int getNextSequence(String collectionName) {

    Counter counter = mongo.findAndModify(
      query(where("_id").is(collectionName)), 
      new Update().inc("seq", 1),
      options().returnNew(true),
      Counter.class);
      
    return counter.getSeq();
  }
}

Counter is just a simple class representing the collection. Please beware the usage of int data type, this will limit to 2^31 entries maximum.

@Document(collection = "counters")
public class Counter {
  @Id private String id;
  private int seq;
  /* getters & setters .. */
}

Then when inserting a new entry (with help of Spring MongoDB Repository support), just set the id field like this before you save it

User newUser = new User();
newUser.setId(counterService.getNextSequence("users"));
newUser.setUsername(username);
newUser.setPassword(password);

userRepository.save(newUser);

You’ll get an incrementing simple numeric id each time you insert new object

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