@Repository
public interface GooglePlayGamesRepository extends MongoRepository <GooglePlayGame, String> {

    GooglePlayGame findByTitleIgnoreCase(String gameTitle);

    @Aggregation("{$sample: {size: ?0} }")
    List<GooglePlayGame> findRandomGames(Long amount);

    List<GooglePlayGame> findByTitleContainsIgnoreCase(String gameTitle);

}

import org.springframework.data.annotation.Id;

@Document(collection = "googleplaygames")
@Getter
@Component
public final class GooglePlayGame implements Serializable {

    @Id
    String title; //это уникальный ключ!
    String genre;
    String price;
...
}

org.springframework.dao.DuplicateKeyException: E11000 duplicate key error collection: 5ef31a21233f9c398f473c6e_botapi.googleplaygames index: title dup key: { title: null };

import org.bson.types.ObjectId;

import org.springframework.data.annotation.Id;
import org.springframework.data.mongodb.core.index.Indexed;

...

@Id
ObjectId id;
@Indexed(unique = true)
String title;

import org.bson.types.ObjectId;
import org.springframework.data.annotation.Id;
import org.springframework.data.mongodb.core.index.Indexed;
import org.springframework.data.mongodb.core.mapping.Document;

@Document(collection = "googleplaygames")
@Builder(builderClassName = "GooglePlayGameBuilder", toBuilder = true)
@AllArgsConstructor
@NoArgsConstructor
@Getter
@FieldDefaults(level = AccessLevel.PRIVATE)
public final class GooglePlayGame implements Serializable {
    @Id
    ObjectId id;
    @Indexed(unique = true)
    String title;
    String currentVersion
...

@Repository
public interface GooglePlayGamesRepository extends MongoRepository <GooglePlayGame, String> {

    @Aggregation("{$sample: {size: ?0} }")
    List<GooglePlayGame> findRandomGames(Long amount);

    List<GooglePlayGame> findByTitleContainsIgnoreCase(String gameTitle);

}

try {
    googlePlayGamesRepository.save(googlePlayGame);
}
catch (org.springframework.dao.DuplicateKeyException e) {
       GooglePlayGame game = googlePlayGamesRepository.findByTitleIgnoreCase(googlePlayGame.getTitle());
       googlePlayGame.setId(game.getId());
       googlePlayGamesRepository.save(googlePlayGame);
}

public <S extends T> S save(S entity) {

     Assert.notNull(entity, "Entity must not be null!");

     if (entityInformation.isNew(entity)) {
          return mongoOperations.insert(entity, entityInformation.getCollectionName());
     }

     return mongoOperations.save(entity, entityInformation.getCollectionName());
}

Object id = saveDocument(collectionName, dbDoc, objectToSave.getClass());

if (!mapped.hasId()) {
	if (writeConcernToUse == null) {
		collection.insertOne(dbDoc);
	} else {
		collection.withWriteConcern(writeConcernToUse).insertOne(dbDoc);
	}
} else if (writeConcernToUse == null) {
	collection.replaceOne(mapped.getIdFilter(), dbDoc, new ReplaceOptions().upsert(true));
} else {
	collection.withWriteConcern(writeConcernToUse).replaceOne(mapped.getIdFilter(), dbDoc,
			new ReplaceOptions().upsert(true));
}
return mapped.getId();