Spring Data MongoRepository Saving Objects with Differing Numbers of Fields

I will explain how varying fields are handled with an example. The following Game.java POJO class represents the object mapping to the game collection document.

public class Game {

    String name;
    List<Actions> actions;

    public Game(String name, List<Actions> actions) {
        this.name = name;
        this.actions = actions;
    }

    public String getName() {
        return name;
    }

    public List<Actions> getActions() {
        return actions;
    }

    // other get/set methods, override, etc..


    public static class Actions {

        Integer id;
        String type;

        public Actions() {
        }

        public Actions(Integer id) {
            this.id = id;
        }

        public Actions(Integer id, String type) {
            this.id = id;
            this.type = type;
        }

        public Integer getId() {
            return id;
        }

        public String getType() {
            return type;
        }

        // other methods
    }
}

For the Actions class you need to provide constructors with the possible combinations. Use the appropriate constructor with id, type, etc. For example, create a Game object and save to the database:

Game.Actions actions= new Game.Actions(new Integer(1000));
Game g1 = new Game("G-1", Arrays.asList(actions));
repo.save(g1);

This is stored in the database collection game as follows (queried from mongo shell):

{
        "_id" : ObjectId("5eeafe2043f875621d1e447b"),
        "name" : "G-1",
        "actions" : [
                {
                        "_id" : 1000
                }
        ],
        "_class" : "com.example.demo.Game"
}

Note the actions array. As we had stored only the id field in the Game.Actions object, only that field is stored. Even though you specify all the fields in the class, only those provided with values are persisted.

These are two more documents with Game.Actions created with type only and id + type using the appropriate constructors:

{
        "_id" : ObjectId("5eeb02fe5b86147de7dd7484"),
        "name" : "G-9",
        "actions" : [
                {
                        "type" : "type-x"
                }
        ],
        "_class" : "com.example.demo.Game"
}
{
        "_id" : ObjectId("5eeb034d70a4b6360d5398cc"),
        "name" : "G-11",
        "actions" : [
                {
                        "_id" : 2,
                        "type" : "type-y"
                }
        ],
        "_class" : "com.example.demo.Game"
}