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Sets - Node.js SDK

On this page

  • Overview
  • Realm Object Models
  • Create an Object With a Set
  • Add Items to a Set
  • Check if a Set has Specific Items
  • Check the Size of a Set
  • Remove an Item from a Set
  • Remove all Items from a Set
  • Traverse a Set

New in version 10.5.0.

Overview

A Realm Set is a special object that allows you to store a collection of unique values. Realm Sets are based on JavaScript sets, but can only contain values of a single type and can only be modified within a write transaction. Sets allow you to perform math operations such as finding the union, intersection, or difference between two sets. To learn more about performing these operations, see the MDN docs for Implementing basic set operations.

Realm Object Models

To define a property type as a Realm Set, specify the data type you want in the set, followed by <>. For instance, for a set made of integer values, specify "int<>".

const characterSchema = {
  name: "Character",
  primaryKey: "_id",
  properties: {
    _id: "objectId",
    name: "string",
    levelsCompleted: "int<>",
    inventory: "string<>",
  },
};

Create an Object With a Set

To create an object with a Realm Set property, you must create the object within a write transaction. When defining your Realm object, initialize the Realm Set by passing an empty array or an array with your initial values.

let playerOne, playerTwo;
realm.write(() => {
  playerOne = realm.create("Character", {
    _id: new BSON.ObjectId(),
    name: "PlayerOne",
    inventory: ["elixir", "compass", "glowing shield"],
    levelsCompleted: [4, 9],
  });
  playerTwo = realm.create("Character", {
    _id: new BSON.ObjectId(),
    name: "PlayerTwo",
    inventory: ["estus flask", "gloves", "rune"],
    levelsCompleted: [1, 2, 5, 24],
  });
});

Add Items to a Set

To add an item to a set, pass the new value to the <Realm.Set>.add() method within a write transaction.

realm.write(() => {
  playerOne.inventory.add("hammer");
  playerOne.levelsCompleted.add(32);
});

Check if a Set has Specific Items

To determine if a set contains a particular value, pass the value to the <Realm.Set>.has() method. The set.has() method will return true if the set contains the value specified.

// check if playerTwo has completed level 3 by calling the `has()` method
// on the Realm Set object
const playerTwoHasCompletedLevelThree = playerTwo.levelsCompleted.has(3);
console.log(
  `Is level three completed by playerTwo: ${playerTwoHasCompletedLevelThree}`
);

Check the Size of a Set

To discover how many items are in a set, you can check the set's size property.

// check how many items playerTwo has in his inventory through the `size`
// property of the Realm Set object
const playerTwoInventorySize = playerTwo.inventory.size;
console.log(`playerTwo has ${playerTwoInventorySize} inventory items`);

Remove an Item from a Set

To remove a specific value from a set, pass the value to the <Realm.Set>.delete() method within a write transaction.

realm.write(() => {
  // remove the compass from playerOne's inventory by calling the
  // `delete()` method of the Realm Set object within a write transaction
  playerOne.inventory.delete("compass");
});

Remove all Items from a Set

To clear the set, run the <Realm.Set>.clear() method within a write transaction.

realm.write(() => {
  // clear all data from the inventory slot of playerTwo by calling
  // the `clear()` method of the Realm Set object in a write transaction
  playerTwo.inventory.clear();
});

Traverse a Set

To traverse a set, use the <Realm.Set>.forEach() method or alternative iteration method.

playerOne.inventory.forEach((item) => {
  console.log(item);
});

Example

Traversing a Set in Order

The order of the Realm Set may be different from the order that the items were added.

You can track the set order by updating an array when a new value is added. For example:

function updateSetAndOrderedSetArray(set, orderedArray, value) {
  const oldSize = set.size;
  set.add(value);
  if (set.size > oldSize) {
    orderedArray.push(value);
  }
}
let playerOne;
let levelsCompletedInOrder = [];
const realm = await Realm.open({
  path: "realm-files/data-type-realm",
  schema: [characterSchema],
});
realm.write(() => {
  playerOne = realm.create("Character", {
    _id: new BSON.ObjectId(),
    name: "PlayerOne",
    inventory: ["potion", "wand", "spell book"],
    levelsCompleted: [],
  });
});
realm.write(() => {
  updateSetAndOrderedSetArray(
    playerOne.levelsCompleted,
    levelsCompletedInOrder,
    5
  );
});
realm.write(() => {
  updateSetAndOrderedSetArray(
    playerOne.levelsCompleted,
    levelsCompletedInOrder,
    12
  );
});
realm.write(() => {
  updateSetAndOrderedSetArray(
    playerOne.levelsCompleted,
    levelsCompletedInOrder,
    2
  );
});
realm.write(() => {
  updateSetAndOrderedSetArray(
    playerOne.levelsCompleted,
    levelsCompletedInOrder,
    7
  );
});
console.log("set ordered", Array.from(playerOne.levelsCompleted)); // not necessarily [5, 12, 2, 7]
console.log("insert ordered", levelsCompletedInOrder); // [5, 12, 2, 7]
// close the realm
realm.close();

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