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    4.0.1 • Public • Published

    Mongoose Id Assigner (IA)

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    A Mongoose Plugin. Easily Manage fields that need an id(unique value) on your mongoose model. This plugin does the work of generating, incrementing, and assigning those values(unique) to those fields.

    This plugin assigns values base on your configurations, be it MongoDB's ObjectId, UUID, Number Increments, String Generators or you provide your custom nextIdFunction.

    It creates a collection with name id_assigner that store the nextId for a configured field. This only happens if you have fields configured with type String or Number. As ObjectId and UUID can be generated locally unique, they do not use this collection for assigning values.

    This is the perfect tool if you wish to work with discriminators and have _id field(and/or other fields) values different amongst the discriminators instances. See examples below for demonstration.

    Table of Content


    yarn add mongoose-id-assigner
    npm install mongoose-id-assigner

    If you wish to use UUIDs FieldTypes, then you need to add this package, uuid.

    yarn add uuid

    Basic Usage

    You could create the plugin from the MongooseIdAssigner Constructor, which returns a instance which you could use to query nextIds.

    from constructor(schema: Schema, options: AssignerPluginOptions): MongooseIdAssigner

    const ExampleIA = new MongooseIdAssigner(exampleSchema, options);
    exampleModel = mongoose.model('Example', exampleSchema);
    // always call the initialise, to ensure assigner initialised before calling getNextId
    // assigners are always auto initialised on every first save
    await ExampleIA.initialise(exampleModel);
    await ExampleIA.getNextId('fieldName'); // nextId

    Alternatively you have the plugin setup by calling the static plugin method.

    static plugin(schema: Schema, options: AssignerPluginOptions): MongooseIdAssigner

    exampleSchema.plugin(MongooseIdAssigner.plugin, options);


    Parameter Type Description
    modelName String(Required) Name of the Model your are working with. If discriminators, then provide baseModel Name.
    fields AssignerFieldsConfigMap (Optional) The configuration Map of the fields you want the assigner to assign ids to. If undefined, then plugin assigns ids to _id field, (ObjectId).
    discriminators DiscriminatorConfigMap (Optional) An Object with key being a discriminatorName and value a Configuration Map for fields on that discriminator that need unique values. Any discriminator without a fieldConfig will use that of the baseModelk

    Points to Note

    • At every Network Assigner init(i.e Assigner with Number, String FieldConfigTypes), the Assigner(for a Model) refreshes and syncs with the db stored options. Take example micro-service cluster, the last app to init always gives most recent field configs, if the db have a field that is not in the most recent field config, it is auto dropped. Therefore always make sure all your micro-service clusters start up with same fieldsConfigs as the last to start rewrites the db and only keeps nextIds.

    • Always Setup plugin before creating Mongoose Model for that schema.

    • Always set FieldConfig Type to reflect schema's path Type, that is if schema Type is Number, then use Number FieldConfigType. See ExampleSchema below with its associated IdAssigners.


    Lets create our Mongoose Schema.

    // schema.js  We would be reusing this schema for brevity (clone)
    import * as mongoose from 'mongoose';
    const ExampleSchema = new mongoose.Schema({
      _id: String,
      photoId: Number,
      emailId: String,
      personId: String,
      uuidField: Buffer,
      uuidFieldString: String,
      uuidFieldBuffer: Buffer,
      objectIdField: mongoose.Schema.Types.ObjectId,

    Configuration methods

    Method 1: Quick Config, Intended for usage with schema.plugin(...). It initialises only when the first document is about to be saved.

    Options Type: AssignerPluginOptions.

    const options: AssignerPluginOptions  = {
      modelName: 'ExampleModel',
      fields: {  // if no _id field config, assigner auto adds _id field with type = "ObjectId"
        uuidFieldString: 'UUID',
    ExampleSchema.plugin(MongooseIdAssigner.plugin, options);
    // Saving the first doc triggers the IdAssigner initialisation for this model.
    const ExampleModel = mongoose.model('ExampleModel', ExampleSchema);
    const doc = await ExampleModel.create({name: 'Mernxl'});
    console.log(doc._id)   --->   '5b57a1d929239e59b4e3d7f3'     // schema field type is String
    console.log(doc.uuidFieldString)   --->   '7729e2e0-8f8b-11e8-882d-2dade78bb893'

    Method 2: Using the MongooseIdAssigner constructor, it takes in the schema as first parameter, then options as second, returning IdAssigner Instance. You can now initialise and use the IdAssigner instance to request nextIds for particular fields.

    Options Type: AssignerOptions.

    const options: AssignerPluginOptions = {
      modelName: 'ExampleModel',
      fields: {
        _id: {
          type: FieldConfigTypes.String,
          separator: 'T',
          nextId: '34T5565',
        uuidFieldBuffer: {
          type: FieldConfigTypes.UUID,
          version: 4,
          asBinary: true,
          versionOptions: {
            rng: (Function) Random # generator function that returns an Array[16] of byte values (0-255) // UUID doc
    const ExampleIA = new MongooseIdAssigner(ExampleSchema, options);
    const ExampleModel = mongoose.model('ExampleModel', ExampleSchema);
    const doc1 = await ExampleModel.create({name: 'Mongoose'});
    console.log(doc1._id);   --->   '34T5565'
    console.log(doc1.uuidFieldBuffer);   --->  Binary { _bsontype: 'Binary', sub_type: 4, position: 36, buffer: B... }
    // it getOnly, does not write off in db
    // TODO: Implement hold nextId with timeout
    console.log((await ExampleIA.getNextId('_id')))   --->   '34T5566'
    const doc2 = await ExampleModel.create({name: 'IdAssigner'});
    console.log(doc2._id);   --->   '34T5566'
    console.log(doc2.uuidFieldBuffer);   --->   Binary { _bsontype: 'Binary', sub_type: 4, position: 36, buffer: B... }

    Working with Discriminators

    You may have a discriminator Instance and want to have different id Types, or fields on one discriminator need to be autogenerated uniquely, here is an example;

    const CharacterSchema = new mongoose.Schema({
      _id: String,
      kind: String,
      someId: Number,
      friends: [String],
    }, {
        discriminatorKey: 'kind',
    const PersonSchema = new mongoose.Schema({
      license: String,
    const DroidSchema = new mongoose.Schema({
      make: String,
    const options: AssignerPluginOptions = {
      modelName: 'example10',
      fields: { 
        someId: 4444,
      discriminators: {
        Person: {
          _id: FieldConfigTypes.ObjectId,
          license: '786-TSJ-000', // default separator `-`
        Droid: {
          _id: FieldConfigTypes.UUID,
          make: {
            type: FieldConfigTypes.String,
            nextId: '18Y4433',
            separator: 'Y',
    const CharacterIA = new MongooseIdAssigner(characterSchema, options);
    const characterModel = mongoose.model('example10', characterSchema);
    const personModel = characterModel.discriminator('Person', personSchema);
    const droidModel = characterModel.discriminator('Droid', droidSchema);
    const character = await characterModel.create({ friends: 'placeholder' });
    const person = await personModel.create({ friends: 'placeholder' });
    const droid = await droidModel.create({ friends: 'placeholder' });
    const person1 = await personModel.create({ friends: 'placeholder' });
    const droid1 = await droidModel.create({ friends: 'placeholder' });
    console.log(character._id)   --->   '5b59d98617e2edc57ede52b8'
    console.log(character.someId)   --->   4444
    console.log((await CharacterIA.getNextId('someId')))  --->   4445
    console.log(person._id)   --->   '5b59d98617e2edc57ede52ba'
    console.log(person.someId)   --->   4445
    console.log(person.license)   --->   '786-TSJ-000'
    console.log(droid._id)   --->   '48db52c0-90df-11e8-b43b-ffe3d317727b'
    console.log(droid.someId)   --->   4446
    console.log(droid.make)   --->   '18Y4433'
    console.log((await CharacterIA.getNextId('make', 'Droid')))  --->   '18Y4434'
    console.log(person._id)   --->   '5b59d98617e2edc57ede52bb'
    console.log(person1.someId)   --->   4447
    console.log(person1.license)   --->   '786-TSJ-001'
    console.log((await CharacterIA.getNextId('license', 'Person')))  --->   '786-TSJ-002'       
    console.log(droid1._id)   --->   'eb185fb0-90df-11e8-800d-dff50a2d22a3'          
    console.log(droid1.someId)   --->   4448
    console.log(droid1.make)   --->   '18Y4434'

    NextId WIP

    It may arise that you need to query and use the nextId to at the front-end. In this case, you just need to get an instance of the Assigner, then use the getNextId method. It is async method as it queries for Number and String cases.

    Strain test

    • Performs the task below on a locally hosted db instance.
    • On CI/CD environment, tests ran using mongodb-memory-server, on v3.4, v3.6, v4.0 and v4.2 on node v12, v10, v8
    // using ts :)
    import * as mongoose from 'mongoose';
    import { AssignerOptions, FieldConfigTypes, localStateStore, MongooseIdAssigner } from './src';
    describe('MongooseIdAssigner', () => {
      it('should be robust enough to avoid duplicates', async () => {
        const options: AssignerPluginOptions = {
          modelName: 'example8',
          fields: {
            _id: '33333',
            photoId: 44444,
            emailId: '55555',
            personId: {
              type: FieldConfigTypes.String,
              nextId: 'SPEC-7382-4344-3232',
              separator: '-',
            uuidFieldString: {
              type: FieldConfigTypes.UUID,
            uuidFieldBuffer: {
              type: FieldConfigTypes.UUID,
              version: 1,
              asBinary: true,
            objectIdField: FieldConfigTypes.ObjectId,
        try {
          const ExampleIA = new MongooseIdAssigner(exampleSchema, options);
          exampleModel = mongoose.model('example8', exampleSchema);
          expect(ExampleIA.readyState).toBe(0); // initialising
          // initialise to ensure that
          // model is set and db is connected
          // before performing heavy tasks
          // or you can set max event listeners to 100 to suppress warnings on waits
          await ExampleIA.initialise(exampleModel);
          const promises = [];
          for (let i = 0; i < 100; i++) {
            promises.push(exampleModel.create({ personId: 'mernxl' }));
          const docs: any[] = await Promise.all(promises);
          for (let i = 0; i < 100; i++) {
            const _id = docs[i]._id;
            const photoId = docs[i].photoId;
            const emailId = docs[i].emailId;
            const personId = docs[i].personId;
            const uuidFieldString = docs[i].uuidFieldString;
            const uuidFieldBuffer = docs[i].uuidFieldBuffer;
            const objectIdField = docs[i].objectIdField;
            expect(typeof photoId).toBe('number');
            expect(typeof emailId).toBe('string');
            expect(typeof uuidFieldString).toBe('string');
            for (const cDoc of docs) {
              if (_id === cDoc._id) {
        } catch (e) {


     * If Options does not contain fields(AssignerFieldsConfigMap),
     * Then setup assigner for _id field, does not use network
    interface AssignerOptions {
      fields?: AssignerFieldsConfigMap;
      discriminators?: DiscriminatorConfigMap;
    interface AssignerPluginOptions {
      modelName: string;
      fields?: AssignerFieldsConfigMap;
      discriminators?: DiscriminatorConfigMap;
     * fieldConfig = string, then nextId = string, default incrementer,
     * fieldConfig = number, then nextId = number, incrementBy = 1
     * fieldConfig = boolean(true), then fieldType = ObjectId
     * fieldConfig = GUID | UUID, then use UUID v1
    interface AssignerFieldsConfigMap {
      [fieldName: string]:
        | FieldConfig
        | string
        | number
        | boolean
        | FieldConfigTypes.GUID
        | FieldConfigTypes.UUID;
     * A map of discriminatorName(modelName) and its own AssignerFieldsConfigMap
    interface DiscriminatorConfigMap {
      [discriminatorName: string]: AssignerFieldsConfigMap;
     * @property {Boolean} noSpace - noSpace, insure we consume all possible values, i.e. we must have 1, 2, 3, 4
     * order doesn't matter but all those keys must be present, no 1, 3, 4, 6.
     * If noSpace is true, then on holdTimeout, that nextId will be use on any newly saving doc, else nextId discarded
     * @property {Number} maxHold[50] - As there may be performance issues when holding ids, maxHold will be set,
     * @property {String} holdTimeout - default timeout string, must be parse-able to number by `ms` plugin
     * @property {Number} holdTimeout - default timeout millis, gotten id stays onHold for this length of time
     * @property {Boolean} holdTimeout - if true, will always getNextId with default timeout of `1 week` else use getOnly on getNextIds
    type FieldConfig = {
      index?: boolean;
      unique?: boolean;
      noSpace?: boolean;
    } & (
      | DefaultFieldConfig
      | StringFieldConfig
      | NumberFieldConfig
      | UUIDFieldConfig);
    enum FieldConfigTypes {
      UUID = 'UUID',
      GUID = 'GUID',
      String = 'String',
      Number = 'Number',
      ObjectId = 'ObjectId',
    export interface DefaultFieldConfig {
      type: FieldConfigTypes.ObjectId;
    export interface StringFieldConfig {
      type: FieldConfigTypes.String;
      nextId: string;
      separator?: string;
      nextIdFunction?: (nextId: string) => string;
    export interface NumberFieldConfig {
      type: FieldConfigTypes.Number;
      nextId: number;
      incrementBy?: number;
      nextIdFunction?: (nextId: number, incrementBy?: number) => number;
    export interface UUIDFieldConfig {
      type: FieldConfigTypes.UUID | FieldConfigTypes.GUID;
      asBinary?: boolean; // default string
      version?: 1 | 4; // supports 1 and 4, default 1
      versionOptions?: any;


    If you find a bug, do well to create an issue so we fix things up in a flash. Have an awesome new feature to add to library, feel free to open a PR

    NextIdFunctions (Incrementer)

    If you have a superb nextIdFunction, and you wish it can be added to the list of nextIdFunctions, feel free to drop a pull request and submit your function.




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