Migrating model mixins | LoopBack Documentation

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Tip: Missing instructions for your LoopBack 3 use case? Please report a Migration docs issue on GitHub to let us know.

Introduction

This document will guide you in migrating custom model mixins, and custom method/remote method mixins in LoopBack 3 to their equivalent implementations in LoopBack 4.

For an understanding of how models in LoopBack 3 are now architecturally decoupled into 3 classes (model, repository, and controller) please read Migrating custom model methods.

In LoopBack 3, it was easy to add property mixins and method mixins.

In LoopBack 4, it is also easy and is accomplished by using a mixin class factory function.

Creating a Property Mixin

This section covers the approach that LoopBack 3 and LoopBack 4 can use to add a property to a model via a mixin.

LoopBack 3 Approach

In LoopBack 3, a developer is able to create a model property mixin by:

placing the mixin logic in a file in a mixins directory
updating the server/model-config.json file with the mixin directory location
updating the model’s json file to include the mixin’s name and a boolean

As an example, we will create a mixin that adds a category property to a model.

Defining The Model Property Mixin category.js

The developer defines a model property mixin in common/mixins/category.js which adds a required property named category to any model.

common/mixins/category.js

module.exports = function (Model, options) {
  Model.defineProperty('category', {type: 'string', required: true});
};

Updating model-config.json

The server/model-config.json needs to contain:

the locations of all models
the location of all mixins
the entry of the model that receives the mixin content (for this example Note)

server/model-config.json

{
  "_meta": {
    "sources": [
      "loopback/common/models",
      "loopback/server/models",
      "../common/models",
      "./models"
    ],
    "mixins": [
      "loopback/common/mixins",
      "loopback/server/mixins",
      "../common/mixins",
      "./mixins"
    ]
  },

 // ... other entries

  "Note": {
    "dataSource": "db"
  }
}

Please see Reference mixins in model-config.js for a short explanation of this file.

Applying The category.js Mixin To A Model

To extend the model Note with the category.js mixin, we need to add a mixins section in common/models/note.json to indicate which mixins should be applied to it.

common/models/note.json

{
  "name": "Note",
  "properties": {
    "title": {
      "type": "string",
      "required": true
    },
    "content": {
      "type": "string"
    }
  },
  "mixins": {
    "Category": true
  }
}

Specifying a value of true for Category will apply the category.js property model mixin to the Note model. A value of false will not apply the mixin.

LoopBack 4 Approach

In LoopBack 4, a developer is able to create a model property mixin by:

creating a base model class which extends Entity
placing the mixin class factory function in a separate file
generating a model using the CLI as usual
adjusting the model file to make use of the mixin class factory function

Defining A BaseEntity Class Which Extends Entity

Let’s define a base model class BaseEntity in src/models/base-entity.ts. It will be used as input to the mixin later.

src/models/base-entity.ts

import {Entity} from '@loopback/repository';
export class BaseEntity extends Entity {}

This is necessary because the Entity class is abstract and doesn’t have a constructor.

Defining The Model Property Mixin Class Factory Function

This mixin class factory function AddCategoryPropertyMixin in src/mixins/category-property-mixin.ts adds the required property category to any model.

src/mixins/category-property-mixin.ts

import {MixinTarget} from '@loopback/core';
import {property, Model} from '@loopback/repository';

/**
 * A mixin factory to add `category` property
 *
 * @param superClass - Base Class
 * @typeParam T - Model class
 */
export function AddCategoryPropertyMixin<T extends MixinTarget<Model>>(
  superClass: T,
) {
  class MixedModel extends superClass {
    @property({
      type: 'string',
      required: true,
    })
    category: string;
  }
  return MixedModel;
}

Note: At the moment, TypeScript does not allow decorators in class expressions. This is why we need to declare the class with a name, and then return it.

Generating A Model Via The CLI

A CLI-generated model named Note with 3 properties: id, title, and content would look like this:

src/models/note.model.ts

import {Entity, model, property} from '@loopback/repository';

@model()
export class Note extends Entity {
  @property({
    type: 'number',
    id: true,
    generated: true,
  })
  id?: number;

  @property({
    type: 'string',
    required: true,
  })
  title: string;

  @property({
    type: 'string',
  })
  content?: string;

  constructor(data?: Partial<Note>) {
    super(data);
  }
}

export interface NoteRelations {
  // describe navigational properties here
}

export type NoteWithRelations = Note & NoteRelations;

Adjusting The Model File To Use AddCategoryPropertyMixin

The model file only requires a few adjustments:

import the BaseEntity class
import the AddCategoryPropertyMixin mixin
Change the class declaration of Note so that it extends the class returned from the mixin function which takes in the BaseEntity superclass as input

src/models/note.model.ts

import {model, property} from '@loopback/repository';
import {AddCategoryPropertyMixin} from '../mixins/category-property-mixin';
import {BaseEntity} from './base-entity';

@model()
export class Note extends AddCategoryPropertyMixin(BaseEntity) {
  @property({
    type: 'number',
    id: true,
    generated: true,
  })
  id?: number;

  @property({
    type: 'string',
    required: true,
  })
  title: string;

  @property({
    type: 'string',
  })
  content?: string;

  constructor(data?: Partial<Note>) {
    super(data);
  }
}

export interface NoteRelations {
  // describe navigational properties here
}

export type NoteWithRelations = Note & NoteRelations;

The required property category has now been added to the Note model via a mixin class factory function.

Creating A Custom Model Method And Remote Model Method Mixin

This section covers the approach that LoopBack 3 can use to add a custom method /remote method to a model via a mixin, and similarly how LoopBack 4 can add a custom method to a repository and controller via a mixin.

LoopBack 3 Approach

The Add a New Model Method And a New Endpoint section of the Migrating custom model methods document explains how a LoopBack 3 developer can define a custom model method named findByTitle on the Note model, and define a remote method to make it available as a new endpoint.

In this section, we will show how a LoopBack 3 developer can define a mixin to accomplish this.

In LoopBack 3, a developer is able to create a custom model method/remote method mixin by:

placing the mixin logic in a file in a mixins directory
updating the server/model-config.json file with the mixin directory location
updating the model’s json file to include the mixin’s name (and options object or boolean)

Defining The Model Method Mixin findByTitle.js

The developer defines a custom model method/remote method mixin in common/mixins/findByTitle.js which adds a custom method findByTitle to any model, and adds a corresponding remote method definition with path /findByTitle as well. An options property returnArgumentName makes it possible to customize the name of the return argument. If it is not specified, the return argument of ‘items’ is used as a default.

common/mixins/findByTitle.js

module.exports = function (Model, options) {
  const returnArgumentName = options.returnArgumentName
    ? options.returnArgumentName
    : 'items';

  Model.remoteMethod('findByTitle', {
    http: {
      path: '/findByTitle',
      verb: 'get',
    },
    accepts: {arg: 'title', type: 'string'},
    returns: {arg: returnArgumentName, type: [Model], root: true},
  });

  Model.findByTitle = function (title, cb) {
    var titleFilter = {
      where: {
        title: title,
      },
    };
    Model.find(titleFilter, cb);
  };
};

For a model named Note, this will expose an endpoint of /Notes/findByTitle.

Ensure model-config.json is set up properly as specified earlier in Updating model-config.json

Applying The findByTitle.js Mixin To A Model

To extend the model Note with the findByTitle.js mixin, we need to add a mixins section in common/models/note.json to indicate which mixins should be applied to it.

common/models/note.json

{
  "name": "Note",
  "properties": {
    "title": {
      "type": "string",
      "required": true
    },
    "content": {
      "type": "string"
    }
  },

  "mixins": {
    "FindByTitle": {
      "returnArgumentName": "notes"
    },

    "Category": true
  }
}

Specifying an options object for FindByTitle is the same as specifying a value of true as it will apply the findByTitle.js custom model method/remote method mixin to the Note model. A value of false will not apply the mixin.

LoopBack 4 Approach

As mentioned in the previous section, the Add a New Model Method And a New Endpoint section of the Migrating custom model methods document explains how a LoopBack 3 developer can define a custom model method named findByTitle on the Note model, and define a remote method to make it available as a new endpoint. It then shows how a LoopBack 4 developer can implement a findByTitle method on the NoteRepository and on the NoteController to accomplish the same thing.

In this section, we will show how a LoopBack 4 developer can define two mixins ( a repository mixin and a controller mixin) to add a findByTitle method to NoteRepository and NoteController respectively.

In LoopBack 4, a developer is able to create a repository and controller method mixin by:

defining a common interface for both mixin class factory functions
placing the mixin class factory functions in separate files
generating a repository and controller using the CLI as usual
adjusting the repository and controller files to make use of its respective mixin class factory function

Defining A Common Interface For The findByTitle Method

Let’s define a common interface FindByTitle in src/mixins/find-by-title-interface.ts.

src/mixins/find-by-title-interface.ts

import {Model} from '@loopback/repository';

/**
 * An interface to allow finding notes by title
 */
export interface FindByTitle<M extends Model> {
  findByTitle(title: string): Promise<M[]>;
}

Defining A Repository Mixin Class Factory Function

In src/mixins/find-by-title-repository-mixin.ts, let’s define the mixin class factory function FindByTitleRepositoryMixin which adds the findByTitle method to any repository.

src/mixins/find-by-title-repository-mixin.ts

import {MixinTarget} from '@loopback/core';
import {Model, CrudRepository, Where} from '@loopback/repository';
import {FindByTitle} from './find-by-title-interface';

/*
 * This function adds a new method 'findByTitle' to a repository class
 * where 'M' is a model which extends Model
 *
 * @param superClass - Base class
 *
 * @typeParam M - Model class which extends Model
 * @typeParam R - Repository class
 */
export function FindByTitleRepositoryMixin<
  M extends Model & {title: string},
  R extends MixinTarget<CrudRepository<M>>,
>(superClass: R) {
  class MixedRepository extends superClass implements FindByTitle<M> {
    async findByTitle(title: string): Promise<M[]> {
      const where = {title} as Where<M>;
      const titleFilter = {where};
      return this.find(titleFilter);
    }
  }
  return MixedRepository;
}

Generating A Repository Via The CLI

A CLI-generated repository for a model Note would look like this:

src/repositories/note.repository.ts

export class NoteRepository extends DefaultCrudRepository<
  Note,
  typeof Note.prototype.id,
  NoteRelations
> {
  constructor(@inject('datasources.db') dataSource: DbDataSource) {
    super(Note, dataSource);
  }
}

Adjusting NoteRepository To Use FindByTitleRepositoryMixin

The repository file only requires a few adjustments:

import the FindByTitleRepositoryMixin mixin class factory function
adjust the declaration of the NoteRepository class to extend the class returned from the mixin function which takes in the DefaultCrudRepository superclass as input.

src/repositories/note.repository.ts

import {FindByTitleRepositoryMixin} from '../mixins/find-by-title-repository-mixin';
import {DefaultCrudRepository} from '@loopback/repository';
import {Note, NoteRelations} from '../models';
import {DbDataSource} from '../datasources';
import {inject, Constructor} from '@loopback/core';

/**
 * A repository for `Note` with `findByTitle`
 */
export class NoteRepository extends FindByTitleRepositoryMixin<
  Note,
  Constructor<
    DefaultCrudRepository<Note, typeof Note.prototype.id, NoteRelations>
  >
>(DefaultCrudRepository) {
  constructor(@inject('datasources.db') dataSource: DbDataSource) {
    super(Note, dataSource);
  }
}

We have now added the findByTitle method to a repository via a mixin class factory function.

Defining A Controller Mixin Class Factory Function

In src/mixins/find-by-title-controller-mixin.ts, let’s define the mixin class factory function FindByTitleControllerMixin which adds the findByTitle method to any controller.

src/mixins/src/mixins/find-by-title-controller-mixin.ts

import {MixinTarget} from '@loopback/core';
import {Model} from '@loopback/repository';
import {FindByTitle} from './find-by-title-interface';
import {param, get, getModelSchemaRef} from '@loopback/rest';

/**
 * Options to mix in findByTitle
 */
export interface FindByTitleControllerMixinOptions {
  /**
   * Base path for the controller
   */
  basePath: string;
  /**
   * Model class for CRUD
   */
  modelClass: typeof Model;
}

/**
 * A mixin factory for controllers to be extended by `FindByTitle`
 * @param superClass - Base class
 * @param options - Options for the controller
 *
 * @typeParam M - Model class
 * @typeParam T - Base class
 */
export function FindByTitleControllerMixin<
  M extends Model,
  T extends MixinTarget<object>,
>(superClass: T, options: FindByTitleControllerMixinOptions) {
  class MixedController extends superClass implements FindByTitle<M> {
    // Value will be provided by the subclassed controller class
    repository: FindByTitle<M>;

    @get(`${options.basePath}/findByTitle/{title}`, {
      responses: {
        '200': {
          description: `Array of ${options.modelClass.modelName} model instances`,
          content: {
            'application/json': {
              schema: {
                type: 'array',
                items: getModelSchemaRef(options.modelClass, {
                  includeRelations: true,
                }),
              },
            },
          },
        },
      },
    })
    async findByTitle(@param.path.string('title') title: string): Promise<M[]> {
      return this.repository.findByTitle(title);
    }
  }

  return MixedController;
}

To customize certain portions of the OpenAPI description of the endpoint, the mixin class factory function needs to accept some options. We defined an interface FindByTitleControllerMixinOptions to allow for this.

It is also a good idea to give the injected repository (in the controller super class) a generic name like this.repository to keep things simple in the mixin class factory function.

Generating A Controller Via The CLI

A CLI-generated controller for the model Note would look like this: (To save space, only a partial implementation is shown)

src/controllers/note.controller.ts

export class NoteController {
  constructor(
    @repository(NoteRepository)
    public noteRepository: NoteRepository,
  ) {}

  @post('/notes', {
    responses: {
      '200': {
        description: 'Note model instance',
        content: {'application/json': {schema: getModelSchemaRef(Note)}},
      },
    },
  })
  async create(
    @requestBody({
      content: {
        'application/json': {
          schema: getModelSchemaRef(Note, {
            title: 'NewNote',
            exclude: ['id'],
          }),
        },
      },
    })
    note: Omit<Note, 'id'>,
  ): Promise<Note> {
    return this.noteRepository.create(note);
  }

  // ...
  // remaining CRUD endpoints
  // ...
}

For a full example of a CLI-generated controller for a model Todo, see TodoController .

Adjusting NoteController To Use FindByTitleControllerMixin

The controller file only requires a few adjustments:

import the FindByTitleControllerMixinOptions interface
import the FindByTitleControllerMixin mixin class factory function
prepare the options for the mixin
adjust the declaration of the NoteController class to extend the class returned from the mixin function which takes in the Object superclass as input.
pass the input options into the mixin
change the name of the injected repository from noteRepository to repository to keep things simple for the mixin class factory function

src/controllers/note.controller.ts

import {Note} from '../models';
import {
  FindByTitleControllerMixin,
  FindByTitleControllerMixinOptions,
} from '../mixins/find-by-title-controller-mixin';
import {Constructor} from '@loopback/core';

import {
  Count,
  CountSchema,
  Filter,
  repository,
  Where,
} from '@loopback/repository';
import {
  post,
  param,
  get,
  getFilterSchemaFor,
  getModelSchemaRef,
  getWhereSchemaFor,
  patch,
  put,
  del,
  requestBody,
} from '@loopback/rest';
import {NoteRepository} from '../repositories';

const options: FindByTitleControllerMixinOptions = {
  basePath: '/notes',
  modelClass: Note,
};

export class NoteController extends FindByTitleControllerMixin<
  Note,
  Constructor<Object>
>(Object, options) {
  constructor(
    @repository(NoteRepository)
    public repository: NoteRepository,
  ) {
    super();
  }

  @post('/notes', {
    responses: {
      '200': {
        description: 'Note model instance',
        content: {'application/json': {schema: getModelSchemaRef(Note)}},
      },
    },
  })
  async create(
    @requestBody({
      content: {
        'application/json': {
          schema: getModelSchemaRef(Note, {
            title: 'NewNote',
            exclude: ['id'],
          }),
        },
      },
    })
    note: Omit<Note, 'id'>,
  ): Promise<Note> {
    return this.repository.create(note);
  }

  // ...
  // remaining CRUD endpoints
  // ...
}

We have now added the findByTitle method to a controller via a mixin class factory function.

This will also expose an endpoint of /notes/findByTitle/{title}.

Summary

As the examples above show, migrating mixins from LoopBack 3 to LoopBack 4 is relatively straightforward using class factory functions.