Creating instance methods in a Sequelize Model using Typescript

Question

I want to extend a Sequelize Model class to add other instance methods but typescript keeps complaining that "Property 'prototype' does not exist on type 'Model'"

const MyModel = (sequelize: Sequelize.Sequelize, dataTypes: Sequelize.DataTypes) => {
  const User = sequelize.define<Instance, Attribute>(
    "users",
    {
      id: {
        type: dataTypes.INTEGER,
        primaryKey: true,
        autoIncrement: true,
      },
      email: {
        type: dataTypes.STRING
      },
      ...
    },
    {
      tableName: "users",
      ...
    },
  );

  User.prototype.verifyUser = function(password: string) {
    ...
  };

  return User;
};

I expect User.prototype.verifyUser to work but typescript complains. How to add to typings?

Answer 1

Following @Shadrech comment, I've an alternative (less hacky and abstract).

export interface UserAttributes {
   ...
}

export interface UserInstance extends Sequelize.Instance<UserAttributes>, UserAttributes {
}

interface UserModelInstanceMethods extends Sequelize.Model<UserInstance, UserAttributes> {

  // Came to this question looking for a better approach to this
  // You'll need root's definitions for invocation and prototype's for creation
  verifyPassword: (password: string) => Promise<boolean>;
  prototype: {
    verifyPassword: (password: string) => Promise<boolean>;
  };
}

const MyModel = (sequelize: Sequelize.Sequelize, dataTypes: Sequelize.DataTypes): UserModelInstanceMethods => {
  const User = sequelize.define<UserInstance, UserAttributes>(
      ...
  ) as UserModelInstanceMethods;

  User.prototype.verifyUser = function(password: string) {
    ...
  };

  return User;
}

Using your model:

sequelize.query("SELECT ...").then((user: UserInstance & UserModelInstanceMethods) => {
  user.verifyPassword(req.body.password) // <= from UserModelInstanceMethods
  user.getDataValue('name') // <= from UserInstance
})

Answer 2

One solution I've seen is where you force type after declaring the Model. So

interface UserModelInstanceMethods extends Sequelize.Model<Instance, Attributes> {
  prototype: {
    verifyPassword: (password: string) => Promise<boolean>;
  };
}

const MyModel = (sequelize: Sequelize.Sequelize, dataTypes: Sequelize.DataTypes) => {
  const User = sequelize.define<Instance, Attribute>(
    "users",
    {
      id: {
        type: dataTypes.INTEGER,
        primaryKey: true,
        autoIncrement: true,
      },
      email: {
        type: dataTypes.STRING
      },
      ...
    },
    {
      tableName: "users",
      ...
    },
  );

  User.prototype.verifyUser = function(password: string) {
    ...
  };

  return User;
} as Sequelize.Model<Instance, Attributes> & UserModelInstanceMethods;

Answer 3

According to the main Sequelize TypeScript Doc, I think that the best way to implement it is using DataTypes.VIRTUAL and skip the property with TypeScript Omit utility on the model creation interface.

Important! Remember the Issue#11675!

• https://stackoverflow.com/a/69289067/717267
• https://github.com/sequelize/sequelize/issues/11675

A simple example:

import {
  Sequelize,
  Model,
  ModelDefined,
  DataTypes,
  Optional,
  // ...
} from 'sequelize';

interface ProjectAttributes {
  id: number;
  ownerId: number;
  name: string;
  readonly createdAt: Date;
  readonly updatedAt: Date;

  // #region Methods
  
  myMethod(name: string): Promise<void>; // <<<===

  // #endregion
}

interface ProjectCreationAttributes extends Omit< // <<<===
  Optional<
    ProjectAttributes,
    | 'id'
    | 'createdAt'
  >,
  'myMethod' // <<<===
> {}

class Project extends Model<ProjectAttributes, ProjectCreationAttributes>
  implements ProjectAttributes {
  public id: ProjectAttributes['id'];
  public ownerId: ProjectAttributes['ownerId'];
  public name: ProjectAttributes['name'];
  public readonly createdAt: ProjectAttributes['createdAt'];
  public readonly updatedAt: ProjectAttributes['updatedAt'];

  public readonly myMethod: ProjectAttributes['myMethod'] // <<<===

   /**
   * Initialization to fix Sequelize Issue #11675.
   *
   * @see https://stackoverflow.com/questions/66515762/configuring-babel-typescript-for-sequelize-orm-causes-undefined-properties
   * @see https://github.com/sequelize/sequelize/issues/11675
   * @ref #SEQUELIZE-11675
   */
  constructor(values?: TCreationAttributes, options?: BuildOptions) {
    super(values, options);

    // All fields should be here!
    this.id = this.getDataValue('id');
    this.ownerId = this.getDataValue('ownerId');
    this.name = this.getDataValue('name');
    this.createdAt = this.getDataValue('createdAt');
    this.updatedAt = this.getDataValue('updatedAt');

    this.myMethod = async (name) => { // <<<===
      // Implementation example!
      await this.update({
        name,
      });
    };
  }

  // #region Methods

  public toString() {
    return `@${this.name} [${this.ownerId}] #${this.id}`;
  }

  // #endregion
}

Project.init(
  {
    id: {
      type: DataTypes.INTEGER.UNSIGNED,
      autoIncrement: true,
      primaryKey: true,
    },
    ownerId: {
      type: DataTypes.INTEGER.UNSIGNED,
      allowNull: false,
    },
    name: {
      type: new DataTypes.STRING(128),
      allowNull: false,
    },


    myMethod: { // <<<===
      type: DataTypes.VIRTUAL(DataTypes.ABSTRACT),
    }
  },
  {
    sequelize,
    tableName: "projects",
  }
);