Sequelizejs: no auto-generation of methods from associations

I am trying to setup a m:n association using feathersjs with sequelize. I am following this 'instruction', but have to adjust to updates feathersjs has gone throught in the meantime.

My hook to associate an already created user with an already created item:

module.exports = function (options = {}) { 
  return function (hook) {
    hook.app.service('users')
      .get(hook.data[0].user_id)
      .then(users => {
        hook.app.service('items')
          .find(hook.data[0].item_id)
          .then(benches => items.addMyusers(users));
      });

    return Promise.resolve(hook);
  };
};

I get:

error: Unhandled Rejection at: Promise  Promise {
  <rejected> TypeError: Cannot read property 'addMyusers' of undefined

Under my user model definition I set the asssocation with:

  users.associate = function (models) {
    users.belongsToMany(models.items, {
      through: {
        model: 'favorites-items',
        unique: false
      },
      foreignKey: 'item_id',
      as: 'myitems',
      constraints: false
    });
  };

under item model definition:

  items.associate = function (models) { 
    items.belongsToMany(models.users, {
      through: {
        model: 'favorites-items',
        unique: false
      },
      as: 'myusers',
      foreignKey: 'user_id',
      constraints: false
    });
  };

Why does addMyusers not exist? Methods like these should be auto-generated when making the association?

As stated in the sequelize docs:

This will add methods getUsers, setUsers, addUser,addUsers to Project, and getProjects, setProjects, addProject, and addProjects to User.

This is not an issue related to sequelize at all.

Look at your error message carefully:

TypeError: Cannot read property 'addMyusers' of undefined

It's not that sequelize didn't add the method, it's that your variable items is undefined, and of course undefined does not have a addMyusers method. From the code you posted, items is not set anywhere in your code, so it is indeed undefined.