您所要求的行为根本无法通过一次更新调用来完成。如果你检查文档中的更新功能，没有包含选项，即sequelize只能为调用更新函数的模型的表构建更新查询。
您必须单独更新关联。我建议您将这些更新放在一个事务中，以避免同时发生对同一对象的多个更新的任何问题。

这里有一个干净的方法来做到这一点：

const { Sequelize, DataTypes } = require("sequelize");
const {
  extendSequelize,
} = require("@hatchifyjs/sequelize-create-with-associations");

(async function main() {
  // create your Sequelize instance
  const sequelize = new Sequelize("sqlite::memory:", {
    logging: false,
  });

  // define your models
  const Merchant = sequelize.define("Merchant", {
    id: { type: DataTypes.INTEGER, primaryKey: true, autoIncrement: true },
    name: DataTypes.STRING,
  });

  const User = sequelize.define("User", {
    id: { type: DataTypes.INTEGER, primaryKey: true, autoIncrement: true },
    firstName: DataTypes.STRING,
    lastName: DataTypes.STRING,
    merchantId: DataTypes.INTEGER,
  });

  Merchant.hasOne(User, {
    as: "user",
    foreignKey: "merchantId",
  });

  User.belongsTo(Merchant, {
    as: "merchant",
    foreignKey: "merchantId",
  });

  // create the tables
  await sequelize.sync();

  // extend Sequelize
  await extendSequelize(Sequelize);

  await Merchant.create({
    name: "Old Merchant",
    user: {
      firstName: "az",
      lastName: "rnd",
    },
  });

  const newMerchant = await Merchant.create({
    name: "New Merchant",
  });

  await User.update(
    { lastName: "Updated", merchant: { id: newMerchant.id } },
    { where: { id: 1 } }
  );
})();