old_contents: module.exports = (sequelize, DataTypes) => {
    const UserGroup = sequelize.define('UserGroup', {
      userId: {
        type: DataTypes.INTEGER,
        allowNull: false
      },
      groupId: {
        type: DataTypes.INTEGER,
        allowNull: false
      },
    }, {
      timestamps: false
    });
    return UserGroup;
  };
  
  
new_contents: module.exports = (sequelize, DataTypes) => {
    const UserGroup = sequelize.define('UserGroup', {
      userId: {
        type: DataTypes.INTEGER,
        allowNull: false
      },
      groupId: {
        type: DataTypes.INTEGER,
        allowNull: false
      },
    }, {
      classMethods: {
        associate: (models) => {
          UserGroup.belongsTo(models.Group, { foreignKey: 'groupId' });
          UserGroup.belongsTo(models.User, { foreignKey: 'userId' });
        }
      },
      timestamps: false
    });
    return UserGroup;
  };
  
  
subject: Modify Server side model to work well with client