belongsToMany 多对多
一个源模型可以连接多个目标模型，目标模型也可以有多个相关的源。

  // 表TenderGuarantee的Model中的关联
TenderGuarantee.associate = () => { 
    TenderGuarantee.belongsToMany(app.mysql.Warrantee, { 
        as: 'warrantee', 
        through: 'tender_warrantee', 
        foreignKey: 'guarantee_id'
    });
 }

// 表Warrantee的Model中的关联
Warrantee.associate = () => { 
    Warrantee.belongsToMany(app.mysql.TenderGuarantee, { 
        as: 'tenderGuarantee', 
        through: 'tender_warrantee', 
        foreignKey: 'warrantee_id' 
    });
}

执行结果: 创建一个新模型tender_warrantee，其包含guarantee_id和warrantee_id两个外键。

as: 对模型重命名
foreignKey: 设置through关系源模型的外键，
otherKey: 设置through关系目标模型的外键。

1. 如果想为连接表(tender_warrantee)添加更多属性，可以建立关联前在sequelize 中为连接表定义一个模型，这会告诉sequelize 使用这个表建立关联而不是创建新表.

module.exports = app => {
  const { CHAR, INTEGER, STRING } = app.Sequelize;

  const TenderWarrantee = app.mysql.define('tender_warrantee', {
    guarantee_id: {
      type: INTEGER(11).UNSIGNED,
      primaryKey: true,
    },
    warrantee_id: {
      type: INTEGER(11).UNSIGNED,
      primaryKey: true,
    },
    guarantee_file: {
      type: STRING,
    },
  }, {
    freezeTableName: true,
    timestamps: true,
    engine: 'InnoDB',
    indexs: [
      {
        fields: [ 'guarantee_id', 'warrantee_id' ],
        unique: true,
      },
    ],
  });

  TenderWarrantee.associate = () => {
    TenderWarrantee.belongsTo(app.mysql.TenderGuarantee, { foreignKey: 'guarantee_id', targetKey: 'guarantee_id' });
    TenderWarrantee.belongsTo(app.mysql.Warrantee, { foreignKey: 'warrantee_id', targetKey: 'warrantee_id' });
  };

  return TenderWarrantee;
};

2. belongsToMany 可以基于through关系查询并可以选择查询属性

TenderGuarantee.findAll({
  include: [{
    model: warrantee,
    through: {
      attributes: ['createdAt'],
      where: {}
    }
  }]
});