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定义关联关系

app/mode/xxx.js 中通过给模型（类）添加associate 属性，属性值为一个function(){}，方法中执行sequelize提供的建立关联关系的方法，例如 belongsTo等

egg-sequelize 插件在loadDatabase的时候会associate()，建立模型之间的关系

module.exports = app => {    const { BIGINT, STRING } = app.Sequelize;    const User = app.model.define('users', {        id: {            type: BIGINT,            primaryKey: true,            autoIncrement: true,        },       team_id: BIGINT,       name: STRING,    });    User.associate = function () {        app.model.User.belongsTo(app.model.Team, { foreignKey: 'team_id' });    };    return User;};

sequelize V4版本修改了：

Removed classMethods and instanceMethods options from sequelize.define. Sequelize models are now ES6 classes. You can set class / instance level methods like this

const Model = sequelize.define('Model', {    ...});// Class MethodModel.associate = function (models) {    ...associate the models};// Instance MethodModel.prototype.someMethod = function () {..}

执行数据库操作，例如 findAll 的时候，如果 include 了 model，执行之前会 model 之间的关联关系。如果没有提前定义，则 ${targetModel.name} is not associated to ${this.name}!

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