loopbackjs：将模型附加到不同的数据源

Question

我已经为我的环境定义了几个使用数据源“db”（mysql）的模型。

有什么方法可以将多个数据源附加到这些模型，以便我能够对不同的数据库执行 REST 操作？

即： 获取 /api/Things?ds="db"

GET /api/Things?ds="anotherdb"

GET /api/Things（将使用默认 ds）

Answer 1

正如@superkau 上面指出的，每个 LoopBack 模型只能附加到单个数据源。

您可以为要使用的每个数据源创建（子类化）新模型。然后，您可以通过唯一的 REST URL 公开这些每个数据源的模型，或者您可以实现一个包装器模型，将方法分派到正确的数据源特定模型。

在我的示例中，我将展示如何为附加到db 和anotherdb 的Car 模型公开每个数据源模型。 Car 模型通过common/models/car.json 和common/models/car.js 以通常的方式定义。

现在您需要定义每个数据源的模型：

// common/models/car-db.js
{
  "name": "Car-db",
  "base": "Car",
  "http": {
    "path": "/cars:db"
  }
}

// common/models/car-anotherdb.js
{
  "name": "Car-anotherdb",
  "base": "Car",
  "http": {
    "path": "/cars:anotherdb"
  }

}

// server/model-config.json
{
  "Car": {
    "dataSource": "default"
  },
  "Car-db": {
    "dataSource": "db"
  },
  "Car-anotherdb": {
    "dataSource": "anotherdb"
  }
}

现在您可以使用以下 URL：

GET /api/Cars:db
GET /api/Cars:anotherdb
GET /api/Cars

上述解决方案有两个限制：您必须为每个数据源定义一个新模型，并且无法使用查询参数选择数据源。

要解决这个问题，您需要一种不同的方法。我再次假设已经定义了一个 Car 模型。

现在您需要创建一个“调度程序”。

// common/models/car-dispatcher.json
{
  "name": "CarDispatcher",
  "base": "Model", //< important!
  "http": {
    "path": "/cars"
  }
}

// common/models/car-dispatcher.js
var loopback = require('loopback').PersistedModel;
module.exports = function(CarDispatcher) {
  Car.find = function(ds, filter, cb) {
    var model = this.findModelForDataSource(ds);
    model.find(filter, cb);
  };

  // a modified copy of remoting metadata from loopback/lib/persisted-model.js
  Car.remoteMethod('find', {
    isStatic: true,
    description: 'Find all instances of the model matched by filter from the data source',
    accessType: 'READ',
    accepts: [
     {arg: 'ds', type: 'string', description: 'Name of the datasource to use' },
     {arg: 'filter', type: 'object', description: 'Filter defining fields, where, orderBy, offset, and limit'}
    ],
    returns: {arg: 'data', type: [typeName], root: true},
    http: {verb: 'get', path: '/'}
  });

  // TODO: repeat the above for all methods you want to expose this way

  Car.findModelForDataSource = function(ds) {
    var app = this.app;
    var ds = ds && app.dataSources[ds] || app.dataSources.default;

    var modelName = this.modelName + '-' + ds;
    var model = loopback.findModel(modelName);
    if (!model) {
      model = loopback.createModel(
        modelName, 
        {},
        { base: this.modelName });
    }

    return model;
  };  
};

最后一点是删除Car并在模型配置中使用CarDispatcher：

// server/model-config.json
{
  "CarDispatcher": {
    dataSource: null,
    public: true
  }
}

Answer 2

默认情况下，您只能基于每个模型附加数据源。这意味着您可以通过 datasources.json 将每个模型附加到不同的数据源。

对于您的用例，您将为多个数据源所需的每个端点添加一个远程挂钩。在您的远程挂钩中，您将执行以下操作：

...
var ds1 = Model.app.dataSources.ds1;
var ds2 = Model.app.dataSources.ds2;

//some logic to pick a data source
if (context.req.params...
...

请参阅http://docs.strongloop.com/display/LB/Remote+hooks 了解更多信息。

Answer 3

对于仍在寻找工作答案的任何人，动态切换数据库的解决方案是编写一个检查请求路径的中间件脚本，然后创建一个新的 DataSource 连接器，传入基于 req.path 变量的变量。例如，如果请求路径是/orders，那么“orders”作为字符串将被保存在一个变量中，然后我们附加一个新的数据源，为“orders”传递该变量。这是完整的工作代码。

'use strict';

const DataSource = require('loopback-datasource-juggler').DataSource;
const app = require('../server.js');

module.exports = function() {
  return function datasourceSelector(req, res, next) {
  // Check if the API request path contains one of our models.
  // We could use app.models() here, but that would also include
  // models we don't want.
  let $models = ['offers', 'orders', 'prducts'];
  // $path expects to be 'offers', 'orders', 'prducts'.
  let $path = req.path.toLowerCase().split("/")[1];

  // Run our function if the request path is equal to one of
  // our models, but not if it also includes 'count'. We don't
  // want to run this twice unnecessarily.
  if (($models.includes($path, 0)) && !(req.path.includes('count'))) {
    // The angular customer-select form adds a true value
    // to the selected property of only one customer model.
    // So we search the customers for that 'selected' = true.
    let customers = app.models.Customer;
    // Customers.find() returns a Promise, so we need to get
    // our selected customer from the results.
    customers.find({"where": {"selected": true}}).then(function(result){
      // Called if the operation succeeds.
      let customerDb = result[0].name;
      // Log the selected customer and the timestamp
      // it was selected. Needed for debugging and optimization.
      let date = new Date;
      console.log(customerDb, $path+req.path, date);
      // Use the existing veracore datasource config
      // since we can use its environment variables.
      let settings = app.dataSources.Veracore.settings;
      // Clear out the veracore options array since that
      // prevents us from changing databases.
      settings.options = null;
      // Add the selected customer to the new database value.
      settings.database = customerDb;
      try {
        let dataSource = new DataSource(settings);
        // Attach our models to the new database selection.
        app.models.Offer.attachTo(dataSource);
        app.models.Order.attachTo(dataSource);
        app.models.Prduct.attachTo(dataSource);
      } catch(err) {
        console.error(err);
      }
    })
    // Called if the customers.find() promise fails.
    .catch(function(err){
      console.error(err);
    });
  }
  else {
    //  We need a better solution for paths like '/orders/count'.
    console.log(req.path + ' was passed to datasourceSelector().');
  }
  next();
  };
};