如何在流星的mongo查询中指定读取首选项 [英] How to specify read preference in Meteor's mongo queries

问题描述

在Meteor Mongo中，如何在Meteor Mongo查询中将readPref指定为primary | secondary.

In Meteor Mongo, how to specify the readPref to primary|secondary in Meteor Mongo Query.

推荐答案

我希望以下内容可以更好地理解流星与Mongo之间的关系.

---

I hope the following provides a better understanding of the relationship between Meteor and Mongo.

---

Meteor为您提供完整的mongo功能.但是，为了方便起见，它提供了一个与mongo集合集成的包装的API 最适合流星环境.因此，如果您通过

Meteor provides you with the full mongo functionality. However for comfort it provides a wrapped API of a mongo collection that integrates best with the Meteor environment. So if you import Mongo via

import { Mongo } from 'meteor/mongo' 

您主要是导入包装的mongo集合，在其中用Meteor光纤执行操作.这些包装的集合的查询返回的游标也不是自然"游标，而是包装的光标进行流星优化.

you primarily import the wrapped mongo collection where operations are executed in a Meteor fiber. The cursor that is returned by queries of these wrapped collections are also not the "natural" cursors but also wrapped cursors to be Meteor optimized.

如果您尝试在未实现的这些实例上访问本机功能，则会收到错误消息.就您而言:

If you try to access a native feature on these instances that is not implemented you will receive an error. In your case:

import { Meteor } from 'meteor/meteor';
import { Random } from 'meteor/random';

const ExampleCollection = new Mongo.Collection('examples')

Meteor.startup(() => {
  // code to run on server at startup
  ExampleCollection.insert({ value: Random.id() })
  const docsCursor = ExampleCollection.find();
  docsCursor.readPref('primary')
}); 

领先

TypeError: docsCursor.readPref is not a function

---

好消息是，您可以访问其下的层Collection.rawCollection()通过a>可以完全访问节点Mongo驱动程序.这是因为Meteor的Mongo.Collection和Cursor最终都在使用此本机驱动程序.

The good news is, you can access the layer underneath via Collection.rawCollection() where you have full access to the node Mongo driver. This is because under the hood Meteor's Mongo.Collection and it's Cursor are making use of this native driver in the end.

现在您将发现另外两个问题:

Now you will find two other issues:

1. readPref在节点mongo游标中命名 cursor.setReadPreference (3.1 API).

1. readPref is named in a node-mongo cursor cursor.setReadPreference (3.1 API).

Cursor.fetch不存在，但被命名为cursor.toArray，它(与许多本机操作一样)返回Promise

Cursor.fetch does not exist but is named cursor.toArray which (as many native operations do) returns a Promise

---

您可以执行以下操作:

import { Meteor } from 'meteor/meteor';
import { Random } from 'meteor/random';

const ExampleCollection = new Mongo.Collection('examples')

Meteor.startup(() => {
  // code to run on server at startup

  ExampleCollection.insert({ value: Random.id() })
  const docsCursor = ExampleCollection.rawCollection().find();
  docsCursor.setReadPreference('primary')
  docsCursor.toArray().then((docs) => {
    console.log(docs)
  }).catch((err)=> console.error(err))
});

---

• 通过使用collection.rawCollection()，您可以访问您可以自己将操作，游标和结果(承诺)集成到您的环境中.好帮手是 Meteor.bindEnvironment 和

  You are on your own to integrate the operations, cursors and results (Promises) into your environment. Good helpers are Meteor.bindEnvironment and Meteor.wrapAsync

  谨防node-mongo驱动程序的API更改.一方面，驱动程序支持的mongo版本，另一方面，流星支持的驱动程序版本.

  Beware of API changes of the node-mongo driver. On the one hand the mongo version that is supported by the driver, on the other hand the driver version that is supported by Meteor.

  请注意，使用本机API可以更轻松地整理"事物，但是它还为您提供了许多新选项.小心使用.

  Note that it is easier to "mess up" things with the native API but it also gives you a lot of new options. Use with care.

  这篇关于如何在流星的mongo查询中指定读取首选项的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！