> For the complete documentation index, see [llms.txt](https://jen-hsuan-hsieh.gitbook.io/asp-net/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://jen-hsuan-hsieh.gitbook.io/asp-net/chapter3rest-api-design-development-and-management/33designing-rest-api/3311building-support-for-partial-responses.md).

# 3.3.11.Building support for Partial Responses

* Partial Responses
  * "API consumer" in control of response
  * 例如mobile的使用者可能不需要取得完整的資料, Partial Responses可以讓使用者指定特定的欄位

    ![](https://163116165-files.gitbook.io/~/files/v0/b/gitbook-legacy-files/o/assets%2F-M4M0G7tIs4o9F9vSrao%2F-M4M0IVB0ax0vs2ocz_r%2F-M4M0XbUS3chkVq6zXko%2F201803185.jpg?generation=1586302966301640\&alt=media)
* Benefits
  * Better performance & optimized resource usage
    * CPU, Memory, Bandwidth
  * API consumer controls the granularity
  * Common API version for all consumers
    * to support multiple devices, use cases - form factors
* Specifiction
  * Depends on the platform/language/DB
  * [Linkedin](https://developer.linkedin.com/docs/rest-api): /people: (id, first-name, last-name)
  * facebook: /friend?fields=id,name,picture
  * Pinterest: /pins?fields=id,link,creator(first\_name)
* Requirements
  * [Visual studio code](https://code.visualstudio.com/docs/?dv=win)
  * [node.js](https://nodejs.org/en/)
  * [mlab](https://mlab.com/signup/)
    * Add collection -> 新增一個叫hotels的collection
    * Add database user -> 新增一個叫xxx的user
    * collection-> index -> add index
      * { "name": 1}, 勾選unique
  * [source code from github](https://github.com/acloudfan/REST-API-Course)
    * 1.clone repository

      ```
      git clone https://github.com/acloudfan/REST-API-Course.git
      cd REST-API-COURSE/
      git checkout partialresponse
      npm install
      ```
    * 2.將資料寫到DB
      * 設定連線字串
        * 到mlab的database頁面下, 複製連線字串 ![](https://163116165-files.gitbook.io/~/files/v0/b/gitbook-legacy-files/o/assets%2F-M4M0G7tIs4o9F9vSrao%2F-M4M0IVB0ax0vs2ocz_r%2F-M4M0XbWHgDWVQ9nleMS%2F201803175.jpg?generation=1586302966447812\&alt=media)
        * 開啟專案, 修改tests/TestHotelsDBOps.js的process.env.DB\_URI
        * 開啟檢視/ 偵錯主控台/ 終端機, 輸入

          ```
             node tests/TestHotelsDBOps
          ```
    * 3.程式碼解析
      * [mongoDB](https://jenhsuan.gitbooks.io/database/content/chapter2-mongodb.html)的作法是用[projection](https://docs.mongodb.com/manual/tutorial/project-fields-from-query-results/)

```
     module.exports = function(router){
    'use strict';

    router.route(URI).get(function(req, res,next){
        console.log("GET Hotels")

        //1. fields
        var fields ={}
        if(req.query && req.query.fields !== undefined){
           fields =  createFields(req.query.fields)
        }

        //2. Setup options
        var options = {fields:fields}
        console.log(options)

        //3. execute the query
        var criteria = {}
        db.select(criteria, options, function(err,docs){

            if(err){
                console.log(err)
                res.status(500)
                res.send("Error connecting to db")
            } else {
                if(docs.length == 0){
                    res.status(404)
                }
                console.log("Retrieved hotels = %d",docs.length)
                res.send(docs)
            }
        });
    });
    }

    // RETRIEVE hotels packages based on criteria & fields
// https://docs.mongodb.com/manual/reference/method/db.collection.find/#find-projection
// options = {
//    fields: {/** Projection **/}
// }
exports.select = function (criteria,options, callback) {
    model.Hotels.find(criteria, function (err, data) {
        callback(err, data)
    }).select(options.fields)
}
```