Applet cloud development joint table data query and application in cloud function

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Applet cloud development joint table data query and application in cloud functions | Multi-table query | Even the table lookup | Aggregate functions

Hello everyone , I'm Ning Yi , Today, let's learn how to query the join table data of cloud development , And teach you how to apply cloud functions , Print the results of our query in the wechat developer tool

Let's set the scene first , Now there are two tables , Let's check the average scores of all the students in the class Mr. Xu took

1、 League table query

Let's first look at how to query , The data connecting the two tables is class In the table id and student In the table class_id

So we should first find out what teacher Xu's class is id, yes 2, And then check student In the table class_id by 2 Of the students , Zhang Er and Li Er , Calculate the average score of the two students

Let's take a look at how to implement such a joint table query in cloud development

You should learn to read cloud development documents as much as possible , However, this document is actually very difficult for novices , If you have any functions that you want me to demonstrate, you can tell me in the bullet screen or comments , If there are many people who want to see , I will issue a separate issue to talk about .

Cloud development documentation , In the development guide – In the database , There is an introduction to join table query , We use lookup Function to realize joint table query

lookup({
  from: < The name of the collection to connect >,
  localField: < Enter the fields of the record to match equally >,
  foreignField: < The fields of the connected set to be matched equally >,
  as: < Output array field name >
})

（1）lookup Join two tables

Apply to the scenario we set above , It's like this , Need to call end Method to mark the end of the definition

lookup({
    
  from: 'student',      // Table to associate student
  localField: 'id',      //class Associated fields in the table 
  foreignField: 'class_id',      //student Associated fields in the table 
  as: 'stu'      // Defines the alias of the output array 
})
.end()

This statement will find the following results , Will find out the information of the class and the information of all the students corresponding to the class

{
    "list":
  [{
    
    "id":1,
    "teacher":" Teacher wang ",
    "cname":" Class one ",
    "stu":[
      {
    
        "sname":" Ning Yi ",
        "class_id":1,
        "score":90
      }
    ]
  },
  {
    
    "id":2,
    "teacher":" Miss Xu ",
    "cname":" Class two ",
    "stu":[
      {
    
        "class_id":2,
        "sname":" Zhang Er ",
        "score":100
      },
      {
    
        "class_id":2,
        "sname":" Li Er ",
        "score":80
      }
    ]
  }]
}

But we only need the data of the students in Mr. Xu's class , So we need to add where Conditions , stay lookup You can't follow it directly where, Need to use match Instead of . Let's improve the above code

（2） Use match Query conditions

.lookup({
    
  from: 'student',
  localField: 'id',
  foreignField: 'class_id',
  as: 'stu'
})
.match({
    
  teacher:" Miss Xu "
})
.end()

Now we just return the student data of Mr. Xu's class , Student data is in stu In the corresponding array

{
    "list":
  [
    {
    "_id":"5e847ab25eb9428600a512352fa6c7c4",
      "id":2,
      "teacher":" Miss Xu ",
      "cname":" Class two ",
      // Student data 
      "stu":[ 
        {
    "_id":"37e26adb5eb945a70084351e57f6d717",
          "class_id":2,
          "sname":" Zhang Er ",
          "score":100
        },
        {
    "_id":"5e847ab25eb945cf00a5884204297ed8",
          "class_id":2,
          "sname":" Li Er ",
          "score":80
        }
      ]
    }
  ]
}

Next, let's continue to optimize the code , Returns the student's average score directly

（3） Return the average student grade directly

If you want to be in a connected table （ In this course student） Do aggregate operations , Just use pipeline Method

however pipeline Cannot be associated with localField、foreignField share , So let's delete localField、foreignField And then pipeline Get student grades in （score） Average value

.lookup({
    
  from: 'student',
  pipeline: $.pipeline()
    .group({
    
      _id: null,
      score: $.avg('$score')
     })
    .done(),
  as: 'stu'
})
.match({
    
  teacher:" Miss Xu "
})
.end()

Now the printed data is like this

{
    "list":
  [
    {
    "_id":"5e847ab25eb9428600a512352fa6c7c4",
      "id":2,
      "teacher":" Miss Xu ",
      "cname":" Class two ",
      "stu":[
        {
    "_id":null,
          "score":90
        }
      ]
    }
  ]
}

But now the output data is a little complicated , If you just want to show teacher and score These two values , Let's do the following

（4） Display only teacher and score These two values

.lookup({
    
  from: 'student',
  pipeline: $.pipeline()
    .group({
      
      _id: null,
      score: $.avg('$score')
    })
    .done(),
  as: 'stu'
})
.match({
    
  teacher:" Miss Xu "
})
.replaceRoot({
    
  newRoot: $.mergeObjects([$.arrayElemAt(['$stu', 0]), '$$ROOT'])
})
.project({
    
  _id:0,
  teacher:1,
  score:1
})
.end()

Now the printed data is like this

{
    "list":
  [
    {
    "score":90,"teacher":" Miss Xu "}
  ]
}

replaceRoot({ newRoot: < expression > }) It's fixed writing , Output the existing field as a new node , We usually use it to turn a two-level array into a one-level array

mergeObjects Is the accumulator operator ,$.arrayElemAt(['$stu', 0]), '$$ROOT’] Will be stu First element in array , That is to say [{"_id":null,"score":90}] Merge to the following nodes of the array , That is to say, with teacher、cname These fields are the same level

project There will be _id Set later as 0, Set the element we want to display to 1, You can control the last output field

2、 Application in cloud function

Next, let's look at how to use cloud functions , Print the above query results in the wechat developer tool

（1） Add data to the cloud database

We open the cloud development console in the wechat developer tool , First create these two tables in the cloud database , Let's create class Table as an example

After creating the form , Let's add records to the table , Add... According to the data in the table above , The data of class 2 is added below

After all the data has been added , Go to the wechat developer tool cloud functions folder and create a folder named test The cloud function of
We created the cloud development project in advance , If you don't know how to create , You can see what I sent before 30 Minutes to create and launch the course of cloud development applet , It teaches you how to create

（2） Create a cloud function and initialize the database

After creation , The system will help us create a test Folder , We turn on test/index.js file , Delete some code created by default , And initialize the database , Like this

//  Cloud function entry file 
const cloud = require('wx-server-sdk')
cloud.init()
// Initialize database 
const db = cloud.database()
const _ = db.command
const $ = _.aggregate

//  Cloud function entry function 
exports.main = async (event, context) => {
    
  // Let's continue to add code here 
}

（3） Edit cloud function entry function

//  Cloud function entry function 
exports.main = async (event, context) => {
    
  return await db.collection('class').aggregate()
  .lookup({
    
    from: 'student',
    pipeline: $.pipeline()
      .group({
    
      _id: null,
      score: $.avg('$score')
    })
    .done(),
  as: 'stu'
})
.match({
    
  teacher:" Miss Xu "
})
.replaceRoot({
    
  newRoot: $.mergeObjects([$.arrayElemAt(['$stu', 0]), '$$ROOT'])
})
.project({
    
  _id:0,
  teacher:1,
  score:1
})
.end()
}

Save the file after editing , And upload and deploy cloud functions

（4） Upload and deploy cloud functions

Right click the cloud function , Select upload and deploy ： Cloud installation depends on （ Do not upload node_modules）

Open the cloud development console — Click cloud function – find test Cloud function click cloud test

In the pop-up test box , Click the run test button directly

The returned results will be printed below , This indicates that the associated table query has been successful