GraphQL - Mutation

In this chapter, we will learn mutation queries in GraphQL.
Mutation queries modify data in the data store and returns a value. It can be used to insert, update, or delete data. Mutations are defined as a part of the schema.
The syntax of a mutation query is given below −
mutation{
   someEditOperation(dataField:"valueOfField"):returnType
}

Let us understand how to add new student record into the datastore using a mutation query.
Create a project folder by the name mutation-app. Change your directory to mutation-app from the terminal. Follow steps 3 to 5 explained in the Environment Setup chapter.
Add schema.graphql file in the project folder mutation-app and add the following code −
type Query {
   greeting:String
}

type Mutation {
   createStudent(collegeId:ID,firstName:String,lastName:String):String
}
Note that the function createStudent returns a String type. This is a unique identifier (ID) which is generated after creating a student.
Create a file resolvers.js in the project folder and add the following code −
const db = require('./db')
const Mutation = {
   createStudent:(root,args,context,info) => {
      return db.students.create({collegeId:args.collegeId,
      firstName:args.firstName,
      lastName:args.lastName})
   }
}
const Query = {
   greeting:() => "hello"
}

module.exports = {Query,Mutation}
The mutation function points to students collection in the datastore. To add a new student, invoke the create method in students collection. The args object will contain the parameters which are passed in the query. The create method of students collection will return the id of a newly created student object.
Create a server.js file. Refer to step 8 in the Environment Setup Chapter. Execute the command npm start in the terminal. The server will be up and running on 9000 port. Here, we use GraphiQL as a client to test the application.
Next step is to open browser and type the URL http://localhost:9000/graphiql. Type the following query in the editor −
//college Id should be matched with data from colleges.json for easy retrieval

mutation {
   createStudent(collegeId:"col-2",firstName:"Tim",lastName:"George")
}
The above query will create a student object in student.json file. The query will return a unique identifier. The response of the query is as shown below −
{
   "data": {
      "createStudent": "SkQtxYBUm"
   }
}
To verify if the student object is created, we can use the studentById query. You can also open the students.json file from data folder to verify the id.
To use studentById query, edit the schema.graphql as given below −
type Query {
   studentById(id:ID!):Student
}

type Student {
   id:ID!
   firstName:String
   lastName:String
   collegeId:String
}
Edit the resolver.js file as given below −
const db = require('./db')
const Query = {
   studentById:(root,args,context,info) => {
      return db.students.get(args.id);
   }
}

const Mutation = {
   createStudent:(root,args,context,info) => {
      return db.students.create({collegeId:args.collegeId,
      firstName:args.firstName,
      lastName:args.lastName})
   }
}

module.exports = {Query,Mutation}
Given below is the query to get student by unique id returned from the mutation query −
{
    studentById(id:"SkQtxYBUm") {
    id
    firstName
    lastName
  }
}
The response from the server is as follows −
{
   "data": {
      "studentById": {
         "id": "SkQtxYBUm",
         "firstName": "Tim",
         "lastName":"George"
      }
   }
}
It is best practice to return an object in mutation. For example, the client application wants to fetch student and college details. In this case, rather than making two different requests, we can create a query that returns an object containing students and their college details.
Add a new method named addStudent which returns object in mutation type of schema.graphql.
Let us learn how to access the college details through student details. Add college type in the schema file.
type Mutation {
   addStudent_returns_object(collegeId:ID,firstName:String,lastName:String):Student

   createStudent(collegeId:ID,firstName:String,lastName:String):String
}

type College {
   id:ID!
   name:String
   location:String
   rating:Float
}

type Student {
   id:ID!
   firstName:String
   lastName:String
   college:College
}
Update a file resolvers.js in the project folder and add the following code −
const Mutation = {
   createStudent:(root,args,context,info) => {

      return db.students.create({
         collegeId:args.collegeId,
         firstName:args.firstName,
         lastName:args.lastName
      })
   },
   
   // new resolver function
   addStudent_returns_object:(root,args,context,info) => {
      const id = db.students.create({
         collegeId:args.collegeId,
         firstName:args.firstName,
         lastName:args.lastName
      })

      return db.students.get(id)
   }
}

//for each single student object returned,resolver is invoked
const Student = {
   college:(root) => {
      return db.colleges.get(root.collegeId);
   }
}

module.exports = {Query,Student,Mutation}
Next, we shall start the server and request query in GraphiQL with the following code −
mutation {
   addStudent_returns_object(collegeId:"col-101",firstName:"Susan",lastName:"George") {
      id
      firstName
      college{
         id
         name
      }
   }
}
The above query adds a new student and retrieves the student object along with college object. This saves round trips to the server.
The response is as given below −
{
   "data": {
      "addStudent_returns_object": {
         "id": "rklUl08IX",
         "firstName": "Susan",
         "college": {
            "id": "col-101",
            "name": "AMU"
         }
      }
   }
}

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 Nilay Mehta

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