Besides retrieving data, APIs also need to support changes or mutations to the data. There generally are three kinds of mutations:

• Creation of new data
• Update for existing data
• Deletion of data

Mutations follow the general structure of queries:

mutation { 
  createHotel(name: "Taj", noRooms: 30) { 
    id 
  } 
} 

Here the mutation has a createHotel root field, and it identifies the mutation uniquely. We are giving it the name and noRooms parameters, with the Taj and 30 values respectively. Like a query, the payload here describes what we want in terms of the properties of the newly created object; here we are asking for the ID of the new person.

Continuing with our hotel example in graphql-go, the mutation root object defines all the various mutations (update, create, delete, and so on). For example, a simple create mutation is defined as follows:

// root mutation 
var rootMutation = graphql.NewObject(graphql.ObjectConfig{ 
   Name: "RootMutation", 
   Fields: graphql.Fields{ 
         "createHotel": &graphql.Field{ 
               Type:        hotelType, 
               // the return type for this field 
               Description: "Create new hotel", 
               Args: graphql.FieldConfigArgument{ 
                     "displayName": &graphql.ArgumentConfig{ 
                           Type: graphql.NewNonNull(graphql.String), 
                     }, 
                     "city": &graphql.ArgumentConfig{ 
                           Type: graphql.NewNonNull(graphql.String), 
                     }, 
                     "noRooms": &graphql.ArgumentConfig{ 
                           Type: graphql.NewNonNull(graphql.Int), 
                     }, 
                     "starRating": &graphql.ArgumentConfig{ 
                           Type: graphql.NewNonNull(graphql.Int), 
                     }, 
               }, 
               Resolve: func(params graphql.ResolveParams) 
                  (interface{}, error) { 
                     // marshall and cast the argument value 
                     displayName, _:= 
                       params.Args["displayName"].(string) 
                     city, _:= params.Args["city"].(string) 
                     noRooms, _:= params.Args["noRooms"].(int) 
                     starRating, _:= params.Args["starRating"].(int) 
 
                     // create in 'DB' 
                     newHotel:= Hotel{ 
                           Id:          randomId(), 
                           DisplayName: displayName, 
                           City:        city, 
                           NoRooms:     noRooms, 
                           StarRating:  starRating, 
                     } 
                     hotels[newHotel.Id] = newHotel 
 
                     // return the new Hotel object 
                     return newHotel, nil 
               }, 
         }, 
   }, 
}) 

Here we use the preceding map described to store the hotel object, after generating an ID for hotel. The ID is generated using the randomId() helper function:

// Random ID Generator 
var letterRunes = []rune("abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ") 
 
func randomId() string { 
   b:= make([]rune, 8) 
   for i:= range b { 
         b[i] = letterRunes[rand.Intn(len(letterRunes))] 
   } 
   return string(b) 
} 

Note that this is not a good way of generating IDs—one big reason is the possibility of collisions. Generally when a DB is used, the ID is autogenerated as the primary key of the object.

The following CURL request shows how to create a hotel using the preceding definition:

curl -g 'http://localhost:8080/graphql?query=mutation+_{createHotel(displayName:"HotelX",city:"NY",noRooms:300,starRating:5){id}}'