Graphql Queries and Mutations

Introduction

We are much familiar that GraphQL is a query language. It is a server-side runtime that is normally served over HTT. We use GraphQL to query data starting any number of changed sources. Here we discuss Graphql Queries and Mutations

In this post, we’re going to discuss the GraphQL mutations operation and try to cover what mutations are, how they compare to queries, and when to use them

Description

There are only two types of operations we can perform in GraphQL:

  • Queries : We use queries to fetch data
  • Mutations: We use mutations to adjust server-side data
  • Mutations signify the state-changing methods in REST for example DELETE, PUT, PATCH, if queries are the GraphQL equivalent to GET calls in REST.
  • Discussions of GraphQL emphasis on data fetching. Though any comprehensive data platform requires a way to modify server-side data as well.
  • Any request might wind up producing some side-effects on the server in REST.
  • On the other hand, with convention it’s optional that one doesn’t use GET requests to adjust data.
  • GraphQL is related a technically any query could be applied to cause a data write.
  • Though, it’s valuable to found a convention that any processes that cause writes should be sent openly by a mutation.
  • We can ask for nested fields, same as in queries, if the mutation field returns an object type.
  • This may be suitable for fetching the new state of an object after an update.

Example:

mutation CreateReviewForEpisode($ep: Episode!, $review: ReviewInput!) {

  createReview(episode: $ep, review: $review) {

    stars

    commentary

  }

}

{

  "ep": "JEDI",

  "review": {

    "stars": 5,

    "commentary": "This is a great movie!"

  }

}

{

  "data": {

    "createReview": {

      "stars": 5,

      "commentary": "This is a great movie!"

    }

  }

}
  • Important to note that in what way createReview field returns the stars and commentary fields of the newly made review.
  • This is particularly useful when mutating current data.
  • For instance, once incrementing a field.
  • For now, we can mutate & query the new value of the field by one request.
  • In this illustration, the review variable we passed in is not a scalar.
  • It’s an input object type. That is a special kind of object type and can be passed in as an argument.

Difference between query and mutation

  • One factual difference between a query and a mutation is at any rate structurally.
  • That difference is the operation type.
  • We practice the word query for queries.
  • We use the word mutation for mutations.
  • One more essential thing to note is that we signal the mutation.
  • We need to invoke by naming it correctly as it happens within our server-side GraphQL API.

Mutations and Input Types

  • We should make an API endpoint a Mutation rather than a Query that alters data. For example inserting data into a database or altering data already in database.
  • This is as modest as creating the API endpoint part of the top-level Mutation type as an alternative of the top-level Query type.
  • Suppose that we have a message of the day server. Anyone may update the message of the day, and anybody can read the current one. The GraphQL schema for this is only:
type Mutation {

setMessage(message: String): String

}

type Query {

getMessage: String

}
  • It’s every so often suitable to have a mutation that maps to a database create or update operation.
  • For example setMessage, yield the same thing that the server stored.
  • The client may learn about those modifications that way, if we adjust the data on the server.
  • Together mutations and queries may be handled by root resolvers.
  • Therefore, the root that implements this schema can just be:
var fakeDatabase = {};

var root = {

setMessage: ({message}) => {

   fakeDatabase.message = message;

   return message;

},

getMessage: () => {

   return fakeDatabase.message;

}

};
  • We don’t requisite anything more than this to implement mutations.
  • We will find many diverse mutations that all accept the same input parameters.
  • Easy illustration is that making an object in a database and updating an object in a database normally take the same parameters.
  • We may use input types for this to make the schema simpler. That can happen by using the input keyword as an alternative of the type keyword.

Example

  • Suppose we have numerous messages instead of a single message of the day.
  • Those are indexed in a database by the id field.
  • Each message has together a content string and an author string.
  • We desire a mutation API together for making a new message and for updating an old message. We may use the schema:
input MessageInput {

content: String

author: String

}

type Message {

id: ID!

content: String

author: String

}

type Query {

getMessage(id: ID!): Message

}

type Mutation {

createMessage(input: MessageInput): Message

updateMessage(id: ID!, input: MessageInput): Message

}
  • Mutations return a Message type here so that the client may get more information.
  • That information is about the newly-modified Message in the similar request as the request that mutates it.
  • Like other objects the Input types can’t have fields.
  • Naming input types by Input on the end is a useful convention.
  • As we will repeatedly want together an input type and an output type that are somewhat different for a single conceptual object.
  • Keeping the data in memory following is approximately runnable code that implements this schema:
var express = require('express');

var { graphqlHTTP } = require('express-graphql');

var { buildSchema } = require('graphql');

// Make a schema, using GraphQL schema language

var schema = buildSchema(`

input MessageInput {

   content: String

   author: String

}

type Message {

   id: ID!

   content: String

   author: String

}

type Query {

   getMessage(id: ID!): Message

}

type Mutation {

   createMessage(input: MessageInput): Message

   updateMessage(id: ID!, input: MessageInput): Message

}

`);

// If Message had any complex fields, we'd put them on this object.

class Message {

constructor(id, {content, author}) {

   this.id = id;

   this.content = content;

   this.author = author;

}

}

// Maps username to content

var fakeDatabase = {};


var root = {

getMessage: ({id}) => {

   if (!fakeDatabase[id]) {

     throw new Error('no message exists with id ' + id);

   }

   return new Message(id, fakeDatabase[id]);

},

createMessage: ({input}) => {

   // Create a random id for our "database".

   var id = require('crypto').randomBytes(10).toString('hex');


   fakeDatabase[id] = input;

   return new Message(id, input);

},

updateMessage: ({id, input}) => {

   if (!fakeDatabase[id]) {

     throw new Error('no message exists with id ' + id);

   }

   // This replaces all old data, but some apps might want partial update.

   fakeDatabase[id] = input;

   return new Message(id, input);

},

};

var app = express();

app.use('/graphql', graphqlHTTP({

schema: schema,

rootValue: root,

graphiql: true,

}));

app.listen(4000, () => {

console.log('Running a GraphQL API server at localhost:4000/graphql');

});


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