Complete Introduction to Fullstack, Type-Safe GraphQL (feat. Next.js, Nexus, Prisma)
In this post, you'll learn how to buildโโfrom scratchโโan entirely type-safe, fullstack web app, using GraphQL with a database attached!
To follow along with the source code, clone this repo.
๐ Update 19/11/2020: The tutorial has been updated to use Nexus Schema as the Nexus Framework has been discontinued
Our tech stack
First, let's have a look at our tools of choice:
- TypeScript - the programming language for both backend and frontend
- React and Next.js - as the frontend framework and middle-end
- Urql GraphQL client - the GraphQL client on the frontend
- PostgreSQL - the database for the app
- Apollo Server - the server framework we'll use to serve the GraphQL API
- Nexus Schema - a library for building code-first GraphQL APIs
-
Prisma Client and
prisma db push
- a toolkit to change the database schema, access, and query the database (Note:prisma db push
is still in a preview state at the moment)
Let's get started! ๐
Step 0: Install VS Code extensions
Before we start, make sure that you have installed these VS Code extensions for syntax highlighting and auto-formatting - Prisma and GraphQL.
Step 1: Spin up a PostgreSQL database
The first thing you'll need is a PostgreSQL database instance to interact with during development.
There are many options for this, but Heroku allows us to host PostgreSQL databases for free with minimal setup required. Check out this post by Nikolas Burk guiding you through the process!
If you have Docker installed and would rather keep your development database local, you can also check out this video I did on how to do this with Docker Compose.
You will be able to get a PostgreSQL URI in this format:
postgresql://<USER>:<PASSWORD>@<HOST_NAME>:<PORT>/<DB_NAME>
Note: if you are using Heroku, you'll get a URI with
postgres
instead ofpostgresql
as the protocol. Both formats should work but we'd prefer to usepostgresql
.
When everything is set up properly, you're good to move on to the next step! ๐
Step 2: Create a Next.js project
Now, create a Next.js project with create-next-app
and enter the directory:
npx create-next-app my-awesome-app --use-npm -e with-typescript
cd my-awesome-app
Git should be automatically initialized by create-next-app
, and your project structure should look like this:
The
create-next-app
might not have the latest TypeScript package pre-installed. If you want to use the latest TypeScript features, runnpm install -D typescript@latest
after the project is bootstrapped.
Step 3: Install Nexus with Prisma
With the Next.js project ready, open a terminal window at the root of the application and install both Nexus Schema and Prisma.
For Prisma, we need @prisma/client
, @nexus/schema
and nexus-plugin-prisma
as a regular dependencies and @prisma/cli
as a dev dependency.
Regular dependencies:
npm i @prisma/client @nexus/schema nexus-plugin-prisma
Dev dependencies:
npm i @prisma/cli
Once the dependencies are installed, initialize Prisma in the project.
npx prisma init
This command will create a prisma
directory. If you have a look inside, you'll see a .env
file and a schema.prisma
file. The schema.prisma
file will hold the database model and the .env
file will hold the database connection string.
Because the database connection string contains sensitive information, it's a good practice to never commit this .env
file with Git, so make sure it's also added to the .gitignore
file.
Adjust the schema.prisma
file to include a User
model:
datasource db {
provider = "postgresql"
url = env("DATABASE_URL")
}
generator client {
provider = "prisma-client-js"
}
model User {
id String @default(cuid()) @id
name String
}
The schema file tells Prisma to use PostgreSQL as the database type, and the database connection URL is defined as an environment variable. It also defines a simple User
data model with an id
and a name
field.
Your project should currently look like this:
Step 4: Wire up Nexus Schema with Next.js
Nexus Schema is a library that allows us to build code-first GraphQL APIs. It's our responsibility to bring along a server to serve that API. For our purposes, we'll use Apollo Server.
There are several varieties of Apollo Server that are used for various purposes. For our project, we'll want apollo-server-mirco
as it's well-suited to serverless deployments.
npm install apollo-server-micro
To create a GraphQL endpoint, create a new file in your project at /pages/api/graphql.ts
. Thanks to the powerful API routes in Next.js, the GraphQL server will be accessible at http://our-app-domain/api/graphql
when the Next.js server is started.
In the /pages/api/graphql.ts
file, write the following boilerplate code:
import { ApolloServer } from 'apollo-server-micro';
// we'll create these in a second!
import { schema } from '../../graphql/schema';
import { createContext } from './../../graphql/context';
const apolloServer = new ApolloServer({
context: createContext,
schema,
tracing: process.env.NODE_ENV === 'development'
});
export const config = {
api: {
bodyParser: false
}
};
export default apolloServer.createHandler({
path: '/api/graphql'
});
Since everything inside the /pages/api/
directory is considered as an API route, it's a good idea to implement the actual schema and resolvers outside this directory.
Now, create a new directory in the project root called /graphql/
and two files within: /graphql/schema.ts
and /graphql/context.ts
.
Inside /graphql/schema.ts
, start by using the makeSchema
function to construct a GraphQL schema with Nexus. We'll also want to use nexus-plugin-prisma
with the CRUD feature enabled:
// graphql/schema.ts
import { objectType, queryType, mutationType, makeSchema } from '@nexus/schema';
import { nexusPrisma } from 'nexus-plugin-prisma';
import path from 'path';
const Query = queryType({
definition(t) {
t.string('hello', { resolve: () => 'hello world' });
}
});
export const schema = makeSchema({
types: [Query],
plugins: [nexusPrisma({ experimentalCRUD: true })],
outputs: {
typegen: path.join(process.cwd(), 'generated', 'nexus-typegen.ts'),
schema: path.join(process.cwd(), 'generated', 'schema.graphql')
},
typegenAutoConfig: {
contextType: 'Context.Context',
sources: [
{
source: '@prisma/client',
alias: 'prisma'
},
{
source: path.join(process.cwd(), 'graphql', 'context.ts'),
alias: 'Context'
}
]
}
});
The call to makeSchema
includes a property called plugins
. This is an array of any plugins we want to use with Nexus Schema and, in our case, we want to use nexus-plugin-prisma
. The configuration we have here tells the plugin to use the CRUD feature which is what allows us to have automatically-generated CRUD resolvers for our API. You can read more on the CRUD feature provided by Nexus Schema.
Next, initialize the PrismaClient
within /graphql/context.ts
and export a function to create the context in Apollo Server.
// graphql/context.ts
import { PrismaClient } from '@prisma/client';
const prisma = new PrismaClient();
export interface Context {
prisma: PrismaClient;
}
export function createContext(): Context {
return { prisma };
}
The file structure should now look something like this:
my-awesome-app/
โโ components/
โโ graphql/
โ โโ context.ts
โ โโ schema.ts
โโ interfaces/
โโ pages/
โโ prisma/
โ โโ .env
โ โโ schema.prisma
โโ utils/
โโ next-env.d.ts
โโ package-lock.json
โโ package.json
โโ tsconfig.json
With these files in place, run the application:
npx next dev
If you go to http://localhost:3000/api/graphql, you'll see the GraphQL Playground up and running (with our "hello world" schema)! ๐
Step 5: Implement your first GraphQL API
With the GraphQL server running in the background and the GraphQL Playground ready at http://localhost:3000/api/graphql, it's time to start implementing the API!
Step 5.1: Define an Object type
Start by defining a User
object type to reflect the database schema. Once defined, add it to the types
array in makeSchema
.
// graphql/schema.ts
import { objectType, queryType, makeSchema } from '@nexus/schema';
const User = objectType({
name: 'User',
definition(t) {
t.model.id();
t.model.name();
}
});
// ...
export const schema = makeSchema({
types: [User, Query]
// ...
});
If you are typing the code above instead of copy n' pasting, you will notice that VS Code will autocomplete the fields (id
, name
) that are available on the User
data model defined earlier in /prisma/schema.prisma
.
Tip: You can also always invoke intellisense with CTRL + SPACE, in case it doesn't automatically show up sometimes, super helpful!
Now, go back to the GraphQL Playground and toggle the Schema side panel - you will see a GraphQL object type User
is generated from the code you just wrote in the /graphql/schema.ts
file.
type User {
id: String!
name: String!
}
Step 5.2: Define the Query type
For the root Query
type, Nexus provides a queryType
function.
To query a list of existing users in the database, you can write a resolver for allUsers
field as follows:
const Query = queryType({
definition(t) {
t.list.field('allUsers', {
type: 'User',
resolve(_parent, _args, ctx) {
return ctx.prisma.user.findMany({});
}
});
}
});
You can do whatever you want in the resolve
function. The Prisma client for your database can be directly accessed as the db
property on the ctx
object. You can read more about the API of Prisma Client in its official documentation.
Tip: You can always use the intellisense feature in VS Code to explore the APIs of Nexus and Prisma Client!
In addition to manually writing resolvers, the Nexus-Prisma plugin conveniently exposes basic "read" operations on the database on t.crud
. The following code will let you find a User
(or a list of User
s) from the database directly.
const Query = queryType({
definition(t) {
t.list.field('allUsers', {
type: 'User',
resolve(_parent, _args, ctx) {
return ctx.prisma.user.findMany({});
}
});
t.crud.user();
t.crud.users();
}
});
The code above will generate a GraphQL root Query
type:
type Query {
allUsers: [User!]
user(where: UserWhereUniqueInput!): User
users(
skip: Int
after: UserWhereUniqueInput
before: UserWhereUniqueInput
first: Int
last: Int
): [User!]!
}
input UserWhereUniqueInput {
id: String
}
Notice that all the related Input
types are also generated for us for free! ๐ฏ
Step 5.3: Define the Mutation type
Similar to the Query
type, a Mutation
type can be defined with the mutationType
function.
๐ Let's have some fun and create a bigRedButton
mutation to destroy all user data in the database.
We also have access to the t.crud
helper here, which exposes the basic "create", "update" and "delete" operations on the database. We then must add Mutation
to the types
array in makeSchema
.
import { objectType, queryType, mutationType, makeSchema } from '@nexus/schema';
// ...
const Mutation = mutationType({
definition(t) {
t.field('bigRedButton', {
type: 'String',
async resolve(_parent, _args, ctx) {
const { count } = await ctx.prisma.user.deleteMany({});
return `${count} user(s) destroyed. Thanos will be proud.`;
}
});
t.crud.createOneUser();
t.crud.deleteOneUser();
t.crud.deleteManyUser();
t.crud.updateOneUser();
t.crud.updateManyUser();
}
});
// ...
export const schema = makeSchema({
types: [User, Query, Mutation]
// ...
});
This will generate a GraphQL schema like below:
type Mutation {
bigRedButton: String
createOneUser(data: UserCreateInput!): User!
deleteOneUser(where: UserWhereUniqueInput!): User
deleteManyUser(where: UserWhereInput): BatchPayload!
updateOneUser(data: UserUpdateInput!, where: UserWhereUniqueInput!): User
updateManyUser(
data: UserUpdateManyMutationInput!
where: UserWhereInput
): BatchPayload!
}
Now, our simple but fully-featured GraphQL API is ready! ๐ฅณ
Step 6: Initialize the database
Before you can do anything with your GraphQL API, you'll need to create tables in the database corresponding to the Prisma schema file.
This can be done by manually connecting to the database and running SQL commands, but I'll show you how to do it with the prisma db push
commnand - the database tool that's part of Prisma 2.
First, save the initial changes of our schema.prisma
file with the command below. At the moment, the prisma db push
command is still in a preview state, so the extra flag --preview-feature
flag is needed.
npx prisma db push --preview-feature
Awesome! With the database prepared, it's time to go back to http://localhost:3000/api/graphql, and have some fun with the your first GraphQL API with Nexus. Let me give you an example to play with!
mutation {
createOneUser(data: { name: "Alice" }) {
id
}
}
Step 7: Set up Urql GraphQL client with Next.js
We'll use Urql as the GraphQL client on the frontend, but you can use any library you like.
First, install the dependencies:
npm install graphql-tag next-urql react-is urql isomorphic-unfetch
Then, create a new file at /pages/_app.tsx
. This is a special Next.js component that will be used to initialize all pages.
import React from 'react';
import { withUrqlClient, NextUrqlAppContext } from 'next-urql';
import NextApp, { AppProps } from 'next/app';
import fetch from 'isomorphic-unfetch';
// the URL to /api/graphql
const GRAPHQL_ENDPOINT = `http://localhost:3000/api/graphql`;
const App = ({ Component, pageProps }: AppProps) => {
return <Component {...pageProps} />;
};
App.getInitialProps = async (ctx: NextUrqlAppContext) => {
const appProps = await NextApp.getInitialProps(ctx);
return { ...appProps };
};
export default withUrqlClient((_ssrExchange, _ctx) => ({
url: GRAPHQL_ENDPOINT,
fetch
}))(
// @ts-ignore
App
);
And that's it! Now you can use the GraphQL client in any page in your Next.js app.
Step 8: Use the GraphQL client
First, create a TSX file at /components/AllUsers.tsx
. This file will have a component that performs an allUsers
GraphQL query and renders the result as a list. This way, we can use the component to fetch all the user info from our PostgreSQL database.
You can create the query first, for example, with the following code. By using gql
, the GraphQL VS Code extension will be able to identify the template string as a GraphQL query and apply nice syntax highlighting to it.
import React from 'react';
import gql from 'graphql-tag';
import { useQuery } from 'urql';
const AllUsersQuery = gql`
query {
allUsers {
id
name
}
}
`;
Since it's known that the data you are going to get is an array of User
objects (thank you, GraphQL schema!), you can also define a new type:
type AllUsersData = {
allUsers: {
id: string;
name: string;
}[];
};
Next, create the React component that will be using the query.
The component encapsulates the following logic:
- If the query is still in a fetching state, the text "Loading..." will be returned
- If an error is occurred during the process, we will display the error
- If the query is no longer fetching and there's no error, the data will be used to render a list of users
const AllUsers: React.FC = () => {
const [result] = useQuery<AllUsersData>({
query: AllUsersQuery
});
const { data, fetching, error } = result;
if (fetching) return <p>Loading...</p>;
if (error) return <p>Oh no... {error.message}</p>;
return (
<div>
<p>There are {data?.allUsers.length} user(s) in the database:</p>
<ul>
{data?.allUsers.map(user => (
<li key={user.id}>{user.name}</li>
))}
</ul>
</div>
);
};
export default AllUsers;
Now, save the TSX file, and mount it onto the home page /pages/index.tsx
:
import Link from 'next/link';
import Layout from '../components/Layout';
import AllUsers from '../components/AllUsers';
const IndexPage = () => (
<Layout title="Home | Next.js + TypeScript Example">
<h1>Hello Next.js ๐</h1>
<p>
<Link href="/about">
<a>About</a>
</Link>
</p>
{/* === Tada! === */}
<AllUsers />
</Layout>
);
export default IndexPage;
Time to spin up the Next.js dev server!
npm run dev
Voilร ! The user list is rendered! ๐ฅณ
Step 9: Auto-generate useQuery
hooks and types
Instead of manually defining all the types we expect to receive via GraphQL, we can also use a very cool package GraphQL Code Generator to generate types directly from the Nexus GraphQL endpoint. This way, you essentially only have to define the types once in the schema.prisma
file as the single-source-of-truth, then all types you'll use in the application can be derived from that schema with little manual effort! ๐
First, copy and refactor the GraphQL queries from the TSX files into the graphql
directory. With the example from Step 8, create a new file at /graphql/queries.graphql.ts
and copy the query from /components/AllUsers.tsx
:
import gql from 'graphql-tag';
export const AllUsersQuery = gql`
query AllUsers {
allUsers {
id
name
}
}
`;
Separating GraphQL operations from components makes it easier to navigate the codebase.
Next, install the packages needed by graphql-code-generator
as dev dependencies:
npm install -D \
@graphql-codegen/cli \
@graphql-codegen/typescript \
@graphql-codegen/typescript-operations \
@graphql-codegen/typescript-urql
Then, create a codegen.yml
file in the project root with the following content:
overwrite: true
schema: 'http://localhost:3000/api/graphql' # GraphQL endpoint via the nexus dev server
documents: 'graphql/**/*.graphql.ts' # parse graphql operations in matching files
generates:
generated/graphql.tsx: # location for generated types, hooks and components
plugins:
- 'typescript'
- 'typescript-operations'
- 'typescript-urql'
config:
withComponent: false # we'll use Urql client with hooks instead
withHooks: true
The configs above will tell graphql-code-generator
to pull the GraphQL schema from http://localhost:3000/api/graphql
, then generate types, Urql useQuery
hooks into a file located at /generated/graphql.tsx
.
Cool, let the code generation begin (in watch mode)!
npx graphql-codegen --watch
You will see some nice, written-by-a-robot code in /generated/graphql.tsx
. How neat!
Now, you can go back to components/AllUsers.tsx
, and replace the manually written AllUsersData
type, the GraphQL query, and the useQuery
hook, with what's in the /generated/graphql.tsx
file:
import React from 'react';
import { useAllUsersQuery } from '../generated/graphql';
const AllUsers: React.FC = () => {
const [result] = useAllUsersQuery();
const { data, fetching, error } = result;
if (fetching) return <p>Loading...</p>;
if (error) return <p>Oh no... {error.message}</p>;
return (
<div>
<p>There are {data?.allUsers?.length} user(s) in the database:</p>
<ul>
{data?.allUsers?.map(user => (
<li key={user.id}>{user.name}</li>
))}
</ul>
</div>
);
};
export default AllUsers;
Revisit the index page of the app at http://localhost:3000, everything works like a charm! ๐
To make the development experience even better, let's optimize the NPM scripts for the project.
First, install the Concurrently NPM module, which is a great tool for running multiple CLI watchers at the same time:
npm install -D concurrently
Then, replace dev
script in the package.json
file with the following:
{
// ...
"scripts": {
// ...
"dev": "concurrently -r \"npx nexus dev\" \"npx next\" \"npx graphql-codegen --watch\""
// ...
}
// ...
}
Now, we can use a single npm run dev
command to launch Nexus, Next.js, and GraphQL Code Generator, all at the same time!
Conclusion
I hope you have enjoyed this tutorial and have learned something useful! You can find the source code in this GitHub repo.
Also, check out the Awesome Prisma listย for more tutorials and starter projects in the Prisma ecosystem!
Top comments (30)
This is a great tutorial!
Any chance you could do another post using this same setup but also including RBAC authentication (maybe with next-auth.js.org/) or some other httpOnly cookie solution?
Sounds like a good idea for a followup post! I'll probably use the nextjs-auth0 package instead of
next-auth
, or a roll-your-own solution.I'm definitely voting for a roll-your-own tutorial!
Thanks for the great post!
Awesome tutorial, Xiaoru. Thank you.
Glad you liked it! ๐
OMG, I'm sorry to bother again but I when I define the graphql schema, I have these extra scalars and t.crud.user()/users() is not generating the Query properly, is not generating the input at all and two scalars are showing up. I defined a dateTime scalar in a previous project. Is this a caching problem?
Could you provide more details of the error message? Also could you check the versions of your Nexus and Nexus Prisma plugin? There was an update for Nexus after the initial version of this post, and the CRUD helpers now are disabled by default.
I updated this tut and the example repo, please check out this PR and let me know if it solves the problems!
BTW there's no global caching as far as I know, so there shouldn't be a problem if you defined some types in another project.
I tried to run "npx nexus dev" at the start of the tutorial, I have recreated the project twice but I keep on getting this error:
4596 โ nexus:glocal The global Nexus CLI you invoked could not hand off to your project-local one becuase it wasn't on disk.
This can happen for example after you have cloned a fresh copy of your project from a remote repository.
Please install your dependencies and try your command again.
Location of the Nexus CLI you invoked: C:\Users\natha\Documents\Web Development\NEXT\nexus-prisma\node_modules\nexus\dist\cli\main.js
Location of your project-local Nexus CLI: C:\Users\natha\Documents\Web Development\NEXT\nexus-prisma\node_modules.bin\main.js
Try installing the versiรณn 0.26.0-next.5 of nexus
Looks like this can happen on Windows. Are you using the native Windows cmd.exe or WSL?
Windows cmd... I should probably migrate over to WSL
Windows support is coming soon to Nexus! However, I'd still recommend you migrate over to WSL or use a Linux dev container of some kind, because most web dev tools and frameworks are created Unix-first.
I'm in the process of downloading WSL 2 now ๐ I was going to make the switch anyway but just put it off. I'll try your tute again once it's set up ๐
Just to follow up, I decided to move to Linux and everything worked well!
Thanks so much for the amazing tutorial, it's awesome now that I've made it :P
(Maybe add a disclaimer for Windows users that they might have trouble, if you want)
Nice, great to hear that! :D
Man this Tutorial is great, very well explained with advanced features that a newbie like me could understand, great Job!
I just want to let you know that I was very confused about the fact that I changed my schema.prisma and the Nexus didn't process the change, I had a big trouble until I understand I had to call
npx prisma generate
to reload the content, so you might want to add this little note on the tutorial, idk.Also, I have a question: The useAllUsersQuery is being processed via SSR by Next.js right? That is awesome because I don't need to type
getServerSideProps
. But who does this magic? And can I change this behaviour to work likegetStaticProps
?Thank you for this amazing work you did, this is going to be the basis for a lot of projects in my company <3
Curious about this as well - this code reads like all the GQL queries are happening client-side. Is that correct?
Hello @hexrcs can you help us on this? Pleeease ๐ Thanks
Thanks for the tutorial, lot's of great info!
I'm confused about something though. You said that using
graphql-code-generator
our source of truth isschema.prisma
.Maybe I'm doing something wrong but I still need to define my objectTypes, queryTypes, mutationTypes and then have my basic queries somewhere right?
And the queries:
I feel that I'm writing a lot of redundant code
Hi there! The source of truth of GraphQL is not
schema.prisma
, but the GraphQL endpoint powered by Nexus.You still need to clearly define how the GraphQL schema looks like (eg. object types), because of security (you don't want to expose everything from your DB, think ID and password) and flexibility (you can write your own resolver instead of being restricted to basic CRUD operations on the database).
However, the Nexus-Prisma plugin provides some simple
.crud
helpers as you can see from the examples, if you just want to directly expose CRUD operations to your GraphQL API. ๐Hey mate, great tutorial. There is one shortcoming in this setup. You are no longer able to spread your nexus schema definition into multiple files. One of the big advantages of the nexus framework is that you no longer need to pull all the dependencies into the schema file manually. This continues to work on localhost:4000/graphql but it no longer works for localhost:3000/api/graphql. The latter endpoint only continues to work when all the objectType, queryType, mutationType, etc. are defined in graphql/schema.ts.
You can try by adapting your repo. Just try to put the objectType for user in a different file. It will break the localhost:3000/api/graphql while localhost:4000/graphql remains in tact. Do you know a workaround for this!
Hi there, thanks for the comment! I'm actually not able to reproduce this - perhaps you forgot to import the files where other parts of the schema are defined?
What I did:
objectType
User
to another file located atgraphql/newFile.ts
- next to the originalschema.ts
import "./newFile.ts"
at the beginning ofgraphql/schema.ts
Note that
schema
from Nexus (is a singleton I believe) is also imported innewFile.ts
, and nothing is exported in that file. Here's what I have:In JS/TS, if you import a file, it is scanned through and run by the interpreter, so
schema.objectType()
is actually executed when the interpreter is done with the lineimport "./newFile.ts"
, so it's not static.Yes, that's exactly what I am talking about,
you now have to write import statements for all schema related files in
graphql/schema.ts
.If you do that then both GraphQL endpoints (localhost:4000/graphql + localhost:3000/api/graphql) work. However, when you omit the manual import statements in
graphql/schema.ts
it still continues to work atlocalhost:4000/graphql
, however it no longer works forlocalhost:3000/api/graphql
. For me not dealing with imports ingraphql/schema.ts
seemed like a nice behaviour of the nexus framework. I was trying to understand if I could avoid the import statements somehow in nextjs localhost:3000/api/graphql?I'm not sure why it would continue to work with Nexus dev server's
/graphql
route, but my guess is that it's due to some caching not being invalidated by Nexus (probably a bug, I'll try to dig into it).The correct behavior should be it fails as well. :)Edit: Normally, Nexus actually scans the entire project for
import {schema} from 'nexus'
, if an entry point is not specified.I think the problem you are encountering is because somehow Nexus imported the external schema after the Nexus
app
has assembled. Actually, when working with Next.js, it's not recommended to have multiple files that import the same part (like use, schema) from Nexus.Awesome tutorial! Are you going to be finishing up your egghead video series on Prisma?
Working on it! :D
Thanks so much, I absolutely love your videos on Prisma!
Wonderful tutorial, Xiaoru, how you have opened my eyes to a great deal of things and forced me to transition from prisma 1 to prisma 2 and God alone knows how my work has been simplified. Esp with the graphql-codegen. My heartfelt congratulations for the work.
This tutorial is awesome but I have a question how we can pass request, response within the context and do I have to pass in both Apollo server and nexus makeSchema. Please help me out. Recently I found this hard to understand the logic behind of context.