GraphQL Custom Resolver Example

This is a simple example of using a custom resolver with Hasura's GraphQL API.

Motivation

Hasura GraphQL Engine provides instant GraphQL APIs over the tables and views of any Postgres database. It also comes with a fine grained access control layer that helps you restrict the data that can be consumed.

However, sometimes you might have to write custom resolvers to capture business logic that is unrelated to the database or needs to execute a custom transaction or write to the database.

In this example, we illustrate how to write custom resolvers and merge them with the Hasura GraphQL Engine. We combine Hasura GraphQL Engine's GraphQL API running at https://bazookaand.herokuapp.com/v1alpha1/graphql with the following custom resolvers:

1. A hello query
2. A count query (that returns a counter from another data source )
3. A increment_counter mutation that increments the value of count.
4. A user_average_age query that makes directly makes an SQL query to Postgres using knex.

You can use this as a boilerplate to write custom resolvers with Hasura GraphQL Engine.

Usage

1. Install the required dependencies.

npm install

2. Set appropriate environment variables for the GraphQL Engine URL, the access key to GraphQL Engine and the Postgres connection string.

# without the /v1apha1/graphql part
export HASURA_GRAPHQL_ENGINE_URL='https://hge.herokuapp.com'
export X_HASURA_ACCESS_KEY='<access_key>'

# Only required for the direct SQL resolver
export PG_CONNECTION_STRING='<postgres-connection-string>' 

3. Run the server

npm start

Deployment

You can deploy this sample boilerplate with:

• Now
• Docker

Deploy using Now

Run these commands to instantly deploy this boilerplate using Now.

git clone https://github.com/hasura/graphql-engine
cd community/boilerplates/custom-resolvers
now -e \
  HASURA_GRAPHQL_ENGINE_URL='https://hge.herokuapp.com' -e \
  X_HASURA_ACCESS_KEY='<access_key>' --npm

Deploy the docker image

This project comes with a Dockerfile.

Implementation Details

We will use Apollo's graphql-tools library to make a working GraphQL Schema out of our custom resolvers. Finally, we will merge these resolvers with the existing Hasura schema so that it can be queried under the same endpoint.

Writing type definitions

The type definitions are written in standard GraphQL format. We need the following queries in our custom logic:

type Query {
  # field hello will return "Hello World" which is a string
  hello: String,

  # field count will return an Int
  count: Int,

  # field user_average_age will return a Float
  user_average_age: Float
}

type Mutation {
  # field "increment_counter" will increment the counter and return type IncrementCounter
  increment_counter: IncrementCounter,

  # IncrementCounter simply returns the new value of the counter
  new_count: Int
}

Writing resolvers

Every resolver is a function that is executed with the following arguments in the order below:

1. root: The root of the current field
2. args: The arguments provided in the query
3. context: The server context, which also consists of headers
4. info: The AST document related to the query made

The resolvers in our case are:

const resolvers = {
  // resolvers for queries
  Query: {
    hello: (root, args, context, info) => {
      // return response
      return 'Hello world!';
    },
    count: (root, args, context, info) => {
      // return response
      return count;
    },
    user_average_age: async (root, args, context, info) => {
      // make SQL query using knex client
      const response = await knexClient('user')
        .avg('age');
      // return response
      return response[0].avg;
    }
  },

  // resolvers for mutations
  Mutation: {
    increment_counter: (root, args, context, info) => {
      // return response
      return { new_count: ++count };
    }
  }
};

Making a new schema out of these custom resolvers

Use makeExecutableSchema() function from the graphql-tools library to make a schema out of the type definitions and resolvers above.

import { makeExecutableSchema } from 'graphql-tools';

const executableCustomSchema = makeExecutableSchema({
  typeDefs,
  resolvers,
});

Merging with existing Hasura schema and serving it

Merge these custom resolvers with the Hasura GraphQL Engine by using the mergeSchemas() function from the graphql-tools library.

import { mergeSchemas } from 'graphql-tools';

const newSchema = mergeSchemas({
  schemas: [
    executableCustomSchema,
    executableHasuraSchema
  ]
});

const server = new ApolloServer({
  schema: newSchema
});

server.listen().then(({ url }) => {
  console.log(`Server running at ${url}`);
});

Check this file to see how it is done.