graphql-fields-list

graphql-fields-list

Add-on to work with GraphQLResolveInfo which helps to extract requested fields list for a particular object resolver. This helps to bypass requested fields data to underlying services or data sources to extract only those minimal parts of data which was requested by end-user.

TypeScript Included!!!

Install

npm i graphql-fields-list

With JavaScript:

const { fieldsList, fieldsMap } = require('graphql-fields-list');

With TypeScript:

import { fieldsList, fieldsMap } from 'graphql-fields-list';

Motivation and Usage

Let's assume we have the following GraphQL schema:

interface Node {
  id: ID!
}
type PageInfo {
  hasNextPage: Boolean!
  hasPreviousPage: Boolean!
  startCursor: String
  endCursor: String
}
type Query {
  node(id: ID!): Node
  viewer: Viewer
}
type User implements Node {
  id: ID!
  firstName: String
  lastName: String
  phoneNumber: String
  email: String
}
type UserConnection {
  pageInfo: PageInfo!
  edges: [UserEdge]
}
type UserEdge {
  node: User
  cursor: String!
}
type Viewer {
  users(
    after: String,
    first: Int,
    before: String,
    last: Int
  ): UserConnection
}

And using the query:

query UserNames query {
    viewer {
        users {
            pageInfo {
                startCursor
                endCursor
            }
            edges {
                cursor
                node {
                    id
                    firstName
                    lastName
                }
            }
        }
    }
}

Our goal is to extract and return ONLY id, firstName and lastName fields from the user data. To achieve that we would need to bypass required fields information to underlying service or database, for example, let's assume we want to select that kind of data from mongodb.

In this case we will need to implement a resolver which will fetch only requested fields from our database like this:

const { connectionFromArray } from 'graphql-relay';
const { fieldsList } = require('graphql-fields-list');
// ... assuming we implement resolver on 'viewer' node:
async resolve(src, args, context, info) {
    // we want to get a clue which user data fields are requested, so:
    const fields = fieldsList(info, { path: 'users.edges.node' });
    // RESULT: fields = ['id', 'firstName', 'lastName']
    // Now we can fetch from mongodb only required part of the data
    // instead of fetching entire user data document (assuming
    // userDb is initialized model of mongoose):
    const users = await userDb.find().select(fields.join(' ')).exec();
    return { viewer: { users: connectionFromArray(users, args) } };
}

In the example above we assume our user model in database contains the same field names as defined by a graphql request. BTW, in a real world, there could be a need to re-map field names from a graphql query to some different names stored in a database. For example, we would need to use automatically created _id field in mongodb as id field in a graphql request. This can be easily achieved specifying a transform map option:

const fields = fieldsList(info, {
    path: 'users.edges.node',
    transform: { id: '_id' },
});
// fields = ['_id', 'firstName', 'lastName']

By the way, in some particular cases there could be a need to retrieve a whole fields name hierarchy from a graphql request. This could be achieved using fieldsMap function:

const { fieldsMap } = require('graphql-fields-list');
// ... inside the resolver as we did above:
const map = fieldsMap(info);
/*
map = {
  users: {
    pageInfo: {
      startCursor: false,
      endCursor: false
    },
    edges: {
      cursor: false,
      node: {
        id: false,
        firstName: false,
        lastName: false
      }
    }
  }
}
*/

Function fieldsMap also accepts path optional argument, which allows to retrieve only a required part of the map:

const map = fieldsMap(info, 'users.pageInfo');
/*
map = {
  startCursor: false,
  endCursor: false
}
*/

For leafs of the fields tree it will return false value, which is usable when you need to detect that the end of a tree branch is reached during traversal.

Both fieldsMap and fieldsList works as expected with graphql query fragmentation, so can be safely used within any possible queries.

License

ISC Licence