neo4j-graphql-js.md

neo4j-graphql.js User Guide

What is neo4j-graphql-js

A package to make it easier to use GraphQL and Neo4j together. neo4j-graphql-js translates GraphQL queries to a single Cypher query, eliminating the need to write queries in GraphQL resolvers and for batching queries. It also exposes the Cypher query language through GraphQL via the @cypher schema directive.

Goals of neo4j-graphql.js

• Translate GraphQL queries to Cypher to simplify the process of writing GraphQL resolvers
• Allow for custom logic by overriding of any resolver function
• Work with graphql-tools, graphql-js, and apollo-server
• Support GraphQL servers that need to resolve data from multiple data services/databases
• Expose the power of Cypher through GraphQL via the @cypher directive

How it works

neo4j-graphql-js aims to simplify the process of building GraphQL APIs backed by Neo4j, embracing the paradigm of GraphQL First Development. Specifically,

• The Neo4j datamodel is defined by a GraphQL schema.
• Inside resolver functions, GraphQL queries are translated to Cypher queries and can be sent to a Neo4j database by including a Neo4j driver instance in the context object of the GraphQL request.
• Any resolver can be overridden by a custom resolver function implementation to allow for custom logic
• Optionally, GraphQL fields can be resolved by a user defined Cypher query through the use of the @cypher schema directive.

Start with a GraphQL schema

GraphQL First Development is all about starting with a well defined GraphQL schema. Here we'll use the GraphQL schema IDL syntax, compatible with graphql-tools (and other libraries) to define a simple schema:

const typeDefs = `
type Movie {
  movieId: ID!
  title: String
  year: Int
  plot: String
  poster: String
  imdbRating: Float
  similar(first: Int = 3, offset: Int = 0): [Movie] @cypher(statement: "MATCH (this)-[:IN_GENRE]->(:Genre)<-[:IN_GENRE]-(o:Movie) RETURN o")
  degree: Int @cypher(statement: "RETURN SIZE((this)-->())")
  actors(first: Int = 3, offset: Int = 0): [Actor] @relation(name: "ACTED_IN", direction:IN)
}

type Actor {
  id: ID!
  name: String
  movies: [Movie]
}


type Query {
  Movie(id: ID, title: String, year: Int, imdbRating: Float, first: Int, offset: Int): [Movie]
}
`;

We define two types, Movie and Actor as well as a top level Query Movie which becomes our entry point. This looks like a standard GraphQL schema, except for the use of two directives @relation and @cypher. In GraphQL directives allow us to annotate fields and provide an extension point for GraphQL. See GraphQL Schema Directive for an overview of all GraphQL schema directives exposed in neo4j-graphql.js

• @cypher directive - maps the specified Cypher query to the value of the field. In the Cypher query, this is bound to the current object being resolved. See Adding Custom Logic for more information and examples of the @cypher GraphQL schema directive.
• @relation directive - used to indicate relationships in the data model. The name argument specifies the relationship type, and direction indicates the direction of the relationship (IN for incoming relationships, OUT for outgoing relationships, or BOTH to match both directions). See the GraphQL Schema Design Guide for more information and examples.

Translate GraphQL To Cypher

Inside each resolver, use neo4j-graphql() to generate the Cypher required to resolve the GraphQL query, passing through the query arguments, context and resolveInfo objects.

import { neo4jgraphql } from 'neo4j-graphql-js';

const resolvers = {
  // entry point to GraphQL service
  Query: {
    Movie(object, params, ctx, resolveInfo) {
      return neo4jgraphql(object, params, ctx, resolveInfo);
    }
  }
};

GraphQL to Cypher translation works by inspecting the GraphQL schema, the GraphQL query and arguments. For example, this simple GraphQL query

{
  Movie(title: "River Runs Through It, A") {
    title
    year
    imdbRating
  }
}

is translated into the Cypher query

MATCH (movie:Movie {title:"River Runs Through It, A"})
RETURN movie { .title , .year , .imdbRating } AS movie
SKIP 0

A slightly more complicated traversal

{
  Movie(title: "River Runs Through It, A") {
    title
    year
    imdbRating
    actors {
      name
    }
  }
}

becomes

MATCH (movie:Movie {title:"River Runs Through It, A"})
RETURN movie { .title , .year , .imdbRating,
  actors: [(movie)<-[ACTED_IN]-(movie_actors:Actor) | movie_actors { .name }] }
AS movie
SKIP 0

@cypher directive

The @cypher directive feature has a dependency on the APOC procedure library, to enable subqueries. If you'd like to make use of the @cypher feature you'll need to install the APOC procedure library.

GraphQL is fairly limited when it comes to expressing complex queries such as filtering, or aggregations. We expose the graph querying language Cypher through GraphQL via the @cypher directive. Annotate a field in your schema with the @cypher directive to map the results of that query to the annotated GraphQL field. For example:

type Movie {
  movieId: ID!
  title: String
  year: Int
  plot: String
  similar(first: Int = 3, offset: Int = 0): [Movie]
    @cypher(
      statement: "MATCH (this)-[:IN_GENRE]->(:Genre)<-[:IN_GENRE]-(o:Movie) RETURN o ORDER BY COUNT(*) DESC"
    )
}

The field similar will be resolved using the Cypher query

MATCH (this)-[:IN_GENRE]->(:Genre)<-[:IN_GENRE]-(o:Movie) RETURN o ORDER BY COUNT(*) DESC

to find movies with overlapping Genres.

Querying a GraphQL field marked with a @cypher directive executes that query as a subquery:

GraphQL:

{
  Movie(title: "River Runs Through It, A") {
    title
    year
    imdbRating
    actors {
      name
    }
    similar(first: 3) {
      title
    }
  }
}

Cypher:

MATCH (movie:Movie {title:"River Runs Through It, A"})
RETURN movie { .title , .year , .imdbRating,
  actors: [(movie)<-[ACTED_IN]-(movie_actors:Actor) | movie_actors { .name }],
  similar: [ x IN apoc.cypher.runFirstColumn("
        WITH {this} AS this
        MATCH (this)-[:IN_GENRE]->(:Genre)<-[:IN_GENRE]-(o:Movie)
        RETURN o",
        {this: movie}, true) | x { .title }][..3]
} AS movie
SKIP 0

This means that the entire GraphQL request is still resolved with a single Cypher query, and thus a single round trip to the database.

Query Neo4j

Inject a Neo4j driver instance in the context of each GraphQL request and neo4j-graphql-js will query the Neo4j database and return the results to resolve the GraphQL query.

let driver;

function context(headers, secrets) {
  if (!driver) {
    driver = neo4j.driver(
      'bolt://localhost:7687',
      neo4j.auth.basic('neo4j', 'letmein')
    );
  }
  return { driver };
}

server.use(
  '/graphql',
  bodyParser.json(),
  graphqlExpress(request => ({
    schema,
    rootValue,
    context: context(request.headers, process.env)
  }))
);

Resources

• Blog post: Five Common GraphQL Problems and How Neo4j-GraphQL Aims To Solve Them - Digging Into the Goals of A Neo4j-GraphQL Integration