For many apps, you can define a fixed schema when the application starts, and define it using GraphQL schema language. In some cases, it's useful to construct a schema programmatically. You can do this using the GraphQLSchema
constructor.
When you are using the GraphQLSchema
constructor to create a schema, instead of defining Query
and Mutation
types solely using schema language, you create them as separate object types.
For example, let's say we are building a simple API that lets you fetch user data for a few hardcoded users based on an id. Using buildSchema
we could write a server with:
var express = require('express'); var graphqlHTTP = require('express-graphql'); var { buildSchema } = require('graphql'); var schema = buildSchema(` type User { id: String name: String } type Query { user(id: String): User } `); // Maps id to User object var fakeDatabase = { 'a': { id: 'a', name: 'alice', }, 'b': { id: 'b', name: 'bob', }, }; var root = { user: function ({id}) { return fakeDatabase[id]; } }; 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');
We can implement this same API without using GraphQL schema language:
var express = require('express'); var graphqlHTTP = require('express-graphql'); var graphql = require('graphql'); // Maps id to User object var fakeDatabase = { 'a': { id: 'a', name: 'alice', }, 'b': { id: 'b', name: 'bob', }, }; // Define the User type var userType = new graphql.GraphQLObjectType({ name: 'User', fields: { id: { type: graphql.GraphQLString }, name: { type: graphql.GraphQLString }, } }); // Define the Query type var queryType = new graphql.GraphQLObjectType({ name: 'Query', fields: { user: { type: userType, // `args` describes the arguments that the `user` query accepts args: { id: { type: graphql.GraphQLString } }, resolve: function (_, {id}) { return fakeDatabase[id]; } } } }); var schema = new graphql.GraphQLSchema({query: queryType}); var app = express(); app.use('/graphql', graphqlHTTP({ schema: schema, graphiql: true, })); app.listen(4000); console.log('Running a GraphQL API server at localhost:4000/graphql');
When we use this method of creating the API, the root level resolvers are implemented on the Query
and Mutation
types rather than on a root
object.
This is particularly useful if you want to create a GraphQL schema automatically from something else, like a database schema. You might have a common format for something like creating and updating database records. This is also useful for implementing features like union types which don't map cleanly to ES6 classes and schema language.