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Constructing Types

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: ({ 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: (_, { 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.

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