fixedMergedSchemas.js

const express = require('express')
const graphqlHTTP = require('express-graphql')
const { makeExecutableSchema, mergeSchemas } = require('graphql-tools')
const _ = require('lodash')
const app = express();

const standaloneResolvers = {
    Query: {
        standalone: (data) => {
            return { foo: "foo" } 
        }
    },
    Standalone: {
        foo: (data) => {
            return "Standalone.foo should always return this string"
        }
    }
}

const dependantResolvers = {
    Query: {
        dependant: () => {
            return { 
                dontNeedExternalTypeField: "baz"
            }
        }
    }
}

const StandaloneSchema = makeExecutableSchema({
    typeDefs: `
        type Query {
            standalone: Standalone
        }
        type Standalone {
            foo: String
        }
    `,
    resolvers: standaloneResolvers
})

const DependantSchema = makeExecutableSchema({
    typeDefs: `
        type Query {
            dependant: Dependant
        }

        type Dependant {
            dontNeedExternalTypeField: String
        }
    `,
    resolvers: dependantResolvers
})

const linkTypeDefs = `
    extend type Dependant {
        needExternalTypeField: Standalone
    }
`

const linkResolvers = (mergeInfo) => ({
    Dependant: {
        needExternalTypeField: {
            resolve(parent, args, context, info) {
                return mergeInfo.delegate('query', 'standalone', {}, context, info)
            }
        }
    }
})

const mergedSchema = mergeSchemas({
    schemas: [StandaloneSchema, DependantSchema, linkTypeDefs],
    resolvers: linkResolvers
})

app.use('/graphql', graphqlHTTP({
  schema: mergedSchema,
  graphiql: true
}))

app.listen(4000)