Schema Stitching in Apollo GraphQL doesn't resolve types from other parts - graphql

Question

I'm trying to make my GraphQL schema composable through schema stitching, but I'm struggling with how to resolve properties of types from a different part.
Here's the schema before decomposing:
type Referee {
id: ID!
stringProp: String!
}
type Referer {
id: ID!
pointer: Referee!
}
type Query {
referers: [Referer]
}
The types both have resolvers, in their respective schemas, that expand object { id } into { id, stringProp } or { id, pointer: { id } }, so that a query
query FromSingleSchema {
referers: {
id
pointer {
id
stringProp
}
}
}
resolves as expected; Query.referers resolves to a list of [{id}] objects, and each of those in turn resolve first into a Referer and then fetches the pointed-to Referee through type resolvers.
Now, I try to decompose the schema:
// schema A
type Referee {
id: ID!
stringProp: String!
}
// schema B
type Referer {
id: ID!
}
type Query {
referers: [Referer]
}
// schema Extensions
extend type Referer {
pointer: Referee!
}
and compose it again:
// both schemaA and schemaB have been created with makeExecutableSchema
import schemaA from './A'
import schemaB from './B'
// schemaExtensions is just a raw GraphQL string
// resolverExtensions is shown below
import { schemaExtensions, resolverExtensions } from './B'
const schema = mergeSchemas({
schemas: [schemaA, schemaB, schemaExtensions],
resolvers: Object.assign({}, resolverExtensions)
})
// resolverExtensions defined as follows:
{
Referer: {
pointer: {
fragment: 'fragment IdFragment on Referee { id }',
resolve: o => ({ id: o.pointerId })
}
}
}
With this, I can run this query without problems:
query OnlyIdFromDecomposedSchemas {
referers: {
id
pointer {
id
}
}
}
but this fails
query FullRefereeFromDecomposedSchemas {
referers: {
id
pointer {
id
stringProp
}
}
}
with the error message
Cannot return null for non-nullable field Referee.stringProp.
What do I need to do for the type resolver for Referee to be able to fill in the rest of the properties once { id } is available, like it does in a single, non-decomposed, schema?

Answer 1

I think you are looking for schema delegation. Schema Delegation is a way to automatically forward a query (or a part of a query) from a parent schema to another schema (called a subschema) that is able to execute the query.
You can use delegateToSchema method like this in your resolver:
{
Referer: {
pointer : {
resolve(parent, args, context, info) {
return info.mergeInfo.delegateToSchema({
schema: schemaA,
operation: 'query',
fieldName: 'referee', // modify according to your query for referee
context,
info,
});
}
}
}
}