How to merge a type whose fields come from two different GraphQL subschemas? - graphql

Question

I'm running a gateway with apollo-server v2 with graphql-tools v7 and have the following services and subschemas, which I've set up as a (contrived) proof-of-concept:
Service 1:
Query {
getUser(input: GetUserInput!): User
}
input GetUserInput {
id: ID!
}
type User {
id: ID!
contacts: Contacts
}
type Contacts {
id: ID!
primary: String
}
Service 2:
Query {
getUser(input: GetUserInput!): User
}
input GetUserInput {
id: ID!
}
type User {
id: ID!
contacts: Contacts
}
type Contacts {
id: ID!
secondary: String
}
The gateway has the combined schema like this:
Query {
getUser(input: GetUserInput!): User
}
input GetUserInput {
id: ID!
}
type User {
id: ID!
contacts: Contacts
}
type Contacts {
id: ID!
primary: String
secondary: String
}
The gateway is configured to stitch the schemas and merge the User type like this:
import { stitchSchemas } from "#graphql-tools/stitch"
...
const schema = stitchSchemas({
subschemas: [
{
schema: service1Schema,
executor: service1Executor,
merge: {
User: {
fieldName: "getUser",
selectionSet: "{ id }",
args: (originalObject) => ({
input: {
id: originalObject.id
}
}),
},
}
},
{
schema: service2Schema,
executor: service2Executor,
merge: {
User: {
fieldName: "getUser",
selectionSet: "{ id }",
args: (originalObject) => ({
input: {
id: originalObject.id
}
}),
},
}
},
]
})
However, when I send a query to fetch the contacts field for a User, only Service 2 is called by the gateway. Instead, I would have expected the gateway to call Service 1 to get its representation of User.contacts and also Service 2 to get its representation of User.contacts and then merge the two into a combined result.
The query I executed:
query GetContacts {
getUser(input: { id: "123" }) {
contacts {
primary
secondary
}
}
}
The result I got (and I see in logs that Service 1 was not called at all):
{
"data": {
"getUser": {
"contacts": {
"primary": null,
"secondary": "Secondary contact from Service 2"
}
}
}
}
The result I expected:
{
"data": {
"getUser": {
"contacts": {
"primary": "Primary contact from Service 1", <-- This should be included in the result.
"secondary": "Secondary contact from Service 2"
}
}
}
}
I have confirmed that I'm able to fetch other fields (e.g. User.name) from Service 1 successfully, so the subschema for Service 1 is configured correctly on the gateway (though probably not the merge options for it).
Since this is similar to the example from the graphql-tools documentation, I'm confused about why the fields of the Contacts type aren't merged as expected in the query response.