GraphQL simple nested query returning null field - node.js

Question

I'm trying to learn GraphQL (& node.js & MongoDB etc.). I cant get this simple nested query to return results :
query getLocationByPerson {
People {
firstName
lastName
service {
location
}
}
}
The result I get is :
{
"data": {
"People": [
{
"firstName": "John",
"lastName": "DOE",
"service": null
}
]
},
"errors": [
{
"message": "Cannot return null for non-nullable field Service.location.",
"locations": [
{
"line": 6,
"column": 7
}
],
"path": [
"People",
0,
"service",
"location"
]
}
]
}
All the code is available here : https://github.com/fabricezerrouki/graphql-playground
Can anyone have a look and help me to figure out what I'm doing wrong?
Thanks.

Answer 1

The problem lies in the way you're setting the service attribute when you create and update a person. For nested types, you want to pass an object containing the subtypes. Right now you're passing it just a string, so when you try to set Service equal to args.serviceId in your createPerson resolver, it doesn't exist, and therefore sets the value of that field to null in MongoDB. Re-factor like so:
In index.js
type Mutation {
createPerson(Xid: String!, firstName: String */ ... */ service: { id: ID!, name: String!, location: String! }): People!
}
In /resolvers/Mutation.js
const Mutations = {
createPerson: async (parent, args) => {
const newPerson = new People({
Xid: args.Xid,
firstName: args.firstName
/* and so on... */
Service: args.service.id
});
}
}
Also, you may run into problems in the future given the way your saving the Service object to MongoDB as a Capital "S" Service, whereas your variable in the queries is lowercase "s" service. You'll have to remember any time you search the database to now directly destructure the the args.service into your Model.find({service}) method. Kind of a niche problem, but I've ran into that exact problem myself, so figured I would mention it.
Edited in response to OP comment
Yes it is possible to do an SQL join-link operation to return the Service object with the Person query, but you'll need to to that in the query resolver and just store the Service id during the mutation. In that case I guess the reason it's null is your calling args.serviceId but the createPerson() parameter is simply "service" so try changing Mutation.js to Service: args.service.
As for the actual join operation, that's a bit more complicated. I would recommend first returning the serviceId string to make sure that works correctly. Then you'll need to delve into the MongoDB aggregation pipeline. More specifically, the $lookup pipeline operator performs a function similar to an SQL join. I end up banging my head against the wall every time I have to deal with aggregation pipelines. It's a pretty advanced subject with a confusing syntax IMO. But there are plenty of tutorials and examples out there for doing exactly what your wanting.