JSON Schema one-to-many relation, in MongoDB Atlas? - mongodb

Question

I'm stumped trying to get a one-to-many relationship established in MongoDB Atlas, via JSON Schema (for use in a GraphQL API).
This is in the Atlas App Services > Data Access > Schema > "Collections" tab.
(It's possible I'm thinking about this wrong, any ideas appreciated.)
So, I have two collections: Node and URL. Each Node can have one or more URLs.
Each URL has three properties: _id (primary key), url, and node:
// URLs (JSON Schema)
{
"title": "url",
"properties": {
"_id": {
"bsonType": "objectId"
},
"node": {
"bsonType": "string"
},
"url": {
"bsonType": "string"
}
}
}
Each URL is matched to a Node (URL.node === Node._id):
// URLs relationships (JSON Schema)
{
"node": {
"ref": "#/relationship/db_cluster/db_name/nodes",
"foreignKey": "_id",
"isList": false
}
}
Each Node has two properties of its own, _id (primary key) and name, and urls which should be an array of linked documents (from the URLs Collection, of course).
// Nodes (JSON Schema)
{
"title": "node",
"properties": {
"_id": {
"bsonType": "string"
},
"name": {
"bsonType": "string"
},
"urls": {
"bsonType": "array",
"items": {
"bsonType": "object",
"properties": {
"_id": {
"bsonType": "objectId"
},
"node": {
"bsonType": "string"
},
"url": {
"bsonType": "string"
}
}
}
}
}
}
So far so good. The above seems to work OK.
The trouble begins when I try and link the two Collections together. I am attempting to link a few URLs to each Node, with this relationship:
// Nodes relationships (JSON Schema)
{
"urls": {
"ref": "#/relationship/db_cluster/db_name/urls",
"foreignKey": "_id",
"isList": true
}
}
However, I can't save the above relationship. I get an error
relationship source property type must match foreign property type
Any ideas? Thanks.