Resolving nested entities in federation

[smikheiev]

Let's say there are types Foo, Bar and Baz looking like:

foo {
  bar {
    baz
  }
}

Type Foo is resolved by subgraph1 and types Bar and Baz are resolved by subgraph2. And subgraph1 calculates and passes IDs of Bar and Baz to subraph2.

So the schema looks something like this:

# subgraph1
type Query {
  foo: Foo!
}
type Foo {
  id: ID!
  bar: Bar!
}
type Bar @key(fields: "id", resolvable: false) {
  id: ID!
  baz: Baz! @shareable
}
type Baz @key(fields: "id barID", resolvable: false) {
  id: ID!
  barID: ID!
}

# subraph2
type Bar @key(fields: "id") {
  id: ID!
  baz: Baz! @shareable
  someBarField: String
}
type Baz @key(fields: "id barID") {
  id: ID!
  barID: ID!
  bazValue: String!
}

Resolver for the foo query is

Query: {
  foo() {
    return {
      id: 'foo-id-from-subgraph1',
      bar: {
        id: 'bar-id-from-subgraph1',
        baz: {
          id: 'baz-id-from-subgraph1',
          barID: 'bar-id-from-subgraph1',
        },
      },
    }
  }
}

and resolvers for Bar and Baz are:

Bar: {
  __resolveReference({ id }) {
    return {
      id,
      someBarField: 'blahblah',
    }
  },
},
Baz: {
  __resolveReference({ id, barID }) {
    return {
      id,
      barID,
      bazValue: 'baz-value-from-subgraph2',
    }
  },
},

Now, if I fetch foo with query

query foo1 {
  foo {
    id
    bar {
      id
      baz {
        id
        barID
        bazValue
      }
    }
  }
}

the result looks fine:

{
  "data": {
    "foo": {
      "id": "foo-id-from-subgraph1",
      "bar": {
        "id": "bar-id-from-subgraph1",
        "baz": {
          "id": "baz-id-from-subgraph1",
          "barID": "bar-id-from-subgraph1",
          "bazValue": "baz-value-from-subgraph2"
        }
      }
    }
  }
}

But if I add someBarField to the query:

query foo2 {
  foo {
    id
    bar {
      id
      someBarField
      baz {
        id
        barID
        bazValue
      }
    }
  }
}

it fails with an error Cannot return null for non-nullable field Bar.baz.

It looks like it's not going to Baz.__resolveReference. Eg, if I add baz field resolver

Bar: {
  __resolveReference({ id }) {
    console.log('__resolveReference for Bar')
    return {
      id,
      someBarField: 'blahblah',
    }
  },
  baz() {
    return {
      id: 'mock',
      barID: 'mock',
      bazValue: 'mock',
    }
  },
},

then query returns

{
  "data": {
    "foo": {
      "id": "foo-id-from-subgraph1",
      "bar": {
        "id": "bar-id-from-subgraph1",
        "someBarField": "blahblah",
        "baz": {
          "id": "baz-id-from-subgraph1",
          "barID": "bar-id-from-subgraph1",
          "bazValue": "mock"
        }
      }
    }
  }
}

bazValue is mock that's taken from baz field resolver 🤯

So if not to fetch someBarField field, then the nested entity of Baz is resolved correctly (using __resolveReference). But if to fetch it - then it's not being resolved.
I wonder if I missed something in the schema, or is the problem somewhere else?

[glasser]

Hi! Federation code such as the subgraph implementation @apollo/subgraph and the query planner lives in https://github.com/apollographql/federation now. That repo doesn't have discussions enabled so I can't directly transfer this, but I think this might be ok as an issue?