Why does the gateway expand interface fragment into multiple implementations fragments? (federation)

For example we have SDL with interface and multiple implementations:

interface Node{
    id: ID!
}

type Brand implements Node{
    id: ID!
    title: String!
}

type Department implements Node{
    id: ID!
    title: String!
}

type Query{
    brand: Brand
}

Then query

{
    brand {
        ... on Node {
            id
        }
    }
}

will be transformed into

Fetch(service: "data") {
    {
      brands {
        ... on Brand {
          id
        }
        ... on Department {
          id
        }
    }
}

So, the result will be error “Fragment cannot be spread here as objects of type "Brand" can never be of type "Department".”

Why don’t we pass fragments as is?

[vitramir]

Looks like I have fixed this in last commit

[rtymchyk]

@vitramir Ah ok, my bad, got confused thinking there was an app-specific fix. The PR makes sense and fixes the problem in our use case.