Federation extending a type but requiring a parent field

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1

Hi,

I want to extend a type to have more information from a subgraph to have some new fields but it requires an argument passed into a parent type. I will use an example below:

I have two subgraphs a book subgraph and a summary subgraph, the summary subgraph extends the chapter type. To achieve this though the summary graph needs access to the book id field and Im unsure the best approach.

# book graph
type Book {
  id: String
  chapters: Chapter
}

type Chapter @key(fields: "?"){
  title: String
  numberOfPages: Int
}

type Query {
  bookById(id: String!): Book
}


# summary graph

type Chapter @key(fields: "?"){
  summary: String
}

I could add an argument into the summary field for book id but this doesnt feel the best as its not strictly needed in the graph.

Alternatively I could add the bookId into the chapter.

I was looking for what is best practice?