Confused why returnPartialData works without a field policy in Apollo Client 3 - graphql

Question

In my application I am searching for products, then clicking into a product to see more detail about it.
I perform a GraphQL query on each page. The SEARCH query returns type [Product], and the PRODUCT query returns type Product.
// Search page
const SEARCH = gql`
query Search($query: String!) {
searchResults: search(query: $query) {
id
name
images
price
}
}
`
// ProductDetail page
const PRODUCT = gql`
query Product($id: Int!) {
product(id: $id) {
id
name
images
optionSetName
options {
id
images
name
}
price
}
}
`
I have enabled returnPartialData on the PRODUCT query, as some of the fields for that product already exist in the cache from the SEARCH query, and I would like to access them before the server request returns.
I thought I would also have to apply a field policy to reference the pre-existing Product, as I don't know how PRODUCT even knows what its return type is.
However, when I do the following:
const { loading, data: { product } = {} } = useQuery(
PRODUCT,
{ variables: { id: productId, isShallow }, returnPartialData: true }
)
console.log(product)
the following is logged to console (the first is from returnPartialData, the second from server):
Somehow the PRODUCT query has associated itself with the existing Product, without me explicitly writing a cache redirect.
I'm confused how this has occurred? It seems like Apollo must have a reference to the GraphQL schema, and has seen the return type of PRODUCT is Product, then automatically used the id arg to reference the existing product.
Using "#apollo/client": "^3.4.1"

Answer 1

Wow, turns out I had made a field policy ages ago and forgotten about it... xD
typePolicies: {
Query: {
fields: {
product: {
read (_, { args, toReference }) {
return toReference({
__typename: 'Product',
id: args.id
})
}
}
}
}
}