I have written a custom merge function for the field products on type Session. It seems the merge function is only being called when I initialise the object Session:1 with its products, and not when I update products later using cache.modify.

My merge function:

const client = new ApolloClient({
  uri: 'http://localhost:8081/graphql',
  cache: new InMemoryCache({
    typePolicies: {
      Session: {
        fields: {
          products: {
            merge (existing, incoming) {
              // this is only being called on useQuery(HydrateSession), not useMutation(UpsertProduct)
              console.log('existing', JSON.stringify(existing, null, 2))
              console.log('incoming', JSON.stringify(incoming, null, 2))
              // remove duplicates when latestProduct has the same id as an existing product — [..., latestProduct]
              if (incoming.filter(p => p.id === incoming[incoming.length - 1].id).length > 1) return existing
              return incoming
            }
          }
        }
      }
    }
  })
})

Initialisation of Session:

const HydrateSession = gql`
  query {
    session {
      id
      products {
        id
      }
    }
  }
`

...

useQuery(HydrateSession)

Updating products later using cache.modify:

const UpsertProduct = gql`
  mutation UpsertProduct($product: ProductInput!) {
    upsertProduct(product: $product) {
      id
    }
  }
`

...

const [upsertProductMutation] = useMutation(UpsertProduct)

const onClick = async () => {
  await upsertProductMutation({
    variables: {
      product: {
        id: 2
      }
    },
    update: (cache, mutationResult) => {
      cache.modify({
        id: 'Session:1',
        fields: {
          products: previous => [...previous, mutationResult.data.upsertProduct]
        }
      })
    }
  })
}

I have a full working example here https://github.com/jsindos/apollo-play, run npm i and then start two separate processes with npm start and npm serve. After clicking the button triggering the mutation, the merge function is not run (as seen by the absence of console.log statements in the console).