Normalised Cache Not Using Data from Different Query about apollo-ios HOT 3 CLOSED

Hi @reubn –

I took a quick look at your minimal test case, and I think that there's a bit of a mis-understanding of how the cache functions. Within your CountryDetailView, you're attempting to create an object of type CountryDetailQuery.Data.Country by fetching a CountryDetailQuery via the cache. However, as you've never fetched CountryDetailQuery before, you don't have a cache entry for that query.

As you said, you're really looking for the country field – which is being supplied by a shared fragment CountryInfo. You can, instead of storing CountryDetailQuery.Data.Country, store CountryInfo. And instead of fetching CountryDetailQuery, you can do a read operation on the cache for a CountryInfo.

Your fetch data function becomes something like this:

  func fetchData() {
    GraphQL.shared.apollo.store.withinReadTransaction { transaction in
      let countryInfo = try transaction.readObject(ofType: CountryInfo.self, withKey: "Country:\(code)")
      self.country = countryInfo
    }
  }

More important that having a fix (which may or may not work for your use-case) is understanding why what you're attempting to do fails.

Imagine, for a moment, that your CountryDataQuery was much more complex:

query CountryDetailQuery($code: ID!) {
  country(code: $code) {
    ...CountryInfo
    population
    officialLanguage
    currentLeader
    governmentType
    # etc
  }
}

At this point, there's a very clear divergence between the AllCountriesQuery.Country and CountryDataQuery.Country data types. Further, when we ask the cache to fetch a whole query, we must have executed that query before in the past. That's because of how queries are keyed.

For example, a CountryDetailQuery with code AD would be keyed as QUERY_ROOT.CountryDetailQuery(code: "AD"), with selections you have keyed similarly (QUERY_ROOT.CountryDetailQuery(code: "AD").country(code: "AD"). When you ask the cache to read for a query, there is no entry in the cache that resolves to that query, as we've never executed it. At its core, returnCacheDataDontFetch will resolve to code that's quite similar to the code I've written above for fetchData but will instead tell the transaction to read for the CountryDetailQuery, not the CountryInfo fragment.

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I'm closing this issue due to inactivity. If this is still unresolved, feel free to provide more information and we can re-open this, or create a new issue.

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