Fragments on unions and interfaces

[alexeygolev]

There is no way to use union type or interfaces with the current client implementation as apollo requires an instance of IntrospectionFragmentMatcher passed into the InMemoryCache and there is no way to hook into the instantiation of the client at the moment.
First, I assumed that #3 might be the way to go. Yet it's not about additional links, we actually need to provide the fragmentMatcher for InMemoryCache here:

aws-mobile-appsync-sdk-js/packages/aws-appsync/src/client.js

Line 100 in 87c52f4

const cache = new InMemoryCache(store);

[alexeygolev]

As far as I can understand both cache implementations use InMemory as a base class, so the fragment matcher logic is untouched. Maybe we could pass the fragmentMatcher with the constructor argument?

[jpstrikesback]

We're doing this via #62 by passing in a set of links and cache

[jpstrikesback]

(the link(s) depend on a cache implementation, and it should be a singleton afaik)

[manueliglesias]

Once PR #96 gets merged, it should be possible to pass custom cache options to the AWS AppSync client like this:

import AWSAppSyncClient from "aws-appsync";

const client = new AWSAppSyncClient({
  url: appSyncConfig.graphqlEndpoint,
  region: appSyncConfig.region,
  auth: {
    type: appSyncConfig.authenticationType,
    apiKey: appSyncConfig.apiKey
  },
  cacheOptions: {
    dataIdFromObject: obj => obj.__typename && obj.id && `MyPrefix:${obj.__typename}:${obj.id}`
  }
});