Support returnPartialData for watched queries again.

[benjamn]

PR #1370 inexplicably removed support for a useful feature: the ability to opt into receiving partial results from the cache for queries that are not fully satisfied by the cache, by passing the returnPartialData: true option to client.watchQuery.

This feature is especially important in conjunction with React (or any other view framework Apollo integrations that use client.watchQuery), since it can prevent rerendering undefined data just before fetching a larger query over the network.

A lot has changed since PR #1370 was merged in March 2017. The author and reviewer of that PR no longer work for Apollo, and I would like to believe that our current development culture would prevent us from ripping out such a useful feature without providing a meaningful replacement.

[hwillson]

Excellent - thanks for bringing this back @benjamn!

[benjamn]

@hwillson Do you think it's safe to remove this method if we're not using it internally?

[hwillson]

@benjamn Yes, for sure!

[hwillson]

I am sooooooo incredibly excited about this PR! Just to call this PR out for those that have notifications enabled on this repo - currently, if you run a query like:

const GET_MEMBER = gql`
  query GetMember($id: ID!) {
    member(id: $id) {
      id
      name
    }
  }
`;

in one component, the results are cached, then you run a superset query like the following in another component:

const GET_MEMBER_WITH_PLANS = gql`
  query GetMemberWithPlans($id: ID!) {
    member(id: $id) {
      id
      name
      plans {
        id
        title
        duration
      }
    }
  }
`;

Apollo Client will not re-use the partial data that was cached from the first query, when it preps and displays the second component. It can't find a cache hit for the full second query, so it fires the full query over the network. After this PR (and after we add in related react-apollo changes), if returnPartialData is set on the second component, the data available to that component will be automatically pre-loaded with the parts of the query that can be found in the cache, before the full query is fired over the network. So we'll be able to show partial data in our components, while the rest of the data is being loaded! 🎉

[DeciderWill]

Hi @benjamn - thank you for taking the time to add this.

We've had an issue with it however. We were unaware of the re-implementation and when updating to 2.6.0 we found a few of our query components were not returning any data and spent a bit of time debugging. Resulting behavior: re-renders a component 3 times with loading states false, false, and finally true. On true the data object is still an empty object. I can see the data has returned successfully and there are no errors. But it is not available on the data prop. Adding returnPartialData={true} resolved the issue. For context, we had two components rendering at the same time making the same query with a variable determining the type of data to be returned ie MetricItem being a union of CpuItem, NetworkItem and MemoryItem.

I'm unsure if this was an intended change that returnPartialData would need to be added and some existing queries would stop working without it?

Thank you.