Switching to a Union Type causes cache.updateQuery to return data as null

[Undistraction]

We are using the graphcache exchange and this has been working really well for us for a while. However we've just made a change to the API which has meant moving from a list containing a single type, to a a list containing a Union type:

fragment ChatMessage_chatMessage on ChatMessage {
  faqs {
    nodes {
      ...Faq_faq
    }
  }
}

To a union:

fragment ChatMessage_chatMessage on ChatMessage {
  faqs {
    nodes {
      ... on Faq {
          ...Faq_faq
          __typename
        }
        ... on Resource {
          ...Resource_resource
          __typename
        }
    }
  }
}

We have a query that gets all the ChatMessages for an Enrolment which is run once the app has initialised (The EnrolmentForChatMessagesQuery referenced below).

When we create a new ChatMessage with createChatMessage, the new message is returned and we append it to the current list of chat messages in an update function. This was working great before switching to a union type and whereas before data was the expected value, it is now always null.

updates: {
    Mutation: {
      createChatMessage(result, { params: { enrolmentId } }, cache) {
        cache.updateQuery(
          {
            query: EnrolmentForChatMessagesQuery,
            variables: {
              id: enrolmentId,
            },
          },
          (data) => {
            // data is now null
            data.enrolment.chatMessages.nodes.push(chatMessage)
            return data
          }
        )
      }
    }
  }

Would really appreciate some help in tracking down the issue as I'm unsure how to debug it. It doesn't seem like a type-change should have this effect, but this is the only change. If I replace the revert this change, data comes back as expected.

Would providing a schema to graphcache help? Our API is behind auth and we couldn't expose our API schema by including a schema in the src so we'd have to introspect at runtime which is not idea.

[Undistraction]

I added a schema and the problem is resolved. It would be good to understand exactly why that fixed the problem.

Given that each item in the list includes a _typename it certainly doesn't feel like a schema should be necessary for this.

[JoviDeCroock]

You should have gotten a warning in the console about heuristic fragment matching basically our cache could not figure out how the data correlated I presume 😅