Handle unresolved elements gracefully

Fixes #376

Co-authored-by: Sylvain Lebresne <[email protected]>

gateway-js/CHANGELOG.md

@@ -4,7 +4,7 @@
 
 > The changes noted within this `vNEXT` section have not been released yet.  New PRs and commits which introduce changes should include an entry in this `vNEXT` section as part of their development.  When a release is being prepared, a new header will be (manually) created below and the appropriate changes within that release will be moved into the new section.
 
-- _Nothing to see here. Stay tuned._
+- Continue resolving when an `@external` reference cannot be resolved. [#376](https://github.com/apollographql/federation/issues/376)
 
 ## v0.45.0
 

gateway-js/src/executeQueryPlan.ts

@@ -523,7 +523,17 @@ function executeSelectionSet(
         const selections = (selection as QueryPlanFieldNode).selections;
 
         if (typeof source[responseName] === 'undefined') {
-          throw new Error(`Field "${responseName}" was not found in response.`);
+          // This method is called to collect the inputs/requires of a fetch. So, assuming query plans are correct
+          // (and we have not reason to assume otherwise here), all inputs should be fetched beforehand and the
+          // only reason for not finding one of the inputs is that we had an error fetching it _and_ that field
+          // is non-nullable (it it was nullable, error fetching the input would have make that input `null`; not
+          // having the input means the field is non-nullable so the whole entity had to be nullified/ignored,
+          // leading use to not have the field at all).
+          // In any case, we don't have all the necessary inputs for this particular entity and should ignore it.
+          // Note that an error has already been logged for whichever issue happen while fetching the inputs we're
+          // missing here, and that error had much more context, so no reason to log a duplicate (less useful) error
+          // here.
+          return null;
         }
         if (Array.isArray(source[responseName])) {
           result[responseName] = source[responseName].map((value: any) =>