overlappingFieldsCanBeMergedRule function

Visitor overlappingFieldsCanBeMergedRule(ValidationCtx context)

Overlapping fields can be merged

A selection set is only valid if all fields (including spreading any fragments) either correspond to distinct response names or can be merged without ambiguity.

See https://spec.graphql.org/draft/#sec-Field-Selection-Merging

Implementation

Visitor overlappingFieldsCanBeMergedRule(
  ValidationCtx context,
) {
  // A memoization for when two fragments are compared "between" each other for
  // conflicts. Two fragments may be compared many times, so memoizing this can
  // dramatically improve the performance of this validator.
  final comparedFragmentPairs = PairSet();

  // A cache for the "field map" and list of fragment names found in any given
  // selection set. Selection sets may be asked for this information multiple
  // times, so this improves the performance of this validator.
  final Map<SelectionSetNode, FieldsAndFragmentNames>

      /// [Map.identity] required since the span is not used in [Node.==]
      /// and, right now, [SelectionSetNode.span] is null anyway
      cachedFieldsAndFragmentNames = Map.identity();

  final visitor = TypedVisitor();

  visitor.add<SelectionSetNode>((selectionSet) {
    final conflicts = findConflictsWithinSelectionSet(
      context,
      cachedFieldsAndFragmentNames,
      comparedFragmentPairs,
      context.typeInfo.getParentType(),
      selectionSet,
    );
    for (final c in conflicts) {
      final reasonMsg = _reasonMessage(c.reason);
      context.reportError(
        GraphQLError(
          'Fields "${c.reason.name}" conflict because ${reasonMsg}.'
          ' Use different aliases on the fields to fetch both if this was intentional.',
          locations: List.of(
            c.fields1.followedBy(c.fields2).map(
                  (e) => GraphQLErrorLocation.fromSourceLocation(
                    (e.span ?? e.name.span)!.start,
                  ),
                ),
          ),
          extensions: _overlappingFieldsCanBeMergedSpec.extensions(),
        ),
      );
    }
  });

  return visitor;
}