hint in error if method only exists on pointer type (fixes #60)

Author: neelance

example/starwars/starwars.go

@@ -598,8 +598,8 @@ func (r *friendsConnectionResolver) Friends() *[]*characterResolver {
 	return resolveCharacters(r.ids[r.from:r.to])
 }
 
-func (r *friendsConnectionResolver) PageInfo() *pageInfoResolver {
-	return &pageInfoResolver{
+func (r *friendsConnectionResolver) PageInfo() pageInfoResolver {
+	return pageInfoResolver{
 		startCursor: encodeCursor(r.from),
 		endCursor:   encodeCursor(r.to - 1),
 		hasNextPage: r.to < len(r.ids),

internal/exec/exec.go

@@ -212,7 +212,11 @@ func (b *execBuilder) makeObjectExec(typeName string, fields map[string]*schema.
 	for name, f := range fields {
 		methodIndex := findMethod(resolverType, name)
 		if methodIndex == -1 {
-			return nil, fmt.Errorf("%s does not resolve %q: missing method for field %q", resolverType, typeName, name)
+			hint := ""
+			if findMethod(reflect.PtrTo(resolverType), name) != -1 {
+				hint = " (hint: the method exists on the pointer type)"
+			}
+			return nil, fmt.Errorf("%s does not resolve %q: missing method for field %q%s", resolverType, typeName, name, hint)
 		}
 
 		m := resolverType.Method(methodIndex)