fix key validations.

lib/graphql/stitching/composer/validate_boundaries.rb

@@ -41,8 +41,9 @@ def validate_as_boundary(supergraph, type, candidate_types_by_location, boundari
           memo[boundary.location][boundary.key] = boundary
         end
 
-        # All non-key fields must be available in at least one boundary location
-        boundary_keys = boundaries.map(&:key).tap(&:uniq!)
+        boundary_keys = boundaries.map(&:key).to_set
+
+        # All non-key fields must be resolvable in at least one boundary location
         supergraph.locations_by_type_and_field[type.graphql_name].each do |field_name, locations|
           next if boundary_keys.include?(field_name)
 
@@ -52,6 +53,13 @@ def validate_as_boundary(supergraph, type, candidate_types_by_location, boundari
           end
         end
 
+        # All locations of a boundary type must include at least one key field
+        supergraph.fields_by_type_and_location[type.graphql_name].each do |location, field_names|
+          if field_names.none? { boundary_keys.include?(_1) }
+            raise Composer::ValidationError, "A boundary key is required for `#{type.graphql_name}` in #{location} to join with other locations."
+          end
+        end
+
         # verify that all outbound locations can access all inbound locations
         resolver_locations = boundaries_by_location_and_key.keys
         candidate_types_by_location.each_key do |location|

test/graphql/stitching/composer/validate_boundaries_test.rb

@@ -74,6 +74,15 @@ def test_permits_no_boundary_query_for_key_only_types
     assert compose_definitions({ "a" => a, "b" => b })
   end
 
+  def test_validates_subset_types_have_a_key
+    a = %{type T { id:ID! name:String } type Query { a(id: ID!):T @stitch(key: "id") }}
+    b = %{type T { name:String } type Query { b:T }}
+
+    assert_error("A boundary key is required for `T` in b", ValidationError) do
+      compose_definitions({ "a" => a, "b" => b })
+    end
+  end
+
   def test_validates_bidirection_types_are_mutually_accessible
     a = %{
       type T { upc:ID! name:String }
@@ -93,6 +102,25 @@ def test_validates_bidirection_types_are_mutually_accessible
     end
   end
 
+  def test_validates_key_only_types_are_mutually_accessible
+    a = %|
+      type T { upc:ID! }
+      type Query { a(upc:ID!):T @stitch(key: "upc") }
+    |
+    b = %|
+      type T { id:ID! }
+      type Query { b(id:ID!):T @stitch(key: "id") }
+    |
+    c = %|
+      type T { id:ID! }
+      type Query { c(id:ID!):T @stitch(key: "id") }
+    |
+
+    assert_error("Cannot route `T` boundaries in a", ValidationError) do
+      compose_definitions({ "a" => a, "b" => b, "c" => c })
+    end
+  end
+
   def test_validates_outbound_types_can_access_all_bidirection_types
     a = %{
       type T { upc:ID! }

test/graphql/stitching/supergraph_test.rb

@@ -290,26 +290,26 @@ def test_route_type_to_locations_favors_longer_paths_through_necessary_locations
     assert routes.none? { |_key, path| path.any? { _1["location"] == "e" } }
   end
 
-  def test_route_type_to_locations_returns_nil_for_unreachable_locations
-    a = %|
-      type T { upc:ID! }
-      type Query { a(upc:ID!):T @stitch(key: "upc") }
-    |
-    b = %|
-      type T { id:ID! }
-      type Query { b(id:ID!):T @stitch(key: "id") }
-    |
-    c = %|
-      type T { id:ID! }
-      type Query { c(id:ID!):T @stitch(key: "id") }
-    |
-
-    supergraph = compose_definitions({ "a" => a, "b" => b, "c" => c })
-
-    routes = supergraph.route_type_to_locations("T", "b", ["a", "c"])
-    assert_equal ["c"], routes["c"].map { _1["location"] }
-    assert_nil routes["a"]
-  end
+  # def test_route_type_to_locations_returns_nil_for_unreachable_locations
+  #   a = %|
+  #     type T { upc:ID! }
+  #     type Query { a(upc:ID!):T @stitch(key: "upc") }
+  #   |
+  #   b = %|
+  #     type T { id:ID! }
+  #     type Query { b(id:ID!):T @stitch(key: "id") }
+  #   |
+  #   c = %|
+  #     type T { id:ID! }
+  #     type Query { c(id:ID!):T @stitch(key: "id") }
+  #   |
+
+  #   supergraph = compose_definitions({ "a" => a, "b" => b, "c" => c })
+
+  #   routes = supergraph.route_type_to_locations("T", "b", ["a", "c"])
+  #   assert_equal ["c"], routes["c"].map { _1["location"] }
+  #   assert_nil routes["a"]
+  # end
 
   describe "#to_definition / #from_definition" do
     def setup