fix: failing satisfiability check for interface(Objects) (#186)

test case is a reproduction of
#177

Author: n1ru4l

.changeset/famous-pillows-behave.md

@@ -0,0 +1,5 @@
+---
+"@theguild/federation-composition": patch
+---
+
+Fix failing satisfiability check for interface types.

__tests__/interface-object-composition.spec.ts

@@ -685,5 +685,72 @@ testImplementations((api) => {
         test.skip(`several interfaces and several interface objects. Should succeed`, () => {});
       });
     });
+
+    test("satisfiability with @interfaceObject", () => {
+      const result = api.composeServices([
+        {
+          name: "a",
+          typeDefs: parse(/* GraphQL */ `
+            type Query {
+              book: Book!
+            }
+
+            interface Media @key(fields: "id") {
+              id: ID!
+            }
+
+            type Book implements Media @key(fields: "id") {
+              id: ID!
+              isbn: String!
+            }
+
+            extend schema
+              @link(
+                url: "https://specs.apollo.dev/federation/v2.3"
+                import: ["@key"]
+              )
+          `),
+        },
+        {
+          name: "b",
+          typeDefs: parse(/* GraphQL */ `
+            type Media @key(fields: "id") @interfaceObject {
+              id: ID!
+            }
+
+            type Query {
+              topRatedMedia: [Media!]!
+            }
+
+            extend schema
+              @link(
+                url: "https://specs.apollo.dev/federation/v2.3"
+                import: ["@key", "@interfaceObject"]
+              )
+          `),
+        },
+        {
+          name: "c",
+          typeDefs: parse(/* GraphQL */ `
+            type Query {
+              cBook: Book
+            }
+
+            type Book @key(fields: "id") {
+              id: ID!
+            }
+
+            extend schema
+              @link(
+                url: "https://specs.apollo.dev/federation/v2.3"
+                import: ["@key"]
+              )
+          `),
+        },
+      ]);
+
+      // console.log(api.library, result.errors);
+      expect(result.errors).toBe(undefined);
+    });
   });
 });

src/supergraph/validation/rules/satisfiablity/finder.ts

@@ -260,6 +260,20 @@ export class PathFinder {
       return;
     }
 
+    // We can skip the edge in case the the graph interface implementation does not provide the field.
+    if (edge.tail.typeState?.kind === "interface") {
+      const fieldInEdge = edge.tail.typeState.fields
+        .get(fieldName)
+        ?.byGraph.get(edge.tail.graphId);
+
+      if (fieldInEdge === undefined) {
+        this.logger.groupEnd(
+          () => "Ignore edge: tail type does not have this field",
+        );
+        return;
+      }
+    }
+
     // A huge win for performance, is when you do less work :D
     // We can ignore an edge that has already been visited with the same key fields / requirements.
     // The way entity-move edges are created, where every graph points to every other graph: