fix(GraphQL): fix for deletion on interfaces with no non Id field

graphql/resolve/delete_mutation_test.yaml

@@ -265,4 +265,30 @@
   dgquery: |-
     query {
       x as deleteX()
-    }
+    }
+
+-
+  name: "Deleting an interface with just a field with @id directive"
+  gqlmutation: |
+    mutation{
+      deleteA(filter:{name:{eq: "xyz"}}){
+        a{
+          name
+        }
+      }
+    }
+  dgquery: |-
+    query {
+      x as deleteA(func: type(A)) @filter(eq(A.name, "xyz")) {
+        uid
+      }
+      a(func: uid(x)) {
+        dgraph.type
+        name : A.name
+        dgraph.uid : uid
+      }
+    }
+  dgmutations:
+    - deletejson: |
+        [{ "uid": "uid(x)"}]
+

graphql/resolve/schema.graphql

@@ -282,3 +282,7 @@ type ThingTwo implements Thing {
     prop: String @dgraph(pred: "prop")
     owner: String
 }
+
+interface A {
+    name: String! @id
+}

graphql/schema/wrappers.go

@@ -518,11 +518,11 @@ func mutatedTypeMapping(s *schema,
 		}
 		// This is a convoluted way of getting the type for mutatedTypeName. We get the definition
 		// for UpdateTPayload and get the type from the first field. There is no direct way to get
-		// the type from the definition of an object. We use Update and not Add here because
-		// Interfaces only have Update.
+		// the type from the definition of an object. We use Delete and not Update or Add here because
+		// Interfaces can't have Add and if there is no non Id field then Update also will not be there.
 		var def *ast.Definition
-		if def = s.schema.Types["Update"+mutatedTypeName+"Payload"]; def == nil {
-			def = s.schema.Types["Add"+mutatedTypeName+"Payload"]
+		if def = s.schema.Types["Add"+mutatedTypeName+"Payload"]; def == nil {
+			def = s.schema.Types["Delete"+mutatedTypeName+"Payload"]
 		}
 
 		if def == nil {