fix graphql get by xid (#4586)

* add test for get xid

* fix get by xid

graphql/resolve/query_test.yaml

@@ -1062,6 +1062,22 @@
       }
     }
 
+-
+  name: "Get with XID where no ID in type"
+  gqlquery: |
+    query {
+      getState(code: "NSW") {
+        name
+      }
+    }
+  dgquery: |-
+    query {
+      getState(func: eq(State.code, "NSW")) @filter(type(State)) {
+        name : State.name
+        dgraph.uid : uid
+      }
+    }
+
 -
   name: "Query editor using code"
   gqlquery: |

graphql/schema/wrappers.go

@@ -493,7 +493,7 @@ func (f *field) IDArgValue() (xid *string, uid uint64, err error) {
 	// 3. ID and XID fields
 	// Therefore, the non ID field is an XID field.
 	for _, arg := range f.field.Arguments {
-		if arg.Name != idField.Name() {
+		if idField == nil || arg.Name != idField.Name() {
 			xidArgName = arg.Name
 		}
 	}
@@ -508,8 +508,7 @@ func (f *field) IDArgValue() (xid *string, uid uint64, err error) {
 		xid = &xidArgVal
 	}
 
-	if idField == nil && xid == nil {
-		// This means that both were optional and were not supplied, lets return here.
+	if idField == nil {
 		return
 	}