From d7feabd79f50f76693322f6e6436126347dcea87 Mon Sep 17 00:00:00 2001
From: minhaj-shakeel <minhaj@dgraph.io>
Date: Mon, 21 Sep 2020 12:21:23 +0530
Subject: [PATCH] fix(GraphQL): fix for deletion on interfaces with no non Id
 field (#6387) (#6417)

Fixes GRAPHQL-655.

For this user schema (interface have no non-Id field):

```
interface A {
   name: String! @id
}

type B implements A {
  age: Int!
}
```
the following delete mutation
```
mutation{
  deleteA(filter:{name:{eq: "xyz"}}){
    a{
      name
    }
  }
}
```
was resulting in the following error:
```
{
  "errors": [
    {
      "message": "Internal Server Error - a panic was trapped.  This indicates a bug in the GraphQL server.  A stack trace was logged.  Please let us know by filing an issue with the stack trace."
    }
  ]
}
```
This PR fixes this bug and now deleting can be performed successfully.

(cherry picked from commit 742259b4a6514b5ce50169d85da0feba6d69cb70)
---
 graphql/resolve/delete_mutation_test.yaml | 28 ++++++++++++++++++++++-
 graphql/resolve/schema.graphql            |  4 ++++
 graphql/schema/wrappers.go                | 10 ++++----
 3 files changed, 36 insertions(+), 6 deletions(-)

diff --git a/graphql/resolve/delete_mutation_test.yaml b/graphql/resolve/delete_mutation_test.yaml
index b2e970b9ad8..bd884725eec 100644
--- a/graphql/resolve/delete_mutation_test.yaml
+++ b/graphql/resolve/delete_mutation_test.yaml
@@ -265,4 +265,30 @@
   dgquery: |-
     query {
       x as deleteX()
-    }
\ No newline at end of file
+    }
+
+-
+  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)"}]
+
diff --git a/graphql/resolve/schema.graphql b/graphql/resolve/schema.graphql
index eef8ed55b4e..ed3211e6e93 100644
--- a/graphql/resolve/schema.graphql
+++ b/graphql/resolve/schema.graphql
@@ -282,3 +282,7 @@ type ThingTwo implements Thing {
     prop: String @dgraph(pred: "prop")
     owner: String
 }
+
+interface A {
+    name: String! @id
+}
\ No newline at end of file
diff --git a/graphql/schema/wrappers.go b/graphql/schema/wrappers.go
index ffdd272acd5..9ab898b2122 100644
--- a/graphql/schema/wrappers.go
+++ b/graphql/schema/wrappers.go
@@ -516,12 +516,12 @@ func mutatedTypeMapping(s *schema,
 		default:
 		}
 		// 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.
+		// for AddTPayload and get the type from the first field. There is no direct way to get
+		// the type from the definition of an object. Interfaces can't have Add and if there is no non Id
+		// field then Update also will not be there, so we use Delete if there is no AddTPayload.
 		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 {