fix(GraphQL): Fix custom(dql: ...) with __typename (GRAPHQL-1098) (#7569

)

This PR fixes the working of `@custom(dql: ...)` with `__typename` and fragments.

Bug Description:
1. Apply GraphQL schema:
```
interface Node {
 id: ID!
}
type User implements Node {
 name: String
}
type Token implements Node {
 key: String
}
type Query {
 queryNodeR: [Node] @Custom(dql: """
  query {
   queryNodeR(func: type(Node)) {
     dgraph.type
     id: uid
     name: User.name
     key: Token.key
   }
  } """)
}
```
2. Add data:
```
mutation {
 addUser(input: [{name: "User1"}]) {numUids}
 addToken(input: [{key: "Token1"}]) {numUids}
}
```
3. Query with both the auto-generated and the custom query and see the difference:
```
query {
 queryNode {
 id
 __typename
 ... UserFrag
 ... TokenFrag
 }
 queryNodeR {
 id
 __typename
 ... UserFrag
 ... TokenFrag
 }
}
fragment UserFrag on User {
 name
}
fragment TokenFrag on Token {
  key
}
```
4. The response being returned was:
```
{
  "data": {
    "queryNode": [
      {
        "id": "0x2",
        "__typename": "User",
        "name": "User1"
      },
      {
        "id": "0x3",
        "__typename": "Token",
        "key": "Token1"
      }
    ],
    "queryNodeR": [
      {
        "id": "0x2",
        "__typename": "Node",
        "name": "User1",
        "key": null
      },
      {
        "id": "0x3",
        "__typename": "Node",
        "name": null,
        "key": "Token1"
      }
    ]
  }
}
```

`queryNodeR` should have had the same result as `queryNode`.

graphql/e2e/custom_logic/custom_logic_test.go

@@ -2572,13 +2572,16 @@ func TestCustomDQL(t *testing.T) {
 	common.SafelyDropAll(t)
 
 	schema := `
-	type Tweets {
+	interface Node {
+		id: ID!
+	}
+	type Tweets implements Node {
 		id: ID!
 		text: String! @search(by: [fulltext, exact])
 		user: User
 		timestamp: DateTime! @search
 	}
-	type User {
+	type User implements Node {
 		screen_name: String! @id
 		followers: Int @search
 		tweets: [Tweets] @hasInverse(field: user)
@@ -2596,6 +2599,17 @@ func TestCustomDQL(t *testing.T) {
 	}
 
 	type Query {
+	  queryNodeR: [Node] @custom(dql: """
+		query {
+			queryNodeR(func: type(Node), orderasc: User.screen_name) @filter(eq(Tweets.text, "Hello DQL!") OR eq(User.screen_name, "abhimanyu")) {
+				dgraph.type
+				id: uid
+				text: Tweets.text
+				screen_name: User.screen_name
+			}
+		}
+	  """)
+
 	  getFirstUserByFollowerCount(count: Int!): User @custom(dql: """
 		query getFirstUserByFollowerCount($count: int) {
 			getFirstUserByFollowerCount(func: eq(User.followers, $count),orderdesc: User.screen_name, first: 1) {
@@ -2708,6 +2722,11 @@ func TestCustomDQL(t *testing.T) {
 	params = &common.GraphQLParams{
 		Query: `
 		query ($count: Int!) {
+		  queryNodeR {
+			__typename
+			... on User { screen_name }
+			... on Tweets { text }
+		  }
 		  queryWithVar: getFirstUserByFollowerCount(count: $count) {
 			screen_name
 			followers
@@ -2740,6 +2759,10 @@ func TestCustomDQL(t *testing.T) {
 	common.RequireNoGQLErrors(t, result)
 
 	require.JSONEq(t, `{
+		"queryNodeR": [
+			{"__typename": "User", "screen_name": "abhimanyu"},
+			{"__typename": "Tweets", "text": "Hello DQL!"}
+		],
 		"queryWithVar": {
 			"screen_name": "abhimanyu",
 			"followers": 5

graphql/schema/completion.go

@@ -72,6 +72,7 @@ func Unmarshal(data []byte, v interface{}) error {
 // At present, it is only used for building custom results by:
 //  * Admin Server
 //  * @custom(http: {...}) query/mutation
+//  * @custom(dql: ...) queries
 //
 // fields are all the fields from this bracketed level in the GraphQL  query, e.g:
 //  {
@@ -117,12 +118,18 @@ func CompleteObject(
 	seenField := make(map[string]bool)
 
 	x.Check2(buf.WriteRune('{'))
-	// @custom(http: {...}) query/mutation results may return __typename in response for abstract
-	// fields, lets use that information if present.
-	typename, ok := res[Typename].(string)
 	var dgraphTypes []string
-	if ok && len(typename) > 0 {
+	if typename, ok := res[Typename].(string); ok && len(typename) > 0 {
+		// @custom(http: {...}) query/mutation results may return __typename in response for
+		// abstract fields, lets use that information if present.
 		dgraphTypes = []string{typename}
+	} else if dgTypeVals, ok := res["dgraph.type"].([]interface{}); ok {
+		// @custom(dql: ...) query results may return dgraph.type in response for abstract fields
+		for _, val := range dgTypeVals {
+			if typename, ok = val.(string); ok {
+				dgraphTypes = append(dgraphTypes, typename)
+			}
+		}
 	}
 
 	for _, f := range fields {