fix(fragment): merge the nested fragments fields (#8075) (#8435)

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## Problem
Fixes
https://discuss.dgraph.io/t/graphql-field-collection-does-not-work-with-dgraph/15646
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## Solution
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Co-authored-by: Naman Jain <naman@dgraph.io>

graphql/e2e/common/fragment.go

@@ -568,5 +568,45 @@ func fragmentInQueryOnObject(t *testing.T) {
 
 	JSONEqGraphQL(t, queryCharacterExpected, string(gqlResponse.Data))
 
+	query2HumanParams := &GraphQLParams{
+		Query: `query  {
+    		queryHuman() {
+        		id
+        		starships {
+            		id
+        		}
+        		...HumanFrag
+    		}
+		}
+
+		fragment HumanFrag on Human {
+    		starships {
+				... {
+					__typename
+					id
+					name
+					length
+					}
+			}
+   	 	}`,
+	}
+	gqlResponse2 := query2HumanParams.ExecuteAsPost(t, GraphqlURL)
+
+	RequireNoGQLErrors(t, gqlResponse2)
+	queryCharacterExpected2 := fmt.Sprintf(`
+	
+		{"queryHuman":[
+			{"id":"%s",
+			"starships":[
+				{"id":"%s",
+				"__typename":"Starship",
+				"name":"Millennium Falcon",
+				"length":2.000000}]
+				}
+			]
+		}
+	`, humanID, newStarship.ID)
+	JSONEqGraphQL(t, queryCharacterExpected2, string(gqlResponse2.Data))
+
 	cleanupStarwars(t, newStarship.ID, humanID, "")
 }

graphql/schema/request.go

@@ -203,6 +203,9 @@ func recursivelyExpandFragmentSelections(field *ast.Field, op *operation) {
 	}, field.SelectionSet, satisfies)
 	field.SelectionSet = make([]ast.Selection, 0, len(collectedFields))
 	for _, collectedField := range collectedFields {
+		if len(collectedField.Selections) > 0 {
+			collectedField.Field.SelectionSet = collectedField.Selections
+		}
 		field.SelectionSet = append(field.SelectionSet, collectedField.Field)
 	}