fix(GraphQL Query): Remove extra fields when querying interfaces (#6596…

…) (#6647)

(cherry picked from commit cec5567)
This PR fixes: GRAPHQL-638

graphql/e2e/common/fragment.go

@@ -160,10 +160,32 @@ func fragmentInQueryOnInterface(t *testing.T) {
 					id
 				}
 			}
+			qcRep1: queryCharacter {
+				name
+				... on Human {
+					name
+					totalCredits
+				}
+				... on Droid {
+					name
+					primaryFunction
+				}
+			}
+			qcRep2: queryCharacter {
+				... on Human {
+					totalCredits
+				}
+				name
+				... on Droid {
+					primaryFunction
+					name
+				}
+			}
 			qc2: queryCharacter {
 				... on Human {
 					name
 					n: name
+
 				}
 			}
 			queryThing {
@@ -274,6 +296,26 @@ func fragmentInQueryOnInterface(t *testing.T) {
 				"id": "%s"
 			}
 		],
+		"qcRep1": [
+            {
+				"name": "Han",
+                "totalCredits": 10
+            },
+            {
+                "name": "R2-D2",
+                "primaryFunction": "Robot"
+            }
+		],
+		"qcRep2": [
+            {
+                "totalCredits": 10,
+                "name": "Han"
+            },
+            {
+                "name": "R2-D2",
+                "primaryFunction": "Robot"
+			}
+		],
 		"qc2":[
 			{
 				"name": "Han",

graphql/resolve/resolver.go

@@ -1265,6 +1265,10 @@ func completeObject(
 	var buf bytes.Buffer
 	comma := ""
 
+	// Below map keeps track of fields which have been seen as part of
+	// interface to avoid double entry in the resulting response
+	seenField := make(map[string]bool)
+
 	x.Check2(buf.WriteRune('{'))
 
 	dgraphTypes, ok := res["dgraph.type"].([]interface{})
@@ -1283,6 +1287,9 @@ func completeObject(
 		if len(dgraphTypes) > 0 {
 			includeField = f.IncludeInterfaceField(dgraphTypes)
 		}
+		if _, ok := seenField[f.ResponseName()]; ok {
+			includeField = false
+		}
 		if !includeField {
 			continue
 		}
@@ -1292,6 +1299,8 @@ func completeObject(
 		x.Check2(buf.WriteString(f.ResponseName()))
 		x.Check2(buf.WriteString(`": `))
 
+		seenField[f.ResponseName()] = true
+
 		val := res[f.DgraphAlias()]
 		if f.Name() == schema.Typename {
 			// From GraphQL spec: