Add findChat() method to ChatData

mongoose.js

@@ -41,4 +41,23 @@ class ChatData {
         }
       })
   }
+
+  findChat (firstParticipantId, secondParticipantId) {
+    var participantIds = [ firstParticipantId, secondParticipantId ]
+    participantIds.sort()
+
+    this.ChatModel.findOne({ participant_ids: participantIds }, function (err, result) {
+      if (err) {
+        return console.error(err)
+      }
+
+      if (result) {
+        var chats = result.chats
+
+        chats.forEach(function (chat) {
+          console.log(chat)
+        })
+      }
+    })
+  }
 }