messages_connection_resolver.go

func (r *chatRoomResolver) MessagesConnection(ctx context.Context, obj *model.ChatRoom, 
  first *int, after *string) (*model.MessagesConnection, error) {
    
  // The cursor is base64 encoded by convention, so we need to decode it first
  var decodedCursor string
  if after != nil {
    b, err := base64.StdEncoding.DecodeString(*after)
    if err != nil {
      return nil, err
    }
    decodedCursor = string(b)
  }

  // Here we could query the DB to get data, e.g. 
  // SELECT * FROM messages WHERE chat_room_id = obj.ID AND timestamp < decodedCursor
  edges := make([]*model.MessagesEdge, *first)
  count := 0
  currentPage := false

  // If no cursor present start from the top
  if decodedCursor == "" {
    currentPage = true
  }
  hasNextPage := false

  // Iterating over the mocked messages to find the current page
  // In real world use-case you should fetch only the required
  // part of data from the database
  for i, v := range r.Messages[obj.ID] {
    if v.ID == decodedCursor {
      currentPage = true
    }

    if currentPage && count < *first {
      edges[count] = &model.MessagesEdge{
        Cursor: base64.StdEncoding.EncodeToString([]byte(v.ID)),
        Node:   &v,
      }
      count++
    }

    // If there are any elements left after the current page
    // we indicate that in the response
    if count == *first && i < len(r.Messages[obj.ID]) {
      hasNextPage = true
    }
  }

  pageInfo := model.PageInfo{
    StartCursor: base64.StdEncoding.EncodeToString([]byte(edges[0].Node.ID)),
    EndCursor:   base64.StdEncoding.EncodeToString([]byte(edges[count-1].Node.ID)),
    HasNextPage: &hasNextPage,
  }

  mc := model.MessagesConnection{
    Edges:    edges[:count],
    PageInfo: &pageInfo,
  }

  return &mc, nil
}