I'm using Rails 6.1.3.2 and trying to make saving of nested attributes more efficient. The database used is MySQL.

Code basis

BoardSet.rb

class BoardSet < ApplicationRecord
   has_many :boards, inverse_of: :board_set, foreign_key: 'board_set_id', dependent: :destroy
   accepts_nested_attributes_for :boards

Board.rb

class Board < ApplicationRecord
   has_many :cells, inverse_of: :board, foreign_key: 'board_id', dependent: :destroy
   accepts_nested_attributes_for :cells

Cell.rb

class Cell < ApplicationRecord
   belongs_to :board, inverse_of: :cells, foreign_key: 'board_id'

There is Grape::API endpoint like this:

params do
  requires :name, type: String, desc: 'BoardSet name'
  optional :boards, as: :boards_attributes, type: Array[JSON] do
    # more params
    optional :cells, as: :cells_attributes, type: Array[JSON] do
      # more params
    end
  end
end
post do
  board_set = BoardSet.new(declared(params, include_missing: false))
  board_set.save!
end

Functionality

Summary of code basis: a BoardSet contains Boards which contain Cells. There is a Grape::API endpoint, where data can be passed to via PUT, which is then saved within the database.

Problem

Saving with board_set.save! is very inefficient. A BoardSet typically contains 40 Boards, where each contain maybe 20 Cells. So in total there are more than 40 * 20 = 800 SQL INSERT statements (a single one for each Cell).

Solution attempts

I've tried these attempts for improvements:

• use the gem activerecord-import which seems to be able to do bulk inserts for such situations. However I think I would need something like BoardSet.import [board_set], recursive: true, but recursive is only supported for PostgeSQL, so not for MySQL, which I'm using.
• first save board_set including boards with empty cells, then transferring the ID of the boards to the cells and save them in bulk. However I didn't manage to remove the cells, I've tried to use cloned_board = board.dup and cloned_board.cells = [] for saving empty copies, but it still saves all cells at cloned_board_set.save! (where cloned_board_set contains cloned boards with emtpy cells).
• I've tried to remove accepts_nested_attributes_for :cells from Board.rb in order to prevent board_set.save! from automatically saving all the cells. However this results in an exception in BoardSet.new(declared(params, include_missing: false)) since the parameter cells_attributes it not known.

Instead of saving all the objects with one line try these 3 steps:

1. Save the board set with only its name i.e. without nested attributes.
2. Save all boards without nested attributes
3. Save all cells

After rails 6 we can do insert all

board_set = BoardSet.new(declared(params[:name], include_missing: false))