Ability to make sorting with different models

[yuri-val]

Example of usage:

I have two models:

• CrewingTypeDocument - sort of categories

class CrewingTypeDocument < ApplicationRecord
  # position_field: :position
  
  has_one :parent, -> { none }, class_name: 'CrewingTypeDocument'
  has_many :children, class_name: 'CrewingDocument'

  # ...
end

• CrewingDocument - sort of items, belongs to categories

class CrewingDocument < ApplicationRecord
  # position_field: :position
  
  belongs_to :parent, class_name: "CrewingTypeDocument"
  has_many :children, -> {none}, class_name: 'CrewingDocument'

  class << self
    def arrange(_options)
      CrewingTypeDocument.order(:position).includes(:children).each_with_object({}) do |item, hash|
        hash[item] = item.children.order(:position)
      end
    end
  end
  
end

In such configuration you can have sorting tree in CrewingDocument model where top level will be CrewingTypeDocument, second level - CrewingDocument.

WARNING: if you want to sort groups and items in same form, you should use same position_field names

RailsAdmin configuration:

RailsAdmin.config do |config|
  config.actions do
    # ...
    nestable
  end

  config.model 'CrewingDocument' do
    # ...
    nestable_tree live_update: :only, position_field: :position
  end
end

Example:

1. sorting items:
   
2. sorting groups:
   

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Style/StringLiterals: Prefer double-quoted strings unless you need single quotes to avoid extra backslashes for escaping.

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Style/RescueModifier: Avoid using rescue in its modifier form.
Style/StringLiterals: Prefer double-quoted strings unless you need single quotes to avoid extra backslashes for escaping.
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