Drag and drop in CSS grid with rails 6, stimulus and rails-ujs

_seating_plan.html.haml

-# https://johnbeatty.co/2018/03/09/stimulus-js-tutorial-how-do-i-drag-and-drop-items-in-a-list/
.grid--draggable{ 'data-controller': 'seating-plan',
                  'data-seating-plan-endpoint': endpoint,
                  'data-action': 'dragstart->seating-plan#onDragStart dragover->seating-plan#onDragOver dragenter->seating-plan#onDragEnter drop->seating-plan#onDrop dragend->seating-plan#onDragEnd' }
  - seating_plan.each do |seat|
    - if seat[:is_empty]
      .grid__item.grid__item--empty{ 'data-row': seat[:row],
                                     'data-col': seat[:col],
                                     style: "grid-row: #{seat[:row]}; grid-column: #{seat[:col]};",
                                     class: ('grid__item--border' if seat[:is_border]) }
    - else
      .grid__item{ draggable: 'true',
                  'data-item-id': seat[:student].id,
                  'data-row': seat[:row],
                  'data-col': seat[:col],
                  style: "grid-row: #{seat[:row]}; grid-column: #{seat[:col]};" }
        .card
          .card-content
            %p.title.has-text-centered= seat[:student].name
          %footer.card-footer
            %p.card-footer-item.has-text-weight-bold
              -# TODO Add sum
              4
            %p.card-footer-item.has-text-grey
              -# TODO Add sum
              10

school_class.rb

# models/school_class.rb

class SchoolClass < ApplicationRecord
  strip_attributes
  has_many :students

  def seating_plan
    row_min, row_max = students.map { |s| s.seat_row }.minmax
    col_min, col_max = students.map { |s| s.seat_col }.minmax
    row_offset = row_min - 2
    col_offset = col_min - 2
    # Can't use double splat operator { **tmp, s.seat_hash => s } with non-symbol (object) keys
    students_index = students.reduce({}) { |tmp, s| tmp.merge({seat_hash(s.seat_row, s.seat_col) => s}) }
    seat_coordinates = (row_min - 1..row_max + 1).to_a.product((col_min - 1..col_max + 1).to_a)
    return seat_coordinates.map do |row, col|
      {
        row: row - row_offset,
        col: col - col_offset,
        is_empty: !students_index.has_key?(seat_hash(row, col)),
        student: students_index[seat_hash(row, col)],
        is_border: row < row_min || row > row_max || col < col_min || col > col_max,
      }
    end
  end

  def seating_plan=(seats)
    # TODO exception handling
    transaction do
      seats.each(&(method :update_seat))
    end
  end

  private

  def seat_hash(row, col)
    {row: row, col: col}.freeze
  end

  def update_seat(student_id:, row:, col:)
    student = students.find(student_id)
    # Bypass validation (when swapping seats, one seat is briefly occupied twice)
    student.attributes = { seat_row: row, seat_col: col }
    student.save! validate: false
  end
end

school_classes_controller.rb

class SchoolClassesController < ApplicationController
  before_action :set_school_class, only: [:seating_plan]

  def seating_plan
    seating_plan = seating_plan_params[:students].map { |s| s.to_h.symbolize_keys }
    puts "Plan: #{seating_plan.inspect}"
    @school_class.seating_plan = seating_plan
    respond_to do |format|
      format.html { redirect_back fallback_location: school_classes_path, notice: t('.notice') }
    end
  end

  private
    # Use callbacks to share common setup or constraints between actions.
    def set_school_class
      @school_class = SchoolClass.find(params[:id])
    end

    # Never trust parameters from the scary internet, only allow the white list through.
    def seating_plan_params
    params.require(:school_class).permit(students: [:student_id, :row, :col])
    end
end

seating_plan.scss

@import 'bulma/bulma';

.grid--draggable {
  display: grid;
  grid-gap: 10px;
  grid-auto-rows: 1fr;
  grid-auto-columns: 1fr;
  padding: 20px 0;

  [draggable] {
    cursor: move; /* fallback if grab is not supported */
    cursor: grab;
  }

  .grid__item--border {
    visibility: hidden;
  }

  &.grid--dragging {
    .grid__item--border {
      visibility: visible
    }

    .grid__item--empty {
      background-color: $grey-lightest;
    }
  }
}

seating_plan_controller.js

import {Controller} from 'stimulus'
import Rails from '@rails/ujs'

// https://johnbeatty.co/2018/03/09/stimulus-js-tutorial-how-do-i-drag-and-drop-items-in-a-list/
export default class extends Controller {
  onDragStart(event) {
    this.element.classList.add('grid--dragging')
    const data = JSON.stringify({
      row: event.target.getAttribute('data-row'),
      col: event.target.getAttribute('data-col'),
    })
    event.dataTransfer.setData('application/drag-key', data)
    event.dataTransfer.effectAllowed = 'move'
    console.log('start')
  }

  onDragOver(event) {
    event.preventDefault()
    console.log('over')
    return true
  }

  onDragEnter(event) {
    event.preventDefault()
    console.log('enter')
  }

  getPosition(el) {
    return [el.getAttribute('data-row'), el.getAttribute('data-col')]
  }

  getPositions() {
    const items = [...this.element.querySelectorAll('.grid__item[draggable]')]
    return items.map(el => ({
      student_id: el.getAttribute('data-item-id'),
      row: this.getPosition(el)[0],
      col: this.getPosition(el)[1],
    }))
  }

  setPosition(el, [row, col]) {
    el.setAttribute('data-row', row)
    el.setAttribute('data-col', col)
    el.style['grid-row'] = row
    el.style['grid-column'] = col
  }

  swap(el1, el2) {
    const el1Pos = this.getPosition(el1)
    const el2Pos = this.getPosition(el2)
    this.setPosition(el1, el2Pos)
    this.setPosition(el2, el1Pos)
  }

  onDrop(event) {
    const { row, col } = JSON.parse(event.dataTransfer.getData("application/drag-key"))
    const targetEl = event.target.closest('[data-row][data-col]')
    const sourceEl = this.element.querySelector(`[data-row='${row}'][data-col='${col}']`)
    this.swap(sourceEl, targetEl)
    event.preventDefault()
    console.log('drop')
  }

  submit(data) {
    Rails.ajax({
      url: this.data.get('endpoint'),
      type: 'put',
      beforeSend(xhr, options) {
        xhr.setRequestHeader('Content-Type', 'application/json; charset=UTF-8')
        // Workaround: add options.data late to avoid Content-Type header to already being set in stone
        // https://github.com/rails/rails/blob/master/actionview/app/assets/javascripts/rails-ujs/utils/ajax.coffee#L53
        options.data = JSON.stringify(data)
        return true
      },
    });
  }

  onDragEnd(event) {
    this.element.classList.remove('grid--dragging')
    this.submit({
      school_class: {
        students: this.getPositions()
      }
    })
    console.log('end')
  }
}