Lucky: Many-to-Many checkboxes

create.cr

class Posts::Create < AdminAction
  include TagParams

  route do
    PostForm.create(params, tag_ids: tag_ids) do |form, post|
      if post
        redirect to: Index
      else
        render NewPage, form: form
      end
    end
  end

  private def tag_ids
    HTTP::Params.parse(request.body.to_s).fetch_all("post:tag_ids")
  end
end

models.cr

class Post < BaseModel
  table :posts do
    column title : String
    column body : String
    has_many taggings : Tagging
    has_many tags : Tag, through: :taggings
  end
end

class Tag < BaseModel
  table :tags do
    column name : String
    has_many taggings : Tagging
    has_many posts : Post, through: :taggings
  end
end

class Tagging < BaseModel
  table :taggings do
    belongs_to post : Post
    belongs_to tag : Tag
  end
end

new_page.cr

class Posts::NewPage < MainLayout
  
  needs form : PostForm

  def content
    form_for Create do
      field(form.title) do |title|
        text_input title
      end
      field(form.body) do |body|
        textarea body
      end
      
      tag_selection(form)

      submit "Save"
    end
  end
  
  private def tag_selection(form)
    post = form.record
    TagQuery.new.name.asc_order.each do |tag|
      label do
        if post.try { |p| p.tags.includes?(tag) }
          input type: "checkbox", name: "post:tag_ids", value: tag.id.to_s, checked: "checked"
        else
          input type: "checkbox", name: "post:tag_ids", value: tag.id.to_s
        end
        text tag.name
      end
    end
  end
end

post_form.cr

class PostForm < Post::BaseForm
  fillable title, body
  needs tag_ids : Array(String)

  after_save update_tags

  def prepare
    validate_required title
  end

  def update_tags(post)
    LuckyRecord::Repo.run do |db|
      ids_constraint = " AND tag_id NOT IN (#{tag_ids.join(", ")})" if tag_ids.any?
      db.exec "DELETE FROM taggings WHERE post_id = #{post.id}#{ids_constraint}"
      now = Time.utc_now
      if tag_ids.any?
        values_for_insert = tag_ids.map do |tag_id|
          "(#{post.id}, #{tag_id}, '#{now}', '#{now}')"
        end
        db.exec "INSERT INTO taggings(post_id, tag_id, created_at, updated_at) VALUES #{values_for_insert.join(", ")} ON CONFLICT DO NOTHING"
      end
    end
  end
end