Saving (associating on new records) many-to-many relationships with Rails / active record

categories_place.rb

class CategoriesPlace < ApplicationRecord
  belongs_to :place
  belongs_to :category

  validates :place, :category, presence: true
end

category.rb

# Category model
class Category < ApplicationRecord
  has_many :categories_places
  has_many :places, through: :categories_places, dependent: :destroy

  validates :name, presence: true
  validates :description, presence: true

  before_save do
    self.slug = name.parameterize
  end
end

controller.rb

# Controller params sample
  def place_params
    params.require(:place).permit(
      :name,
      :description,
      categories_places_attributes: [
        :category_id
      ]
    )
  end

place.rb

class Place < ApplicationRecord
  belongs_to :user
  has_many :categories_places
  has_many :categories, through: :categories_places
  
  # This does the trick (Create new Place record and assign existing categories to it)
  accepts_nested_attributes_for :categories_places

  validates :name, presence: true
  validates :description, presence: true

  validates_associated :categories_places

  before_save do
    self.slug = name.parameterize
  end
end

POST https://localhost/places
{
  "name": "Nameless",
  "description": "rails test",
  "categories_places_attributes": [
  	{ "category_id": 1 }
  ]
}