class Company < ApplicationRecord
  has_many :describes, dependent: :destroy
  has_many :descriptors, through: :describes, source: :metadatum
end

class Metadatum < ApplicationRecord
  has_many :describes, dependent: :destroy
  has_many :descriptees, through: :describes, source: :company

  ...
end

class Describe < ApplicationRecord
  belongs_to :company
  belongs_to :metadatum
end

class CompaniesController < ApplicationController
  ...

  def destroy
    @company = Company.find(params[:id])
    @company.destroy
    redirect_to companies_url
  end

  ...
end

ActiveRecord::Schema.define(version: <some version #) do

  create_table "companies", force: :cascade do |t|
    t.string   "name"
    t.string   "description"
    t.datetime "created_at",  null: false
    t.datetime "updated_at",  null: false
    t.index ["name"], name: "index_companies_on_name", unique: true
  end

  create_table "describes", id: false, force: :cascade do |t|
    t.integer  "company_id"
    t.integer  "metadatum_id"
    t.datetime "created_at",   null: false
    t.datetime "updated_at",   null: false
    t.index ["company_id"], name: "index_describes_on_company_id"
    t.index ["metadatum_id"], name: "index_describes_on_metadatum_id"
  end

  create_table "metadata", force: :cascade do |t|
    t.string   "name"
    t.string   "description"
    t.datetime "created_at",  null: false
    t.datetime "updated_at",  null: false
    t.index ["name"], name: "index_metadata_on_name", unique: true
  end

end

class CreateCompanies < ActiveRecord::Migration[5.0]
  def change
    create_table :companies do |t|
      t.string :name
      t.string :description

      t.timestamps
    end
    add_index :companies, :name, unique: true
  end
end

class CreateMetadata < ActiveRecord::Migration[5.0]
  def change
    create_table :metadata do |t|
      t.string :name
      t.string :description

      t.timestamps
    end
    add_index :metadata, :name, unique: true
  end
end

class CreateDescribes < ActiveRecord::Migration[5.0]
  def change
    create_table :describes, id: false do |t|
      t.references :company, foreign_key: true
      t.references :metadatum, foreign_key: true

      t.timestamps
    end
  end
end

@company.destroy

undefined method `to_sym' for nil:NilClass Did you mean? to_s

def create
  @company = Company.new(company_attributes)

  params[:metadata][:ids].each do |m|
    if !m.empty?
      @company.descriptors << Metadatum.find(m)
    end
  end

  if @company.save

  ...
end