A simple, sql only, multi model search

[ActiveRecord, SQL] Multi-model sql-only search with UNION

The idea is, to have models adhere to a convention, which stats, that there has to be a field "searchable_tokens",
which includes searchable tokens. Now, we can simply have a (virtual) model/table `search_results`, which can be
unioned with "casted selects" of each model we want to search. The casted selects make the columns of each model to fit
into the schema of the search result.

Mixed with ActiveRecord's polymorphic associations, the result is an ActiveRecord relation with elements, which points to
their respective subject.

a1.rb

# db/migrate/20170317064203_create_search_results.rb
#
# This table is not meant to have any rows, it's more like a virtual table
#
class CreateSearchResults < ActiveRecord::Migration[5.0]
  def change
    create_table :search_results do |t|
      t.integer :subject_id
      t.string  :subject_type
      t.text    :searchable_tokens
    end
  end
end

a2.rb

# app/models/search_result.rb
#
class SearchResult < ApplicationRecord
  belongs_to :subject, polymorphic: true
end

rails c # .rb

# - the selects force the results in a schema that can be unioned with the search_results table
# - because we declared the model to be polymorphic associated to a subject, the actual hit can be accessed via `subject`

# If your search_results table has timestamps, than you have to select created_at, updated_at, too
companies = Company.where("searchable_tokens LIKE '%micha%'").select("NULL as id, id AS subject_id, 'Company' AS subject_type, searchable_tokens")
users = User.where("searchable_tokens LIKE '%micha%'").select("NULL as id, id AS subject_id, 'User' AS subject_type, searchable_tokens")

saerch_result = SearchResult.all.union(companies).union(users)

search_result.first.subject # => Company
search_result.second.subject # => User