admin_bounds_generator.rb

require 'rails/generators/active_record'

module ActiveRecord
  module Generators
    class AdminBoundsGenerator < ActiveRecord::Generators::Base
      source_root File.expand_path('../templates', __FILE__)
      puts 'here'
      puts behavior
      def copy_migration
        migration_template 'migration.rb', "db/migrate/admin_bounds_create_#{table_name}.rb"
        puts behavior
        puts 'wehaoeuaoenutahoeuntaohe'
      end
    end
  end
end

admin_bounds_generator_spec.rb

require 'spec_helper'
require 'generators/active_record/admin_bounds_generator'

# if ENV['DETECTIVE']
#   RSpec.configure do |c|
#     c.around(:example) do |ex|
#       orig_stdout = $stdout
#       ex.run
#       expect($stdout).to be(orig_stdout)
#     end
#   end
# end

module ActiveRecord
  module Generators
    describe AdminBoundsGenerator, type: :generator do
      destination File.expand_path('../../tmp', __FILE__)

      before(:all) do
        prepare_destination
      end

      it 'creates a migration' do
        run_generator %w(admin_bound)
        assert_migration 'db/migrate/admin_bounds_create_admin_bounds.rb', /def change/
      end
    end
  end
end

output

here
/Users/rick/Organisations/playtimestudios/admin_bounds/lib/generators/active_record/admin_bounds_generator.rb:8:in `<class:AdminBoundsGenerator>': undefined local variable or method `behavior' for ActiveRecord::Generators::AdminBoundsGenerator:Class (NameError)
	from /Users/rick/Organisations/playtimestudios/admin_bounds/lib/generators/active_record/admin_bounds_generator.rb:5:in `<module:Generators>'
	from /Users/rick/Organisations/playtimestudios/admin_bounds/lib/generators/active_record/admin_bounds_generator.rb:4:in `<module:ActiveRecord>'
	from /Users/rick/Organisations/playtimestudios/admin_bounds/lib/generators/active_record/admin_bounds_generator.rb:3:in `<top (required)>'
	from /Users/rick/Organisations/playtimestudios/admin_bounds/spec/generators/active_record_generator_spec.rb:2:in `require'
	from /Users/rick/Organisations/playtimestudios/admin_bounds/spec/generators/active_record_generator_spec.rb:2:in `<top (required)>'
	from /Users/rick/Organisations/playtimestudios/admin_bounds/vendor/bundle/gems/rspec-core-3.3.2/lib/rspec/core/configuration.rb:1327:in `load'
	from /Users/rick/Organisations/playtimestudios/admin_bounds/vendor/bundle/gems/rspec-core-3.3.2/lib/rspec/core/configuration.rb:1327:in `block in load_spec_files'
	from /Users/rick/Organisations/playtimestudios/admin_bounds/vendor/bundle/gems/rspec-core-3.3.2/lib/rspec/core/configuration.rb:1325:in `each'
	from /Users/rick/Organisations/playtimestudios/admin_bounds/vendor/bundle/gems/rspec-core-3.3.2/lib/rspec/core/configuration.rb:1325:in `load_spec_files'
	from /Users/rick/Organisations/playtimestudios/admin_bounds/vendor/bundle/gems/rspec-core-3.3.2/lib/rspec/core/runner.rb:102:in `setup'
	from /Users/rick/Organisations/playtimestudios/admin_bounds/vendor/bundle/gems/rspec-core-3.3.2/lib/rspec/core/runner.rb:88:in `run'
	from /Users/rick/Organisations/playtimestudios/admin_bounds/vendor/bundle/gems/rspec-core-3.3.2/lib/rspec/core/runner.rb:73:in `run'
	from /Users/rick/Organisations/playtimestudios/admin_bounds/vendor/bundle/gems/rspec-core-3.3.2/lib/rspec/core/runner.rb:41:in `invoke'
	from /Users/rick/Organisations/playtimestudios/admin_bounds/vendor/bundle/gems/rspec-core-3.3.2/exe/rspec:4:in `<top (required)>'
	from /Users/rick/Organisations/playtimestudios/admin_bounds/vendor/bundle/bin/rspec:23:in `load'
	from /Users/rick/Organisations/playtimestudios/admin_bounds/vendor/bundle/bin/rspec:23:in `<main>'