SomethingBuilder

gistfile1.rb

class SomethingBuilder
  def initialize(something)
    @something = something
  end

  def build
    build_something_after_something_else
  end

  private

  def build_something_after_something_else
    # Update model A
    # Insert to model B
    # Insert to model A and B mapping
  end
end

class AController < ApplicationController
  def create
    # Do the usual stuff here...
    # Then, use the service!
    SomethingBuilder.new(@a).build
  end
  
  # Other controller actions here...
end

class BController < ApplicationController
  def create
    # Do the usual stuff here...
    # Then, use the service!
    SomethingBuilder.new(@b).build
  end
  
  # Other controller actions here...
end