gistfile1.rb

module ActiveRecord
  class Base
    class << self
      alias_method :old_method_missing, :method_missing

      def method_missing(method_id, *arguments, &block)
        if /^find_or_build_by_([_a-zA-Z]\w*)$/ =~ method_id.to_s
          names = $1.split('_and_')
          relation.find_or_build_by(names, *arguments)
        else
          old_method_missing(method_id, *arguments, &block)
        end
      end

      def find_or_build_by(names, *arguments)
        find_attributes = {}
        values = arguments[0]
        names.each {|k| find_attributes[k] = values[k] }

        record = where(find_attributes).first

        puts relation.inspect
        
        if record
          record.sanitize_for_mass_assignment(values).each {|k, v| record.send("#{k}=", v)}
        else
          record = relation.build(values)
          relation.push(record)
        end

        puts relation.inspect

        return record
      end
    end
  end
end

> c = Company.new; c.employees.find_or_build_by_id(name: 'Alfred')
=> #<Employee id: nil, name: "Alfred", city: nil, company_id: nil> 
> c.employees
 => [] 
> c.employees.relation
 => [#<Employee id: nil, name: "Alfred", city: nil, company_id: nil>] 

I expect c.employees contains the newly created employee (as contained in c.employees.relation)

I believe that my problem is that my code is in class methods, where it should belong to instance methods.