Question

In Rails/ActiveReocrd is there a way to replace one instance with another such that all the relations/foreign keys get resolved.

I could imagine something like this:

//setup
customer1 = Customer.find(1)
customer2 = Customer.find(2)

//this would be cool
customer1.replace_with(customer2)

supposing customer1 was badly configured and someone had gone and created customer2, not knowing about customer1 it would be nice to be able to quickly set everything to customer 2

So, also this would need to update any foreign keys as well

User belongs_to :customer
Website belongs_to :customer

then any Users/Websites with a foreign key customer_id = 1 would automatically get set to 2 by this ‘replace_with’ method

Does such a thing exist?

[I can imagine a hack involving Customer.reflect_on_all_associations(:has_many) etc]

Cheers,
J

Answer 1

Something like this could work, although there may be a more proper way:

Updated: Corrected a few errors in the associations example.

class MyModel < ActiveRecord::Base

  ...

  # if needed, force logout / expire session in controller beforehand.
  def replace_with (another_record)
    # handles attributes and belongs_to associations
    attribute_hash = another_record.attributes
    attribute_hash.delete('id')
    self.update_attributes!(attribute_hash)

    ### Begin association example, not complete.

    # generic way of finding model constants
    find_model_proc = Proc.new{ |x| x.to_s.singularize.camelize.constantize }
    model_constant = find_model_proc.call(self.class.name)

    # handle :has_one, :has_many associations
    have_ones = model_constant.reflect_on_all_associations(:has_one).find_all{|i| !i.options.include?(:through)}
    have_manys = model_constant.reflect_on_all_associations(:has_many).find_all{|i| !i.options.include?(:through)}

    update_assoc_proc = Proc.new do |assoc, associated_record, id|
      primary_key = assoc.primary_key_name.to_sym
      attribs = associated_record.attributes
      attribs[primary_key] = self.id
      associated_record.update_attributes!(attribs)
    end

    have_ones.each do |assoc|
      associated_record = self.send(assoc.name)
      unless associated_record.nil?
        update_assoc_proc.call(assoc, associated_record, self.id)
      end
    end

    have_manys.each do |assoc|
      associated_records = self.send(assoc.name)
      associated_records.each do |associated_record|
        update_assoc_proc.call(assoc, associated_record, self.id)
      end
    end

    ### End association example, not complete.

    # and if desired..
    # do not call :destroy if you have any associations set with :dependents => :destroy
    another_record.destroy
  end

  ...

end

I’ve included an example for how you could handle some associations, but overall this can become tricky.