Question

I would like to
1. Grab an object 1 and object 2 from the database (both have 2 has_many associations each)
2. Create object 3 which is a clone of object 1 and therefore has no associations yet
2. Duplicate object 2’s associations and add them to object 3’s associations
3. Use the new object 3 for operations in memory
4. Exit the process with no permanent change to the database – object 1 and object 2 still have their original associations when the process exits

What instead is happening

1. I grab an object 1 and object 2 from the database (both have 2 has_many associations each)
2. I create object 3 which is a clone of object 1 and therefore has no associations yet
3. I duplicate object 2’s associations and add them to object 3’s associations
4. I use the new object 3 for operations in memory
5. I exit the process HOWEVER there is now a permanent change to the database – object 2 no longer has its associations because their keys have been changed to the (Temporary) object 3 id.

Here is my code. Note that all contains an array with an arbitrary # of objects in it

object1 = all.last.clone #we take the most recently created object
all.each do |instance|
  instance_association1 = (instance.association1).dup
  object1.association1 += instance_association1 #BUG this moves the association
  object1.association1.uniq!
  instance_association2 = (instance.association2).dup
  object1.association2 += instance_association2
  object1.association2.uniq!
end

Note that now when I say:
all.last.association1, I get an empty array.

Help!!

Answer 1

MongoMapper’s associations may be a little overzealous with saving. I want to overhaul it sometime, but it’s not an easy problem.

The code that’s going to tell you when saves happen is many_documents_proxy.rb. When you do my_association = [...], replace is the method that’s called.

The only method that doesn’t do any saving is build, so you may be able to build up your temporary object like so:

tmp = all.last.clone
all.each do |instance|
  instance.association1.each { |doc| tmp.association1.build(doc.attributes) }
  tmp.association1.uniq!
  # ...
end

More generally, you can convert all your associated documents to arrays and not worry about saving…

associations1 = []
associations2 = []
all.each do |doc|
  associations1 += doc.associations1.to_a
  associations2 += doc.associations2.to_a
end
assocations1.uniq!
assocations2.uniq!

HOWEVER, watch out! Ruby’s uniq method looks like it uses Ruby’s #hash method to compute equality, which may not give you the results you want in this case. Do a few tests to make sure that my_obj.hash == my_obj2.hash if my_obj == my_obj2. See this discussion for strategies on implementing your own #hash method, if that’s the route you need to go.