Heisenberg principal in ActiveRecord::Relation: observing relation changes it

I have isolated a case where calling ActiveRecord::Relation#all (or evidently anything operation that forces evaluation) changes its state to that it subsequently produces incorrect results. A self-contained test example is attached. Observed in Ruby 1.9.2 / Rails 3.0.0

The general synopsis:

A RefThing table has two fks: obj1_id and obj2_id. The following method correctly create a RefThing and returns 1:

def works(obj1, obj2)
  relation1 = RefThing.where(:obj1_id => obj1.id)
  relation1.where(:obj2_id => obj2.id).create
  relation1.size
end

The following method returns zero instead, even though it correctly creates a RefThing. Note the ONLY difference is the call to relation1.all before the create:

def broke(obj1, obj2)
  relation1 = RefThing.where(:obj1_id => obj1.id)
  relation1.all    # forces evaluation of relation1
  relation1.where(:obj2_id => obj2.id).create
  relation1.size
end

[As an aside: it's not just that relation1.size is returning 0 -- it's simply empty, and attempts to use it for accessing fields from RefThing are futile. And yes, it took me a long time to isolate this from a much larger code body! :)]