rails_loading_problem.rb

class Foo
  has_many :bars
end

class Bar
  belongs_to :foo
  belongs_to :qux
end

class Qux
  # no inverse to bar
end

puts Foo.first(:include => {:bars => :qux}).bars
# Loads Foo (LIMIT 1)
# Loads Bars where foo_id = Foo.id
# Loads Quxes where ID in (Bars.map(&:qux_id))
# 3 queries, as expected

# BUT

puts Foo.first(:include => {:bars => :qux}).bars.map(&:qux)
# Loads Foo (LIMIT 1)
# Loads Bars where foo_id = Foo.id
# Loads Quxes where ID in (Bars.map(&:qux_id))
# Loads Qux where ID == x
# Loads Qux where Id == x+1
# ...
# For each Qux in Bars

# W. T. F.?

# this is the Rails 3.0 version, same behaviour as above, n+1 queries:
puts Foo.includes(:bars => :qux).first.bars.map(&:qux)

## UPDATE!
# The problem only happened where foo.bars.qux returned nil (i.e. bar had a qux_id references a Qux that
# no longer existed). Because Qux is nil, Rails tries to get it by querying the database. So its really 
# a data integrity issue; if you expect Bars to always have a Qux, make sure your model reflects that 
# with appropriate validation! The ActiveRecord behaviour in this case makes total sense.