m = Factory(:milestone) just wasn't working. Don't know why.
I had the following factories:
factories.rb
Factory.define :milestone do |m|
m.association :rulemaking
m.association :phase
m.association :activity
m.association :date_category
m.milestone Date.today
m.association :revision#_number, :factory => :revision
end
Factory.define :revision do |r|
r.revision_number { Factory.next(:integer)}
r.revision_date Date.today
end