Factory Girl: How to associate a record to another record without creating a new record?

I'm using Factory Girl/Rspec2/Rails 3.

In factories.rb, I have:

Factory.define :user do |user|
  user.name       'Some guy'
  user.email      'some_guy@somewhere.org'
  user.password   'password'
end

Factory.define :org_admin, :parent => :user do |user|
  user.email 'org_admin@somehwere.org'
end

Factory.define :user_with_membership_request, :parent => :user do |user|
  user.email 'user_with_membership_request@somehwere.org'
end

Factory.define :organization do |org|
  org.name        'MEC'
  org.description 'Mountain Equipment Co-op'
end

Factory.define :membership do |membership|
  membership.user { Factory(:user) }
  membership.organization { Factory(:organization) }
end

Factory.define :admin_membership, :parent => :membership do |membership|
  membership.user { Factory(:org_admin) }
  membership.is_admin true
  membership.status 'active'
end

Factory.define :membership_request, :parent => :membership do |membership|
  membership.user { Factory(:user_with_membership_request) }
  membership.status 'requested'
end

and then in my rspec test I have:

  it 'should accept the membership request' do
    @org_admin = Factory(:org_admin)
    test_sign_in(@org_admin)
    @organization = Factory(:organization)
    @membership_request = Factory(:membership_request)

    put :update, :organization_id => @organization.id, :id => @membership_request.id, :acceptance => 'approve'
    ...
  end

When I run the test, I get:

 Failure/Error: @membership_request = Factory(:membership_request)
 Validation failed: Name has already been taken

I understand the reason for the failure is because FactoryGirl is creating another organization (with the same name).

But what I'd like to do is create several memberships all associated with the same organization. How do I do that?

Thanks.

Sean