Modeling a debate app

gistfile1.txt

def User
has_many :propositions, :as => :debateable
has_many :oppositions, :as => :debateable
end

def Proposition
belongs_to :debateable, :polymorphic => :true
has_one :opposition, :as => :debateable
end

def Opposition
belongs_to :debateable, :polymorphic => :true
end


# These active record models need to create a world such that:

# A User has many Propositions and Oppositions. However, an Opposition may only be created by User1 in response to a existing Proposition that has already been created by User2. Each Proposition can have only one Opposition.

# The app is meant to display a Proposition along-side its corresponding Opposition. So basically:
# Users should be able to create as many Propositions as they want.
# Users can only create an Opposition in response to an existing Proposition.
# Each Proposition can have either 0 or 1 Oppositions.
# A Proposition and its corresponding Opposition cannot both be owned by the same user.

# Please advise--I'm not sure if I'm on the right track with the associations I've outlined above.