Conditional has_one union depending on a boolean value

I'm implementing a evidence login to a site that has wholly oAuth login.
The User has a union to a Facebook_account denote where it stores facebook
data. we wish this charge to be contingent on value in a User model. ie.
has_one :facebook_account, if: :fb_login?Is this illusory or am we doing
something wrong?Using rails 4.0.4Thanks