ruby on rails - HABTM filter_or_create usage -

This question is asking a similar thing: But I'm new to rail and let me apply it to my problem.

I can easily create a new collaboration between a new location and user and location:

  @location = location.new ( Params [: location] .merge (: user_ids => current_user.id))   
 How can I change it:  
 
 New  Location  and association is created if  location  is not present  
  location  already exists, then a new union has been created.   
?  
 My models are like this:  
  location :  
  class space < ActiveRecord :: Base attr_accessible: name ,: user_ids has_and_belongs_to_many: user expired   
  user :  
  class user & lt; ActiveRecord :: Base attr_accessible: Email, Password: password_confirmation: remember_me has_and_belongs_to_many: end of locations   
 So I want to know how to create a union and how to check what I want to do Should Just An Association  
 There is only  location_id  and  user_id  fields in my association table.  
 Many thanks.  
  
 
 
 You want to take a look at: AR :: B # first_or_sartay / AR :: B # First_or_initialize