How to enter unique associations only?

问题

Running the following code to add an association enters multiple entries each time the code is ran:

store.categories << category

Is there a way to make it only enter unique associations between the two models in the db?

回答1:

Directly from the rails guides, hope it helps:

class Person
  has_many :readings
  has_many :posts, :through => :readings, :uniq => true
end

回答2:

Ignoring duplicates only seem to work with begin and rescue logic:

begin
  stores.categories << category
rescue
  puts "Duplicate entry ignored"
end

来源：https://stackoverflow.com/questions/8032342/how-to-enter-unique-associations-only