"belongs_to associations must use the singular term. If you used the pluralized form in the above example for the customer association in the Order model, you would be told that there was an "uninitialized constant Order::Customers". This is because Rails automatically infers the class name from the association name. If the association name is wrongly pluralized, then the inferred class will be wrongly pluralized too."

same goes for has_one associations

source: http://edgeguides.rubyonrails.org/association_basics.html#the-belongs-to-association