I recently encountered an error in Rails where I was trying to connect a Category model to a Transaction model. I had a has_one associtation and encountered the following error:

class Category < ActiveRecord::Base
  has_one :transaction
end

"You tried to define an association named transaction on the model Category, but this will conflict with a method transaction already defined by Active Record. Please choose a different association name."

This is caused by ActiveRecord already using a method named "transactions". To handle this issue, choose an appropriate alternative name for the association and specify the model class and foreign key manually.

has_one :association_name, :foreign_key: "key_name", class_name: "ModelClass"

In my case this was implement as:

class Category < ActiveRecord::Base
  #has_one :transaction
  has_one :owner, foreign_key: "category_id", class_name: "Transaction"
end