How to search in rails from two models?

Doing JOINs the Rails way

• How can I do a Rails find JOIN without SQL, using the two tables' existing has_and_belongs_to_many relationships? I have two classes Sample and User, which have: has_and_belongs_to_many :users, :uniq => true has_and_belongs_to_many :samples, :uniq => true relationships in their respective model files. This sets up the many-to-many relationship between the two classes. Let's say I want to associate a Sample instance with a User instance: Sample.find_by_id(params[:sample][:id]).users << User.find_by_id(params[:user_id]) So far so good. But I'd like to avoid duplicates of a combination of sample_id and user_id. Therefore, I'd like to search on samples_users to find a previously-entered record which matches those two id numbers. I could do this with a SQL JOIN but I'd like to learn the Rails way, so that I don't have to write custom SQL that will likely break during development. How to I use the find method on Sample to get any matches with a specified sample_id and user_id?>

• Answer:

  This kind of sidesteps your question, but avoiding duplicates can be done at the database level by creating a unique index across sample_id and user_id in the samples_users table like so: add_index :samples_users, [:sample_id, :user_id], :unique => true There's a chance you'll need a begin / rescue for saving - I'm not sure if it just returns false when things don't work out or if AR throws an exception.