Having figured this problem in rails all by myself made me so happy that I wanted to write it down and not let myself forget.

Problem

I have a One-to-Many table relationship and the many table needs to have each record have a different value for the field of "letter" for parent row created.

Solution

By using the validates_uniqueness_of in the model of the child table and defining the scope. The validation will limit the check of an existing row to a smaller range.

Example

validates_uniqueness_of :letter, :scope => :parent_field_id

Going Further

I had another problem that required the use of validates_uniqueness_of but it required checking a value for a field that had a true/false value. I needed to have many rows exist with the field being false, but onle one row contain true. The following validation works very good for this situation:

validates_uniqueness_of :is_correct, :scope => :parent_column_id, :if => Proc.new { |answer| answer.is_correct }

The scope limits the validation of the child row to be localized for the parent record only when the :if statement executes the block contains the "is_correct" field.

What I like about this statement is that the if block can be any type of check you want. For example if you want to check on a different field than the one you are deciding is unique on.

Closing

Please be aware this is the only way I know how to accomplish this task. If you have any suggestions on how to accomplish what I described above please add your comments below.