Searching a Related model with a scope

Hi, everybody!

I am trying to figure out how to add a related model to an anonymous
scope to complete a search. Right now, I have this:
      scope = Record.scoped({})
      scope = scope.conditions "color = ?", params[:color] unless
params[:color] == '--Select Color--'
      scope = scope.conditions "size = ?", params[:size] unless
params[:size] == '--Select Size--'
      scope = scope.conditions "shape = ?", params[:shape] unless
params[:shape] == '--Select Sample Rate--'
      scope = scope.conditions "quantity = ?", params[:quantity]
unless params[:quantity] == '--Select Quantity--'
      scope = scope.conditions "file_short = ?", params[:length]
unless params[:length] == '--Select Length--'
      scope = scope.conditions " = ?", params[:widget]
unless params[:widget] == '--Select Name--'
      @records = scope

Records belong to Pairs, and Pairs belong to Runs. Everything will
work fine unless a widget is selected, and I need to have it located,
at which point it returns a "No such column" from SQL. Is there a
work around to allow you to get to a related model?