You can use the following
Product.where('id != ?', params[:id])
Which will generate what you are looking for, while parameterizing the query.
With Rails 4, the following syntax has been added to support not clauses
Product.where.not(id: params[:id])
Add multiple clauses with chaining…
Product.where.not(id: params[:id]).where.not(category_id: params[:cat_id])