Figured it out.
I need to be working with the join object in the instance variable rather than the joined object.
Find the @contents
which belong to this @miniset
and then get the @miniature
info from there. Makes much more sense.
<% @miniset.contents.where(miniset_id: @miniset).each do |content| %>
<%= link_to content.miniature.name, content.miniature %>
x<%= content.quantity %>
<% end %>
Found some very complicated answers to similar questions but this is dead simple. Hope it helps someone.