class Seller < ApplicationRecord
 has_many :buyer_sellers
 has_many :buyers, :through => :buyer_sellers
end

class Buyer < ApplicationRecord
 has_many :buyer_sellers
 has_many :sellers, :through => :buyer_sellers
end

class BuyerSeller < ApplicationRecord
 belongs_to :buyer
 belongs_to :seller
end

<%= form_for(buyer) do |f| %>
 <%= some fields %>

 <%= f.fields_for :buyer_seller do |ff| %>
  <% ff.date_select :start_date %>
 <% end %>

<% end %>

Buyer Load (3.1ms)  SELECT  "buyers".* FROM "buyers" WHERE "buyers"."id" = $1 LIMIT $2  [["id", 1], ["LIMIT", 1]]
Unpermitted parameter: buyer_seller