@conversation = Conversation.includes(:messages)
                            .get_coach_conversations(@seller)

def self.get_seller_conversations(seller)
  @conversations = seller.conversations
                         .includes(buyer: [:purchases, :user])
                         .joins(:messages)
                         .where(messages: {only_for_buyer: false})
                         .distinct
  new.sorted_conversations(@conversations)
end

def sorted_conversations(conversations)
  conversations.sort_by { |c| c.messages.last.created_at }
               .reverse
end

json.array!(@conversations.map do |c|
  ...
  ...
  already_client: c.buyer.purchases
                         .where(seller: @seller)
                         .where('subscription = ? OR product_price > ?',
                                 true, 0)
                         .exists?
end)