SELECT DISTINCT products.detail_name FROM (SELECT DISTINCT product_category_id, detail_name FROM products WHERE detail_name ILIKE '%шин%' ORDER BY name) products

@words = ProductCategory.all.flat_map do |pc|
      pc.products.where("detail_name ILIKE '%#{params[:oem]}%'").limit(1)
    end

    respond_to do |format|
      format.html
      format.json { render json: @words.map(&:detail_name) }
    end

products = Product.where('detail_name ILIKE %qqq%).pluck(:product_category_id, :detail_name).uniq(&:first).map(&:second)

SELECT MAX(product_category_assigns.product_id) AS product_id 
FROM "product_categories" 
INNER JOIN "product_category_assigns" ON "product_category_assigns"."product_category_id" = "product_categories"."id" 
WHERE "product_categories"."active" = 't' 
GROUP BY product_categories.id;