https://gist.github.com/tkawa/4745785 の件、FormBuilder(form_forみたいなやつ)がいらなければ、gemなくてもこんなふうに書けばすむというメモ。

filter_params.rb

class FilterParams < ActionController::Parameters
  include ActiveModel::Model

  validates :date, format: /\A\d{4}-\d{2}-\d{2}\Z/
  validates :q,    length: { maximum: 20 }

  def initialize(params = {})
    params.reverse_merge!(per: '10')
    super(params)
  end

  def read_attribute_for_validation(key)
    self[key]
  end
end

posts_controller.rb

class PostsController < ApplicationController
  def index
    @posts = if filter_params.valid? # バリデーション実行
      Post.date(filter_params[:date]).search(filter_params[:q]).page(filter_params[:page]).per(filter_params[:per])
    else
      Post.none
    end
  end
 
  def create
    @post = Post.new(post_params)
    if @post.save
      redirect_to @post, notice: 'Post was successfully created.'
    else
      render action: "new"
    end
  end
 
  private
    def post_params
      # これは普通のstrong_parametersの使い方
      params.require(:post).permit(:title, :body, :tag, :date)
    end
 
    def filter_params
      FilterParams.new(params).permit(:date, :q, :page, :per)
    end
end