ransack在合并https://github.com/activerecord-hackery/ransack/pull/390后支持它。你应该声明ransackable_scopes方法来添加搜索可见的作用域。
来自手册
继续上一节，按作用域搜索需要在模型类上定义一个ransackable_scopes白名单。白名单应该是一个符号数组。默认情况下，所有类方法（例如scopes）被忽略。作用域将被应用于匹配真值，或者如果作用域接受一个值，则应用于给定值：

class Employee < ActiveRecord::Base
  scope :activated, ->(boolean = true) { where(active: boolean) }
  scope :salary_gt, ->(amount) { where('salary > ?', amount) }

  # Scopes are just syntactical sugar for class methods, which may also be used:

  def self.hired_since(date)
    where('start_date >= ?', date)
  end

  private

  def self.ransackable_scopes(auth_object = nil)
    if auth_object.try(:admin?)
      # allow admin users access to all three methods
      %i(activated hired_since salary_gt)
    else
      # allow other users to search on `activated` and `hired_since` only
      %i(activated hired_since)
    end
  end
end

Employee.ransack({ activated: true, hired_since: '2013-01-01' })

Employee.ransack({ salary_gt: 100_000 }, { auth_object: current_user })

Ransack让你为此创建自定义 predicate ，不幸的是文档留下了改进的空间，但是结帐：https://github.com/ernie/ransack/wiki/Custom-Predicates
另外，我相信你试图解决的问题是在他们的问题跟踪。有一个很好的讨论正在进行：https://github.com/ernie/ransack/issues/34

我写了一个名为siphon的gem，它可以帮助你将参数转换为activerelection作用域。将其与ransack相结合可以实现这一点。
你可以read full explanation here。同时，这是它的要点

视图

= form_for @product_search, url: "/admin/products", method: 'GET' do |f|
  = f.label "has_orders"
  = f.select :has_orders, [true, false], include_blank: true
  -#
  -# And the ransack part is right here... 
  -#
  = f.fields_for @product_search.q, as: :q do |ransack|
    = ransack.select :category_id_eq, Category.grouped_options

好了，现在`params[:product_search]`持有范围，`params[:product_search][:q]`拥有ransack goodness。我们现在需要找到一种方法，将数据分发到表单对象。因此，首先让ProductSearch在控制器中吞下它：

### 控制器

products_controller.rb

def index
@product_search = ProductSearch.new(params[:product_search])
@products ||= @product_formobject.result.page(params[:page])
end

### Form对象

product_search.rb

class ProductSearch
include Virtus.model
include ActiveModel::Model

These are Product.scopes for the siphon part

attribute :has_orders, Boolean
attribute :sort_by, String

The q attribute is holding the ransack object

attr_accessor :q

def initialize(params = {})
@params = params || {}
super
@q = Product.search( @params.fetch("q") { Hash.new } )
end

siphon takes self since its the formobject

def siphoned
Siphon::Base.new(Product.scoped).scope( self )
end

and here we merge everything

def result
Product.scoped.merge(q.result).merge(siphoned)
end
end