我想出：

我创建了一个实例方法来使用 find_by来检索用户的电影评论方法评论模型：

  class User< ActiveRecord :: Base 
 .... 
 def movie_review（album）
 Review.find_by（user_id：self，album_id：album）
 end 
 end 
 

设置回调时，此方法也派上用场：

  class ReviewsController< ApplicationController 
 before_action：limit_review，only：[：new，：create] 
 .... 
 private 
 def limit_review 
 user_review = current_user.movi​​e_review（@movie） 

如果user_review.present？ 
 redirect_to edit_movie_review_path（@movie，user_review）
 end 
 end 
 end 
 

创建了一个帮助方法，用于显示编辑或创建的相关链接。非常感谢Austio和他的建议：

 模块评论Helper 
 def create_or_edit_review_path（movie）
 user_review = current_user.movi​​e_review（movie）if user_signed_in？ 

如果user_signed_in？ &安培;&安培; user_review.present？ 
 link_to编辑评论，edit_movie_review_path（movie，user_review）
 else 
 link_to写评论，new_movie_review_path 
 end 
 end 
 end 
 

最后这是我在我的视图模板中调用帮助器：

  .... 
<％= create_or_edit_review_path（@album）％> 
 

I have my app setup where users can write reviews for a movie. What I'd like to do is limit the user to create only one review per movie. I've managed to accomplish this in my reviews controller as so:

class ReviewsController < ApplicationController
  before_action :has_reviewed, only [:new]
  ....
  def has_reviewed?
    if Review.where(user_id: current_user.id, movie_id: @movie.id).any?
      redirect_to movie_reviews_path

      flash[:notice] = "You've already written a review for this movie."
    end
  end
end

Where I'm now having trouble is translating this same logic into my index view template with the helper methods of Devise and CanCanCan at my disposal.

<% if user_signed_in? && ... %> # current_user has already created a review for this movie
  <%= link_to "Edit Review", edit_movie_review_path(@movie, review) %>
<% else %>
  <%= link_to "Write a Review", new_movie_review_path %>
<% end %>

Also: Is there any way to improve the lookup in my has_reviewed? method? I feel like there's a better way to write it but can't determine the most appropriate fix.

Here's what I came up with:

I created an instance method to retrieve a user's movie review using the find_by method on the Review model:

class User < ActiveRecord::Base
  ....
  def movie_review(album)
    Review.find_by(user_id: self, album_id: album)
  end
end

This method also comes in handy when setting up my callback:

class ReviewsController < ApplicationController
  before_action :limit_review, only: [:new, :create]
  ....
  private
    def limit_review
      user_review = current_user.movie_review(@movie)

      if user_review.present?
        redirect_to edit_movie_review_path(@movie, user_review)
      end 
    end
end

Created a helper method for showing the appropriate link to edit or create a review. Big thanks to Austio and his suggestion:

module ReviewsHelper
  def create_or_edit_review_path(movie)
    user_review = current_user.movie_review(movie) if user_signed_in?

    if user_signed_in? && user_review.present?
      link_to "Edit review", edit_movie_review_path(movie, user_review)
    else
      link_to "Write a review", new_movie_review_path
    end
  end
end

And at last this is how I call the helper in my view template(s):

....
<%= create_or_edit_review_path(@album) %>