Ruby on Rails-NilClass的未定义方法

Question

I'm creating a picture-rating app where users can click on pictures and rate them on a scale from 1 to 5. I'm trying to calculate the average rating of a picture. Before when users clicked on a rating value, that value became the picture's rating.

Rating: 5

If a user clicked on 1, the rating would change to 1

Rating: 1

When reality, the rating should have been 3.

(5 + 1) / 2
=> 3

Here's what I've accomplished so far in implementing this feature.

I added a migration to create two new columns for my Pictures Table

rails g migration AddRatingsToPictures ratings_count: integer, rating_total: integer

Both the new attributes, ratings_count and rating_total are integer types, meaning they are assigned a nil value at default.

p = Picture.first
p.attribute_names
=> ['id', 'title', 'category', 'stars', 'updated_at', 'created_at', 
'ratings_count', 'rating_total']
p.ratings_count
=> nil
p.rating_total
=> nil

My only problem is the NilClass Error.

Here is my update method in my PicturesController.

def update
  @picture = Picture.find(params[:id])
  @picture.ratings_count = 0 if @picture.stars.nil?
  @picture.rating_total = @picture.stars
  @picture.rating_total += @picture.stars if @picture.stars_changed?
  @picture.ratings_count += 1 if @picture.rating_total_changed?
  if @picture.update_attributes(picture_params)
    unless current_user.pictures.include?(@picture)
      @picture = Picture.find(params[:id])
      current_user.pictures << @picture
      redirect_to @picture, :flash => { :success => "Thank you! This picture has been added to your Favorites List" }
    else
      redirect_to :action => 'index'
      flash[:success] = 'Thank you! This picture has been updated' 
    end
  else
    render 'edit'
  end
end

Here is my picture_param method in my PicturesController

 def picture_params
  params.require(:picture).permit(:title, :category, :genre, :stars)
end

Here is what the two new columns do

ratings_count: Calculates the number of times a picture has been rated
rating_total: Calculates the sum of the stars a picture has received

In the above code, I first set the ratings_count to 0 if the picture doesn't have a rating. This means that the picture hasn't been rated yet.

I then need to initially set the rating_total to the number of stars a picture has. If a user changed the star rating, I would add those stars to the rating_total. And if the total increased, that's my cue to increase the number of ratings.

Obviously, to calculate the average, I'd do something like this.

(@picture.rating_total / @picture.ratings_count).to_f

Now, I think I have the right idea but I know why this doesn't work. When columns are created with an integer value, by default they are set to nil. This leads to a NilClass Error when I load the web page.

undefined method `/' for nil:NilClass

Here is my code in the View

<li><strong>Rating:</strong> <%= pluralize((@picture.rating_total / @picture.ratings_count), 'Star') %></li>

Answer 1

This will handle the case of uninitialized (nil) values in your attributes...

def update
  @picture = Picture.find(params[:id])
  if @picture.stars_changed?
    @picture.ratings_count = (@picture.ratings_count || 0) + 1
    @picture.rating_total = (@picture.rating_total || 0) + ( @picture.stars || 0)
  end

You don't need an array of ratings or ratings persisted to database, assuming you only count votes where the rating changes, you can accumulate the count and the total and divide the two (which is, in fact, what you're doing so I'm preaching to the converted).

Although it seems to me that if I change a picture from 5 to 1 and it only changes to 3, I'm gonna keep clicking 1 :)

Answer 2

You could set the default value on the migration when you created it. But no worries, you can create a new migration to change it:

# Console
rails g migration change_default_for_ratings_count_and_rating_total

# Migration Code
class ChangeDefaultForRatingsCountAndRatingTotal < ActiveRecord::Migration

  def change
    change_column :pictures, :ratings_count, :integer, default: 0
    change_column :pictures, :rating_total,  :integer, default: 0
  end
end

Keep in mind that some databases don't automatically assign newly updated default values to existing column entries, so maybe you will have to iterate over every picture already created with nil values and set to 0.

Answer 3

Ok, the main reason it is not working is because

• you fetch the picture
• you check the stars from the database, and the NOT the passed form-parameters
• you do update_attributes, which if I am not mistaken, used to set attributes and then save the complete object, but since rails 4 only updates the passed attributes (which is what you would expect)

One small remark: keeping the rating correct is a function I would place in the model, NOT in the controller.

Furthermore, how to handle the if nil, initialise to zero I wrote a short blogpost about. In short: overrule the getter.

So I would propose the following solution. In your model write

class Picture < ActiveRecord::Base


  def ratings_count
    self[:ratings_count] || 0
  end

  def ratings_total
    self[:ratings_total] || 0
  end


  def add_rating(rating)
    return if rating.nil? || rating == 0

    self.ratings_count += 1
    self.ratings_total += rating
    self.stars = self.ratings_total.to_f / self.ratings_count
    self.save
  end

  def rating
    return 0 if self.ratings_count == 0
    self.ratings_total.to_f / self.ratings_count
  end

and then the code in your controller becomes much cleaner and readable:

def update
  @picture = Picture.find(params[:id])

  stars = picture_params.delete(:stars)

  if @picture.update_attributes(picture_params)
    @picture.add_rating stars
    unless current_user.pictures.include?(@picture)
      current_user.pictures << @picture
      redirect_to @picture, :flash => { :success => "Thank you! This picture has been added to your Favorites List" }
    else
      redirect_to :action => 'index'
      flash[:success] = 'Thank you! This picture has been updated' 
    end
  else
    render 'edit'
  end
end

I first delete the :stars from the parameters, because I do not want to save those, I want to use those for the add_rating . I then try to update_attributes , which will fail if there are any failing validations, and if that is ok, I will add_rating which itself will handle nil or zero correctly. Well granted: I do not know how you handle a "non-rating" (nil? zero?). It is possible a rating of zero should be added, because it will add a rating, but most UI I know do not allow to select 0 as rating, so you might want to change the zero handling.

Answer 4

Ok, an alternative...

Do an after_initialize so the fields are never, never, ever nil. Even if you're creating a new Picture object, they'll be initialized as zero. Problem will go away.

  class Picture << ActiveRecord::Base

    after_initialize do |picture|
      picture.ratings_count ||= 0
      picture.rating_total ||= 0
    end

    ...
  end