How to use scopes in rails to query the database for a subset of information and display it

rubyonrails-talk
1

am a beginner and I appreciate an answer to help me understand where my knowledge gap is:

The app is to display a post. The posts belong to a category (appetizers, entrees…) My thought was to use scopes to display all the appetizers on the posts in one view and then have the entrees in another view and so on.

The models:


class Post < ActiveRecord::Base
attr_accessible :body, :category_id, :title
belongs_to :category
scope :appetizers, -> { where(post.category.name => "Appetizers")}
end
class Category < ActiveRecord::Base
attr_accessible :name
has_many :posts
end

In the view I want to loop through the posts where the category name is “Appetizers”.

  <table>
<tr>
<th>Title</th>
<th>Body</th>
<th>Category</th>
</tr>
<% @post.appetizers.each do |app| %>
<tr>
<td><%= app.title %></td>
<td><%= app.body %></td>
<td><%= app.category.name%></td>
</tr>
<% end %>
</table>

I am getting an “undefined method” error. I have tried searching for an example here that explains what is a correct solution. I also tried creating a method in the Post model like this:

  def appetizers_list
@appetizer_list = Post.appetizers.all
end

Then call the method in the view:

 <% @appetizer_list.each do |app| %>
<tr>
<td><%= app.title %></td>
<td><%= app.body %></td>
<td><%= app.category.name%></td>
</tr>

Obviously I am confusing what needs to be done after creating the scope and how to get the view to “see” it. Or if I need a scope at all and it can be done in a more simple way.

So is there a best practices for retrieving subsets of data in rails and displaying them to the view?

2

what and where is the reported error?

3

As tamouse suggests it would have been better if you could have told us the exact error and which line it is on, but if the error is on this line, and is saying calling method name for nil, then it is because one of the posts does not have an associated category (so app.category is nil)

Colin

4

Apologies to both Colin and Tamouse. Indeed as Colin surmised the error is:

undefined method `each’ for nil:NilClass

Extracted source (around line #13):

10:

11:

12:

13: <% @appetizers_list.each do |app| %>

14:

15: <%= app.title %>

16: <%= app.body %>

I did try what was suggested in this blog post - http://www.jacopretorius.net/2012/02/scopes-in-rails.html which involves creating a controller action adding a custom route and creating a link_to in the view. This worked but at the expense of adding more to the controller. Here is that solution:

Action in Post Controller:

def appetizers

@posts = Post.appetizers

render ‘index’

end

**Route: **

get ‘posts/appetizers’ => ‘posts#appetizers’, :as => ‘appetizers_posts’

Change in Post Model:

scope :appetizers, → { where(:category_id => 3)}

And change in the view to be a link:

<% @posts.each do |post| %>

<%= post.title %> <%= post.body %> <%= post.category.name%>

<% end %>

<%= link_to ‘Appetizers’, appetizers_posts_path %>

So this works but it seems like it is not the best way to do it - seems like the controller would get bloated if there are a lot of categories. Another issue is that in the scope I have to use category_id => 3 to get the appetizers to list. I would like at least to have "post.category.name => “Appetizers” so to be more clear about what is being scoped but this give me an error:

undefined method `key?’ for nil:NilClass

I have a feeling there are many ways to display the appetizer listing in the view. Adding actions to the controller and adding routes seems not elegant. Can you suggest an alternative and or point to the concept I seem to be missing in understanding how to display a subset of information from the database in the view. I would like to use scopes and perhaps a method in the model…

Again, sorry for not posting the error, I appreciate any comments you can give on the subject.

5

So this works but it seems like it is not the best way to do it - seems like the controller would get bloated if there are a lot of categories. Another issue is that in the scope I have to use category_id => 3 to get the appetizers to list. I would like at least to have "post.category.name => “Appetizers” so to be more clear about what is being scoped but this give me an error: undefined method `key?’ for nil:NilClass

Rails doesn’t understand that post.category.name in the where clause (not in relation to a specific post) means to add a condition on the category’s name. You’d have to do something like

scope :appetizers, → {joins(:category).where(:categories => {:name => ‘Appetizers’})}

I have a feeling there are many ways to display the appetizer listing in the view. Adding actions to the controller and adding routes seems not elegant. Can you suggest an alternative and or point to the concept I seem to be missing in understanding how to display a subset of information from the database in the view. I would like to use scopes and perhaps a method in the model…

I’d suggest adding a single route/action for displaying the posts from any category. You can still make it look pretty by setting up slugs for categories and fetching categories by slug rather than id (friendly_id is one of many gems that do this).

You might also consider doing something like this, instead of using scopes (assuming you have this by category action)

@posts = Category.find(params[:category_id]).posts

Fred

6

Thank you Fred.

This worked great:

Rails doesn’t understand that post.category.name in the where clause (not in relation to a specific post) means to add a condition on the category’s name. You’d have to do

something like

scope :appetizers, → {joins(:category).where(:categories => {:name => ‘Appetizers’})}

I understand it a little but will have to learn more about why it works.

I would like to do this:

I’d suggest adding a single route/action for displaying the posts from any category.

I will do some reading to see how I need to set it up as I think it would help me avoid creating a new route for each category.

I took a gander at friendly_id and it look interesting however, I want to get the basics down before I make it pretty…

I tried playing around with this idea:

@posts = Category.find(params[:category_id]).posts

I think what you mean is to create a method in the Post controller that will return a list of a specific category. How would I call the method in the view so that the it picks up the category_id. I have done the following:

In the Post controller:

def category_show

@posts = Category.find(params[:category_id]).posts

render ‘index’

end

in the route.rb

get ‘posts/category_show’ => ‘posts#category_show’, :as => ‘category_show’

In the view: (THIS IS WRONG)

<% @posts.each do |post| %>

<%= post.title %> <%= post.body %> <%= post.category.name%> <%= link_to 'category_show', category_show_path %>

<% end %>

This results in a error:

Couldn’t find Category without an ID

Fred - thank you for getting me this far. Any comments on the above would be appreciated.