How do you order by a custom model method that has no attribute in SQL?

Previously I ordered my posts as this:

@posts = Post.find(:all, :order => "created_at DESC")

But now I want to replace created_at with a custom method I wrote in the Post model that gives a number as its result.

My guess:

@posts = Post.find(:all, :order => "custom_met开发者_C百科hod DESC")

which fails..

It fails because you are asking your db to do the sorting.

@posts = Post.all.sort {|a,b| a.custom_method <=> b.custom_method}

Note that this becomes non-trivial when you want to start paging results and no longer wish to fetch .all. Think about your design a bit before you go with this.

Just to expand on @Robbie's answer

Post.all.sort_by {|post| post.custom_method }.reverse

As the first answer noted, order is an Active Record command that essentially does a SQL query on your database, but that field doesn't actually exist in your database.

As someone else commented, you can more cleanly run the Ruby method sort_by by using the ampersand (more info here):

Post.all.sort_by(&:custom_method)

However, things do get complicated depending on what you want to do in your view. I'll share a case I recently did in case that helps you think through your problem. I needed to group my resource by another resource called "categories", and then sort the original resource by "netvotes" which was a custom model method, then order by name. I did it by:

• Ordering by name in the controller: @resources = Resource.order(:name)
• Grouping by category in the outer loop of the view: <% @resources.group_by(&:category).each do |category, resources| %>
• Then sorting the resources by votes in the partial for resources: <%= render resources.sort_by(&:netvotes).reverse %>

The view is a bit confusing, so here is the full view loop in index.html.erb:

<% @resources.group_by(&:category).each do |category, resources| %>
  <div class="well">
    <h3 class="brand-text"><%= category.name %></h3>
    <%= render resources.sort_by(&:netvotes).reverse %>
  </div>
<% end %>

And here is the _resource.html.erb partial:

<div class="row resource">
  <div class="col-sm-2 text-center">
    <div class="vote-box">
      <%= link_to fa_icon('chevron-up lg'), upvote_resource_path(resource), method: :put %><br>
      <%= resource.netvotes %><br>
      <%= link_to fa_icon('chevron-down lg'), downvote_resource_path(resource), method: :put %>
    </div>
  </div>
  <div class="col-sm-10">
    <%= link_to resource.name, resource.link, target: "_blank" %>
    <p><%= resource.notes %></p>
  </div>
</div>

This is a bit more complicated than what I like but this I like to keep my sort to stay as a active record model so its bit more complicated than just

Post.all.sort_by {|post| post.custom_method }

what I do is:

ids = Post.all.sort_by {|post| post.custom_method }.map(&:ids)
Post.for_ids_with_order(ids)

this is a custom scope in the Post model

#app/models/post.rb
class Post < ApplicationRecord
  ...
    scope :for_ids_with_order, ->(ids) {
    order = sanitize_sql_array(
      ["position(id::text in ?)", ids.join(',')]
    )
    where(:id => ids).order(order)
  }

  ...
end

I hope that this help

Well, just Post.find(:all) would return an array of AR objects. So you could use Array.sort_by and pass it a block, and since those records are already fetched, you can access the virtual attribute inside the block that sort_by takes.

RDoc: Enumerable.sort_by

Keep in mind that sort_by will return an Array, not an ActiveRecord::Relation, which you might need for pagination or some other some view logic. To get an ActiveRecord::Relation back, use something like this:

order_by_clause = Post.sanitize_sql_array(<<custom method expressed in SQL>>, <<parameters>>)
Post.all.order(Arel.sql(order_by_clause))

in rails 3 we can do this as: Post.order("custom_method DESC")
When upgrading app from rails2 to rails3