Rails convention - placing logic in view vs controller vs partial - ruby-on-rails

Question

Messages are displayed green if sent by the current user, and blue otherwise. Following Rails convention, where does that logic belong?
Introdution
The user will visit /group/:id to see the list of messages, so the corresponding view is views/groups/show.html.erb and the corresponding controller is controllers/groups_controller.rb.
The message we want to display are in an array in #group, as #group.messages. The array is sorted by timestamp.
The code to style the color of the message is not important, but for simplicity purposes we will say there are two class selectors (one for from and one for to) and we can simply add a class attribute to the div that a message is within to change its color.
Both the user's sent and received messages are held in the array #group.messages.
If we have an individual message stored in message, we can test if it was sent by the current user with:
if session[:user_id] == message.user_id
Problem
Messages are ordered by timestamp and will need to be displayed in that order. For this reason, I can't see any clean way of handling the logic in the controller.
I would like to keep as much logic as possible out of the views and especially out of the partials, but after considering the options for rendering sent and received messages in different ways, the cleanest option I've found is to put the logic in the message partial.
Handling the logic in the message partial:
<% if message.user.id == session[:user_id] %>
<div class="to">
<p> <%= message.body %> </p>
</div>
<% else %>
<div class="from">
<p> <%= message.body %> </p>
</div>
<% end %>
Pros:
This method handles the logic with one if statement that is clean and simple
It allows us to make the code DRY because we won't have to use the logic anywhere else if we want it on other pages
Since every message only has a body, we don't have to make another partial to display messages without this formatting
Cons:
The logic is in the partial! I think people I'm working with or other programmers or even myself would first look in the controller then in the view then in the partial to make any changes or see the code
This doesn't feel like normal Rails convention
Handling the logic in the view:
Possibly two clean solutions -
1) Style the messages inside the logic or
2) Render a different partial for sent/received messages
Styling inside the logic:
<% #group.messages.each do |message| %>
<% if message.user.id == session[:user_id] %>
<div class="to">
<p> message.body </p>
</div>
<% else %>
<div class="from">
<p> message.body </p>
</div>
<% end %>
<% end %>
Rendering different partials:
<% #group.messages.each do |message| %>
<% if message.user.id == session[:user_id] %>
<%= render :partial => '/messages/sent_message', :message => message %>
<% else %>
<%= render :partial => '/messages/received_message', :message => message %>
<% end %>
<% end %>
Pros:
Either view solution keeps the logic out of the partial
It makes sense that showing something as one color or another is decided in the view
The view solution using two partials is clean and allows us to avoid styling within logic which also means that we can change the style within the partials and affect the look of messages everywhere.
Cons:
Both view options mean that our code is no longer DRY. Using these methods will mean that if we want the same functionality on 3 other pages, we will have to write the same code 3 more times
It makes sense that a view shouldn't be deciding anything
The view solution using two partials means that we will crowd the views/messages folder with partials, and still not have a default partial for rendering messages
Both of the view solutions just feel dirty in my opinion
My main points about my solutions -
No option allows for the logic to be held within the controller
Placing the logic inside the view means that to provide the same functionality on multiple pages, the same code will be written in more than one place
The option that looks the cleanest and makes the most sense to me means putting logic inside a partial, and there must be a better way.. right?
None of the solutions seem like they follow Rails convention
Which of the three options I coded best follow Rails convention?
Is it possible to place the logic in the controller?
Is there a better way to design this so that there is a clear solution following Rails convention?

Answer 1

What you probably have realized is that each of the three versions you described is either not DRY or not scalable. You've done a great job analyzing pros and cons of each option, so there is very little for me to add there. :)
To add presentation functionality to your models, Rails community uses Presenters. There is a great article on Presenters here that explains more about them.
Basically, you'll want to have one partial for message:
<div class=<%=#presenter.css_class%>>
<p> <%= message.body %> </p>
</div>
Then Presenter:
class MessagesPresenter
def initialize(message, current_user)
#message = message
#current_user = current_user
end
def css_class
message.user == current_user ? 'to' : 'from'
end
private
attr_reader :message, :current_user
end
And controller:
#presenter = MessagesPresenter.new(#message, current_user)
Voila! The presenter is available in both views and partials and is a great place to stash all presentation logic.

Answer 2

Since the only difference in these examples in the CSS class, you're repeating yourself quite a bit. Can't you add or remove a class on the tag depending on whether the tag belongs to the current_user or not?
This is really a presentation issue, and you can handle this simple logic for displaying the correct CSS tag using decorators (http://johnotander.com/rails/2014/03/07/decorators-on-rails/). I recommend using Draper (https://github.com/drapergem/draper).
First, for simplicity, add a current_user helper method to application_controller.rb to return the authenticated user.
Add a Decorator:
MessageDecorator.rb
def recipient_class
user_id == current_user.id ? "to" : "from" # (user_id delegates to message object)
end
Now your views can have much cleaner logic
Views
Message Partial:
<div class="<%= message.recipient_class %>">
<p><%= message.body %></p>
</div>
collection partial in the main view:
<%= render partial: "message", collection: #messages, as: :message %>
Finally, call decorate on messages in your controller action:
#messages = #group.messages.decorate
EDIT
You can also use a simple helper method rather than a decorator:
def css_class_for_message(message)
message.user_id == current_user.id ? "to" : "from"
end