Rails noob: getting value from database and showing on application.html.erb

Question

brief update I might not even bother with a form to submit the data to the database and instead just use Heroku console, so my only concern is retrieving it.

Original Question

I've read all the books but now that I'm trying to implement I'm getting confused

I created a model named Total (not the best name, but that's another problem) with three columns, profit, number and fees. Here's the console printout

t = Total.first
 => #<Total id: 1, profit: 500, number: 7, fees: 40, created_at: "2012-01-12 04:21:33", updated_at: "2012-01-12 04:21:58">

(Note, I assume that this entry will be available to present at localhost:3000 if made on the development database)

The easiest place for me to present this data (due to my webpage layout) is in application.html.erb so I've got a little table like this. When you look at this table, you can see how I wish to present the data by calling 'profit' on an instance variable @t.profit @t.number and @t.fees.

<table class = "condensed-table" >
        <thead>
          <tr>

            <th>Profit </th>
            <th>Number of Trades</th>
            <th> Fees</th>
          </tr>
        </thead>
        <tbody>
          <tr>


            <td>$<%= @t.profit  %> </td>
            <td><%=  @t.number %></td>
            <td><%=  @t.fees %></td>
          </tr>
          </tbody>
      </table>

So now it's only a question of getting the data from the database, and creating the instance variable so I can call @t.profit, just like I did in the console

ruby-1.9.2-p290 :019 > t.profit
 => 500

So with a Total.rb model and presentation in application.html.erb, I figured I needed to do this ....

@t = Total.find(params[:id])

to get the data from the database, but what should I call the method and where should i put it to make it work?

For example, if I had a pages controller and a pages view, I could just create an index action in the view like

def index
@t = Total.find(params[:id])
end

and then I could just do @t.profit (i assume). However, if I did something like that my table to present the data wouldn't be available on every part of my site.

I tried to put it in application_controller.rb

 def index

    @t = Total.find(params[:id])

  end

and got this error message

NoMethodError in Posts#index

Showing /Users/me/Sites/enki2/app/views/layouts/application.html.erb where line #73 raised:

undefined method `profit' for nil:NilClass

Extracted source (around line #73):

Your last sentence is very confusing. Can you explain a bit more about why you think that you wouldn't be able to use that variable throughout your site? (hint: you can) — sosborn
– sosborn, Commented Jan 12, 2012 at 4:55
@sosborn well what he wants is for it to be available to every view, not just a specific resource's view. That's how I understand it, anyway. — Srdjan Pejic
– Srdjan Pejic, Commented Jan 12, 2012 at 5:00
The problem with what you want is that there will be no params[:id] available with every view and even in those that where it will be, it'll differ based on context. I'm unsure why you need this application-wide. — Srdjan Pejic
– Srdjan Pejic, Commented Jan 12, 2012 at 5:01
I want it application wide, because I want it presented at the top of the page no matter what other content the user is looking at. Hey @sosborn, well if an index action in pages_controller.rb would have worked, then it should also work in application.html.erb, but I got the error message you see in the updated OP. — Leahcim
– Leahcim, Commented Jan 12, 2012 at 5:03
@SrdjanPejic would you be able to write an answer explaining what I should be doing then? — Leahcim
– Leahcim, Commented Jan 12, 2012 at 5:04

PinnyM · Accepted Answer · 2012-01-12 05:04:38Z

1

You can do this via a filter hook - it allows a method (or several methods) to be called for all actions, and if needed, for all controllers. You can also explicitly exclude it from firing for certain actions and controllers. See - http://apidock.com/rails/ActionController/Filters/ClassMethods/before_filter

However, in your case you probably won't have params[:id] available in every action call for every controller. If the table needs to be included as part of your general layout, you will probably need an id that doesn't change. You can use the acts_as_singleton plugin for this - https://github.com/stephencelis/acts_as_singleton. Then you can use this in the partial itself:

    <% @t = Total.instance -%>
    <td>$<%= @t.profit  %> </td>
    <td><%=  @t.number %></td>
    <td><%=  @t.fees %></td>

Or you can simply assume it will always be the first:

    <% @t = Total.first -%>
    <td>$<%= @t.profit  %> </td>
    <td><%=  @t.number %></td>
    <td><%=  @t.fees %></td>

answered Jan 12, 2012 at 5:04

PinnyM

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5 Comments

Leahcim Over a year ago

Thanks, does your second example Total.first require the plugin as well?

PinnyM Over a year ago

Nope, that's included free with Rails :)

Leahcim Over a year ago

Thanks, but what's with the hypen " -%> "?

Srdjan Pejic Over a year ago

<% -%> is an execution block inside an ERB template. It doesn't print out or convert anything, it just runs the code. Also, if these are currency amounts, you'll want to use the number_to_currency helper and remove the $ sign.

PinnyM Over a year ago

What @Srdjan Pejic said. In plain english, the carriage return character at the end of the line would cause the outputted html to look a bit weird, so using the extra hyphen (-%>) tells ERB to strip out the carriage return for this line

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