XML Reference Guide

Mar 14, 2003

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At this point we've got the very basics of a working Rails application, and using the scaffolding we can make changes to the database. Now we're going to look at taking control of that process. We'll start by editing an existing question, and then we'll move on to adding and deleting questions.

When we last left our application, the "edit" action was being handled by the scaffolding. To change that, start by creating a template that references the specific question we want to edit:

<html>
<head>
   <title>Questions, questions questions</title>
</head>

<body>
   <h2>Edit a question</h2>

   The existing question is "<%= @thisQuestion.questionText %>"

</body>
</html>

As before, if we access this page, we have a problem:

The problem lies in the fact that we haven't provided a specific object from which we want the questionText attribute. To solve the problem, create the edit action in the Questions controller, question_controller.rb:

class QuestionController < ApplicationController

   model :question
   
   scaffold :question

   def index
   end

   def list
      @questions = Question.find_all
   end

   def edit
      @thisQuestion = Question.find(@params["id"])
   end

end

As you may remember, the URL includes the id of the question we're working with. As such, we can extract it from the params array. (We'll look at this array in detail in the moment, but for now just take it on faith that the id value is in it.) The find method expects to receive the ID of the object you're looking for in any model, whether it's the Question, Survey, or Answer. It's just the way that Rails has been designed; models automatically have the find method, which takes the id of the particular object, goes to the database, and populates an object with the data it finds for that row.

(To digress for a moment, you can also find a group of objects, which are passed as an array, using the find_all method. With no arguments, this method finds all records. You also have the option of providing a filter, such as "id > 25" or "status = 'finished'" to limit the records you get back.)

Once we find it, we assign it to the thisQuestion variable, which gets passed to the template:

So now we have the correct question, so let's build an editing form around it in the "edit" template:

<html>
<head>
   <title>Questions, questions questions</title>
</head>
<body>

   <h2>Edit a question</h2>

  <form method="post" 
       action="/question/editExistingQuestion/<%= @thisQuestion.id %>">
    Edit the question: <br />   
                                             
    <%= text_field("theExistingQuestion", "questionText", 
                      "value" => @thisQuestion.questionText) %>                
    <br />

    <input type="submit" value="Save Question">
  </form>

</body>
</html>

OK, let's start at the top, with the form element. If you're familiar with HTML, you know that the action attribute represents the URL to which the data will be sent. In Rails, this means we want the URL that looks something like this:

http://localhost:3000/question/editExistingQuestion/12

In this case, the we're dealing with a question object, for which we want to perform the editExistingQuestion action, on the object with an id of 12. To replicate that in the action attribute, we construct the tag dynamically, asking Rails to add the id for the current object.

Next we have the text_field method, which takes the following arguments. First, the name of an array to which the data will belong. This will make more sense in a moment, when we see the actual data. Second, we have the name of the actual piece of data. This name should match the attribute name for the object, so Rails knows what to do with it. Finally, we have an optional HTML option that allows us to add an arbitrary attribute and value. When you have Rails render the page, either by entering the URL manually, or clicking on a "Change" link, it creates the actual form for you:

<html>
<head>
   <title>Questions, questions questions</title>
</head>

<body>
   <h2>Edit a question</h2>

  <form method="post" action="/question/editExistingQuestion/12">
    Edit the question: <br />                                                  
    <input id="theExistingQuestion_questionText" 
            name="theExistingQuestion[questionText]" size="30" type="text" 
            value="What is your name?" />                
    <br />

    <input type="submit" value="Save Question">
  </form>

</body>
</html>

Notice that Rails has taken care of housekeeping issues such as creating the actual input element and adding the appropriate attributes. Notice also that the name element hints that we are actually dealing with an array. The page itself looks perfectly normal:

Okay, so let's see what happens if we submit the form. First we need to create an action in the controller:

...
   def edit
      @thisQuestion = Question.find(@params["id"])
   end

   def editExistingQuestion
      render_text @params.inspect
   end

end

In this case, we are simply telling Rails to print out a representation of the parameters that are being submitted with the request so that we can see them. If we then go ahead and submit the form, we should see something like this:

{"action"=>"editExistingQuestion", 
"theExistingQuestion"=>{"id"=>"12", "questionText"=>"What is your name?"}, 
"controller"=>"question"}

Notice that the parameters are actually an array that has as one of its members (specifically, the theExistingQuestion member) another array. Now let's do something with that data:

...
   def editExistingQuestion
      thisQuestion = Question.find(@params["id"])
      thisQuestion.attributes = @params["theExistingQuestion"]
      if thisQuestion.save
         render_text "Question saved.  Thank you."
      else
         render_text "Sorry, no can do."
      end
   end

end

First, as before, we're finding the specific object we want to work with. Once we have it, we can specifically assign the theExistingQuesiton array to the attributes array for the object, and everything will fall into place. That's why we made sure that the name of the field matched the name of the attribute we wanted to set.

Once we've populated the data, all we have to do to save it to the database. We can do that with the save method, which returns true if the operation is successful. Try it by making a change and submitting the form:

To see if it worked, click the "back" button twice, and then reload the page if necessary:

Well, it worked, but it wasn't very convenient. Fortunately, we can tell Rails to send us back to the list of questions automatically:

...
   def editExistingQuestion
      thisQuestion = Question.find(@params["id"])
      thisQuestion.attributes = @params["theExistingQuestion"]
      if thisQuestion.save
         redirect_to(:action => "list")
      else
         render_text "Sorry, no can do."
      end
   end

end

You can redirect to any action, such as "new", or even a custom action that you've created.

Adding a new record is just as easy. Start by adding a form to the list template:

<html>
<head>
   <title>Questions, questions questions</title>
</head>

<body>
   <h2>Existing questions</h2>

   <% @questions.each do |@thisQuestion| %>
       <%= @thisQuestion.questionText %> 
       ( <%= link_to("Change", :action => "edit", :id => @thisQuestion.id) %> )
       <br />

   <% end %>

  <br />
  <form method="post" action="addNewQuestion">
    Add a new question:                                                   
    <%= text_field("newQuestion", "questionText") %>                
    <input type="submit" value="Save Question">
  </form> 

</body>
</html>

The principle is the same as editing the record, but because we don't have a specific object to deal with, we can be a bit more general in our action. We'll create the new action in a moment. The form, meanwhile, is just a typical form:

Now we just need to save the question when its submitted. Create the new addNewQuestion action:

...
   def editExistingQuestion
      thisQuestion = Question.find(@params["id"])
      thisQuestion.attributes = @params["theExistingQuestion"]
      if thisQuestion.save
         redirect_to(:action => "list")
      else
         render_text "Sorry, no can do."
      end
   end

   def addNewQuestion
      thisQuestion = Question.new
      thisQuestion.attributes = @params["newQuestion"]
      if thisQuestion.save
         redirect_to(:action => "list")
      else
         render_text "Sorry, no can do."
      end
   end

end

The process is virtually identical to editing an existing object, except that instead of populating it by finding a record in the database, we're using the Question class to create a new object. We can then populate it and save it, moving back to the list of existing questions if all goes well.

So what about deleting existing records? It's also a fairly simple process. Start by creating a "delete" link on the list template:

<html>
<head>
   <title>Questions, questions questions</title>
</head>

<body>
   <h2>Existing questions</h2>

   <% @questions.each do |@thisQuestion| %>
       <%= @thisQuestion.questionText %> 
       ( <%= link_to("Change", :action => "edit", :id => @thisQuestion.id) %> )
       ( <%= link_to("Delete", :action => "destroy", :id => @thisQuestion.id)%> )

       <br />
   <% end %>

  <br />
  <form method="post" action="addNewQuestion">
    Add a new question:                                                   
    <%= text_field("newQuestion", "questionText") %>                
    <input type="submit" value="Save Question">

  </form> 

</body>
</html>

The only difference between this link and the previous one is the action to which it takes you. In Rails, to delete an object, you "destroy" it, so we'll keep with that as the action name.

Actually deleting it is a straightforward process:

...
      if thisQuestion.save
         redirect_to(:action => "list")
      else
         render_text "Sorry, no can do."
      end
   end


   def destroy
      thisQuestion = Question.find(@params["id"])
      if thisQuestion.destroy
         redirect_to(:action => "list")
      else
         render_text "Sorry, no can do."
      end
   end

end

As in the case of editing the object, we first find the appropriate one in the database, but from there we use the destroy method instead of save.

So that takes care of creating, retrieving, editing, and deleting objects from the database, which is, let's face it, most of what you do in a dynamic web application. With a little tweaking for your own purposes, you should be ready to go.

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