Rails%203%20In%20Action

Leaving a comment
It only selects the id column from the comments table and limits the result set to 1,
which results in a super-fast query to check if there are any comments. You used
exists? back in chapter 8 when you checked if a ticket had any assets. You could use
empty? here instead, but that would load the comments association in its entirety and
then check to see if the array was empty. If there were a lot of comments, then this
would be slow. By using exists?, you stop this potential performance issue from cropping
up.
Inside this div, if there are comments, you call render and pass it the argument of
@ticket.comments. On the end of that call, select on it.
You use select here because you don’t want to render the comment object you’re
building for the form at the bottom of the page. If you left off the select,
@ticket.comments would include this new object and render a blank comment box.
When you call select on an array, you can pass it a block that it will evaluate on all
objects inside that array and return any element that makes the block evaluate to anything
that’s not nil or false.
The argument you pass to select is called a Symbol-to-Proc and is a shorter way of
writing this:
{ |x| x.persisted? }
This is a new syntax versions of Ruby >= 1.8.7 and used to be in Active Support in Rails
2. It’s a handy way of writing a shorter block syntax if you’re only looking to call a single
method on an object.
The persisted? method checks if an object is persisted in the database by checking
if it has its id attribute set and will return true if that’s the case and false if not.
By using render in this form, Rails will render a partial for every single element in
this collection and will try to locate the partial using the first object’s class name.
Objects in this particular collection are of the Comment class, so the partial Rails will try
to find will be at app/views/comments/_comment.html.erb, but you don’t have this file
right now. Let’s create it and fill it with the content from the following listing.
Listing 10.8 app/views/comments/_comment.html.erb
Here you’ve used a new method, div_for. This method generates a div tag around
the content in the block and also sets class and id attributes based on the object
passed in. In this instance, the div tag would be the following:
The class method from this tag is used to style your
comments so that they will look like figure 10.3 when
the styles from the stylesheet are applied.
Figure 10.3 A comment
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