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Now that we know how to detect errors and

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redirect back to a form when we do see
an error we have three tasks left.

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First, we'll need a place to display
those errors in our Thymeleaf templates.

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Second, we need to make error messages

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available to the templates
from our controller.

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And last we'll also make available to
our Thymeleaf templates any data that

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the user had previously entered, so
that it doesn't have to be retyped, ready?

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Let's do it.

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First, let's tackle the issue of
redisplaying the data a user has entered

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if validation has failed, so
that it doesn't have to be retyped.

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This is where we can take
advantage of that timely

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object binding we talked
about earlier in the course.

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Let me open the category
form to remind you of that,

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here in the form element
is where we bound.

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That object called category and
inside this form element,

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that is some of its child elements.

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Namely, the input and select element

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refer to some of the properties of that
bound object using this asterisk here.

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Now as long as Thymeleaf has an object
name category in the model map,

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it will use the categories properties in
pre populating the form field values.

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In our case, the name and the color code.

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So back to the category controller.

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What we need to do is add this category
that's passed into this controller method

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here to the model map.

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The problem is that we're redirecting, so

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that if we were to include a model
parameter here and add it as an attribute,

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that attribute wouldn't survive past
the processing of this current request.

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But we do need it to survive the redirect
that is we need it to be available

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on the next request,
the one that we're redirecting to.

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That is this categories/ add request.

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Thankfully, Spring offers a solution for
this and

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that is the redirect attributes object.

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If we add a redirect attributes
parameter to this method,

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any flash attributes added will
survive exactly one redirect.

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So let me add a RedirectAttributes
parameter here.

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RedirectAttributes and
I will call it the same thing.

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And now, if we do detect errors,
let's include the category object that

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was submitted so
that it's available upon redirect.

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So here, I'm gonna add the category
if invalid data was received.

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So the way we'll do that is the following.

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We will use the add flash attribute method
that will make the attribute survive

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exactly one redirect and any subsequent
requests won't include this exact data.

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Here's how it looks, we'll use that
redirect attributes parameter value and

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we will addFlashAttribute just like this.

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We'll call it category and
we'll add whatever category

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was passed into this method which
would be a compilation of values that

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came from the POST Request that is from
the form data that the user submitted.

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Before we take our changes for a spend,
let's take a peek at one more item.

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Think about this if evaluation fails,

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we're gonna redirect
back to this add form.

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Now let's go to the method
that renders this view and

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keep in mind that we're adding a category
object here to be available for

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exactly one redirect.

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Let's go up to the method that
processes the form itself.

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Let me find that one.

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Here it is, formNewCategory.

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Looking closely,

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it appears that even if we redirected with
the category item included in the model.

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We are replacing it here with a newly
constructed category object which will

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blow away all of the data that was
previously entered by the user.

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So let's make sure not to overwrite
that if one does exist and

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the way we do that is the following.

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We say, if the
(!model.containsAttribute("category")),

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then we'll add a new
category to the model.

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And if it does contain a model
attribute named category.

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Well, we won't add another one because
that must mean a user had submitted

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invalid data.

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And we added a category attribute
that included that category

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object with invalid data and that will
be the one we want to use in the form.

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All right, with that in place,
let's reboot this app.

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And see if we successfully
redirect if there are errors and

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include the previously
entered data in the form.

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So I will stop my previous instance
of the app and i will rerun that.

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Looks like it compiled successfully and
we get all the way down

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to the message that says
the application started successfully.

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Cool, let me switch to Chrome now.

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Let me try to add a category here and
I will specify a color, but

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I will completely omit the Category Name.

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So i will Add that and great!

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We're back at the form with the color
that we previously chose, so

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we don't have to choose that again.

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Now let's check out that size validation
by trying to add a category with

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both say two characters, say Hi.

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And I will add that and there again,
we're back at the form seeing

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the data we already entered so
we don't have to re-enter that.

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Now finally,

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let's make sure we can still successfully
add a category of all data is valid.

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Let me finish this here with Hibernate and
click Add, great!

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We're done with that part and next,

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we'll tackle the display of validation
messages in our Thymeleaf templates.