[TYPO3-mvc] Fluid form problem

[Pascal Geldmacher]

Hey,

I've implemented a simple form view with fluid :

<f:form action="create" method="POST" name="post" object="{post}" 
arguments="{blog: blog}">
	<f:form.textbox property="title" />
	<f:form.textarea property="description" cols="46" rows="8" />
	<f:form.textbox property="author" /><br/>
	<f:form.textbox property="date"></f:form.textbox>
	
	<f:for each="{blog.categories}" as="category">
		<f:form.Checkbox property="categories" value="{category.uid}" 
/>{category.name}
	</f:for>
	<f:form.textarea property="twitterMessage" cols="46" rows="8" />
	<f:form.textbox property="trackback_url" /><br/>
	<f:form.submit value="Speichern" />
</f:form>

post is my model Post. The post object has a splObjectStorage with 
categories. The problem is that the single information how "title" or 
"description" will stored in the post table but it dont save the 
selected categories in the MM relation with the information in the post 
table how many categories this post has.
Could it be possible, that i must create the complete post in many 
steps. First the post information then the categories to the post and 
and and ?

thanks for help,

Pascal