[Flow] Validation triggered when trying to show object in rest controller

[Christian Loock]

Hello,

I am currently trying to implement a simple REST Controller. I defined a 
route like so:

-
   name:'Project REST by FileSystem Name'
   uriPattern:'REST/project/{project.fileSystemName}'
   defaults:
     '@format':'json'
     '@package':'RA.ReleaseManager'
     '@controller':'projectRest'
   routeParts:
     project:
       objectType:'RA\ReleaseManager\Domain\Model\Project'

If I now try to call the controller, the validation of the Project Model 
is triggered:

Validation failed while trying to call 
RA\ReleaseManager\Controller\ProjectRestController->showAction().

In the Model, I have defined Validation for a property:

/**
  * The name, working copies of this project get in the file system, eg. flow_vkf_admin
  *@varstring
  *@Flow\Validate(type="RegularExpression", options={"regularExpression"="/^[^_]*_[^_]*_[^_]*$/"})
  *@Flow\Validate(type="NotEmpty")
  */
protected$fileSystemName;

But I wonder, why this is even triggered, and why it throws an error, 
since it is a model, that has been created before using the MVC stack, 
running through validation just fine. Unfortunatly it doesnt give me any 
further information on why the validation fails. I already looked into 
the logs, with no success in finding what is going wrong here.

So this let's me with a couple of questions:

1. Why is Validation even triggered on a safe (GET) request.
2. How is it possible that the validation fails, for an object that 
should be loaded from the database, according to the routing.
3. Where can I see why the validation fails in a REST controller?