Validate Model after Evaluate

This is kind of example.

Let's have an @Entity with field

Code:

---

@NotBlank(message = "You have to upload your avatar")
private String avatar;

---

In flow, there is action handling, according to manual section:
http://static.springsource.org/sprin...multipart-form

Code:

---

<transition on="proceed" to="company">
    <evaluate expression="registerService.uploadAvatar(flowScope.userModel, fileUploadHandler)"
        result="flowScope.publisher" result-type="">
    </evaluate>
</transition>

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The problem is, model comes through validation before this "<evaluate>" is reached, and so the field avatar cannot be filled up with URL, and so the validation fails.

I'd like to validate model on transition but after processing this "<evaluate>"

How can we do it?