Symfony validation with dynamic message

Question

I have been looking around and was wondering if it is possible to validate a form but dynamically add error message.

For example:

Test\TesterBundle\Model\Products:
    constraints:
        - Propel\PropelBundle\Validator\Constraints\UniqueObject:
            fields: [Url, SKU, Title]
            message: Field already exists

This if the Url isn't unique it will output Field already Exists. However, it doesn't state which field already exists.

My first thought was to use [fields] inside the message which through an error:

Test\TesterBundle\Model\Products:
    constraints:
        - Propel\PropelBundle\Validator\Constraints\UniqueObject:
            fields: [Url, SKU, Title]
            message: [fields] already exists

I then tried a simple %s to see if it already inserted this but just output the string.

Is there an already created method, would I need to extend the UniqueObject Constraint and create a new one that outputs the field name aswel, or would I need to create a new function to do this?