synapsestudios/synapse-base — pull request #75 by paulstatezny (2014-05-23 19:38:33+00:00)
Title: Create RowExists Validator

## Create RowExists Validator

### Acceptance Criteria
1. `RowExists` validator constraint exists for Symfony Validator component.
1. Constraint takes a mapper upon construction.
1. Constraint calls `findById` on the mapper with the given value.
1. If `findById` does not return an `AbstractEntity`, constraint adds a violation.