title: is there chance could trigger validation to a certain field of a data model by using SQLAlchemy
body: <pre><code>class DataXX(Base):
  __tablename__ = 'DataXX'
  
  MyID = Column(Integer, primary_key=True, unique=True)
  AnForeignID = Column(ForeignKey('xxxx.xxx'), nullable=False)
  </code></pre>
  <p>Then I am thinking to have a validate function to constrains required field <code>AnForeignID</code> by having the func inside of data class (below)</p>
  <pre><code>@validates('AnForeignID')
  def validate_func(self, key, value):
  # somehow this function never triggered, any help will apprecaite
  </code></pre>