title: Filtering column values in flask-admin with one to many relationship
body: <p>I am using flask-admin to create an admin interface. In my admin panel I have 2 tables first one is locations and the other one is demands which has a one to many relationship with location. I am trying to filter the location combobox on my demands form so users can only select active ones but I couldn't manage it. Is it possible? if yes how can I do it.</p>
  
  <p>I tried to do it with form_ajax_refs but I failed.</p>
  
  <pre class="lang-py prettyprint-override"><code>class Location(db.Model):
      __tablename__ = 'Locations'
      id = db.Column(db.Integer(), primary_key=True)
      name = db.Column(db.String(80), unique=True)
      active = db.Column(db.Boolean)
  
  class Demand(db.Model):
      __tablename__ = 'Demands'
      id = db.Column(db.Integer(), primary_key=True)
      name = db.Column(db.String(80), unique=True)
      delivery_from_id = db.Column(db.Integer, db.ForeignKey('Locations.id'))
      delivery_from = db.relationship(Location,
                                      backref=db.backref('demands_from', lazy='dynamic'),
                                      foreign_keys=[delivery_from_id],
                                      )
  
  class DemandView(ModelView):
          form_ajax_refs = {
              'delivery_from': QueryAjaxModelLoader('delivery_from', db.session, Location, fields=['name', 'active'],
                                                    filters=["active=True"]),
          }
  
  admin.add_view(ModelView(Location, db.session, 'Locations'))
  admin.add_view(DemandView(Demand, db.session, 'Demands'))
  </code></pre>