title: Flask-Admin Cannot find reverse relation
body: <p>I already have an application using my SQLAlchemy model, and I'm now trying to add a flask admin website on top to manage some of the data in the DB. This model is already setup and working throughout the existing application so I know the relationships are configured correctly. 
  In my DB the data_package table has a FK to the supplier table, and this has been added as a relationship. However, when I create a flask model view to allow editing of the data_package I'm receiving the error:</p>
  
  <blockquote>
    <p>Exception: Cannot find reverse relation for model </p>
  </blockquote>
  
  <p>I've added a subset of the code below, there are a lot more columns and logic but this is the bare minimum I have to try and test a fix to the issue.The issue is happening because I am adding <strong>inline_models = (Supplier,)</strong> which I need as I want to have a select to allow the supplier to be changed for a data package.</p>
  
  <p>The line of code throwing the exception is <a href="https://github.com/flask-admin/flask-admin/blob/master/flask_admin/contrib/sqla/form.py#L558" rel="noreferrer">https://github.com/flask-admin/flask-admin/blob/master/flask_admin/contrib/sqla/form.py#L558</a>. The reason for this is the relationship direction is defined as ONETOMANY, and there is a check to ensure the direction is MANYTOONE or MANYTOMANY</p>
  
  <pre><code>Base = declarative_base()
  
  
  class Supplier(Base):
      __tablename__ = 'supplier'
      id = Column(Integer, primary_key=True)
      name = Column(String(100), nullable=False)
  
  class DataPackage(Base):
      __tablename__ = 'data_package'
      id = Column(Integer, primary_key=True)
      name = Column(String(100), nullable=False)
  
      supplier_id = Column(Integer, ForeignKey('supplier.id'), index=True, nullable=False)
      supplier = relationship("Supplier", backref='packages')
  
  class DataPackageAdminView(ModelViewOnly):
      form_columns = ['name', 'supplier']
      inline_models = (Supplier,)
  
  def create_app():
      settings.configure_orm()
  
      app = Flask(__name__)
      app.config.from_pyfile('config.py')
  
      @app.route('/')
      def index():
          return '&lt;a href="/admin/"&gt;Click me to get to Admin!&lt;/a&gt;'
  
      admin = Admin(app, name='Test Editor', template_mode='bootstrap3')
      admin.add_view(DataPackageAdminView(DataPackage, settings.Session, category='Data Sources'))
  
      return app
  
  if __name__ == '__main__':
      app = create_app()
      app.run(debug=True)
  </code></pre>