code: def _discover_ideal_backend(orm_backend):
      
      if orm_backend:
          return orm_backend
  
      if peewee is not MISSING and sqlalchemy is not MISSING:
          raise RuntimeError('Both PeeWee and SQLAlchemy detected as installed, '
                             'but no explicit backend provided! Please specify '
                             'one!')
      if peewee is not MISSING:
          return _PEEWEE_BACKEND
      elif sqlalchemy is not MISSING:
          return _SQLALCHEMY_BACKEND
      else:
          return MISSING
docs: Auto-discover the ideal backend based on what is installed.
  
      Right now, handles discovery of:
        * PeeWee
        * SQLAlchemy
  
      Args:
          orm_backend (str): The ``orm_backend`` value that was passed to the
              ``create_app`` function. That is, the ORM Backend the User
              indicated they wanted to use.
  
      Returns:
          str|fleaker.missing.MissingSentinel: Returns a string for the ideal
              backend if it found one, or :obj:`fleaker.MISSING` if we couldn't
              find one.
  
      Raises:
          RuntimeError: Raised if no user provided ORM Backend is given and BOTH
              PeeWee and SQLAlchemy are installed.