def get_model_classes():
    """
    Retrieve a list of all the defined model classes.

    :rtype: ``list``
    """
    global MODEL_CLASSES

    if MODEL_CLASSES:
        return MODEL_CLASSES

    result = []
    for module_name in MODEL_MODULE_NAMES:
        module = importlib.import_module(module_name)
        model_classes = getattr(module, "MODELS", [])
        result.extend(model_classes)

    MODEL_CLASSES = result
    return MODEL_CLASSES

    Retrieve a list of all the defined model classes.

    :rtype: ``list``
    
st2common/st2common/models/db/__init__.py