MongoDB插入/更新/删除数组的元素，该元素是另一个数组的元素

Consider the following document:

请考虑以下文档：

{
    "entity_id" : 10,
    "features" : 
    [ 
    {
        "10" : "Test System 2"
    }, 
    {
        "20" : "System 2 Description"
    }, 
    {
        "30" : ["Free", "Monthly", "Quaterly"]
    }, 
    {
        "40" : ["Day", "Swing"]
    }
],
}
{