MongoDB组并将所有子文档合并到父文档中

Question

将文档分组并使用子文档中的一个字段作为关键字将每个组下的列表合并到父文档。

以下是我的文件

{ "_id" : "1", "a" : "11", "b" : "o11", "c" : "s11" }
{ "_id" : "2", "a" : "22", "b" : "o22", "c" : "s22" }
{ "_id" : "3", "a" : "11", "b" : "o12", "c" : "s12" }
{ "_id" : "4", "a" : "11", "b" : "o13", "c" : "s13" }
{ "_id" : "5", "a" : "22", "b" : "o23", "c" : "s23" }

我想按'a'分组并将子文档列表合并到一个文档中，如下所示：

{
    {a:"11", o11: { "_id" : "1", "a" : "11", "b" : "o11", "c" : "s11" }, o12: { "_id" : "3", "a" : "11", "b" : "o12", "c" : "s12" }, o13: { "_id" : "4", "a" : "11", "b" : "o13", "c" : "s13" }},
    {a:"22", o22: { "_id" : "2", "a" : "22", "b" : "o22", "c" : "s22" }, o23: { "_id" : "5", "a" : "22", "b" : "o23", "c" : "s23" }
}