在Aggregation Pipeline Stages中，有一个$lookup函数，具体的用法如下
{
$lookup:
{
from: <collection to join>,
localField: <field from the input documents>,
foreignField: <field from the documents of the "from" collection>,
as: <output array field>
}
}
而如果localField是 DBRef里面的$id，那么则不能lookup
$lookup:
{
from: "tableA",
localField: "tableA.$id",
foreignField: "_id",
as:"tableA"
}
}
会返回如下错误
assert: command failed:
{ "ok" : 0, "errmsg" : "FieldPath field names may not start with '$'.", "code" : 16410 }
原因是管道函数中限制了fieldName不能以$开始，所以此处会报错。
When change notification is first stage in aggregation any data manipulation that's done in that pipeline may need to project or check/use the value of "o.$__" field ($set, $unset, etc).
解决办法:在lookup前加入下面管道代码生成一个去掉$的field
{$addFields:{"newfoo":
{$arrayToObject:{$map:{
input:{$objectToArray:"$foo"},
in:{
k:{$cond:[
{$eq:[{"$substrCP":["$$this.k",0,1]},{$literal:"$"}]},
{$substrCP:["$$this.k",1,{$strLenCP:"$$this.k"}]},
"$$this.k"
]},
v:"$$this.v"
}
}}}
}}
此处的newfoo为新增的field的别名，foo为DBRef的field,其它代码为固定写法，也就是可以复制此处代码修改两个地方，效果如下
before
{
"foo":{
"$ref":"foo",
"$id":"123"
}
}
after
{
"foo":{
"$ref":"foo",
"$id":"123"
},
"newfoo":{
"ref":"foo",
"id":"123"
}
}
那么我们就可以开心的使用lookup了。
参考官方还在OPEN的defectSERVER-14466,也许以后可以用$开头作为fieldName使用，但是暂时的代替方法为上述方法.
