mongo query doesn't work with $ in field name - mongodb

Question

Command to get raw_data:
db.raw_data.find({'cat':'like'},
{'properties':1}).limit(1).pretty()
data:
{
"_id" : ObjectId("5656b9a0c2492dec3442da52"),
"properties" : {
"subcategory" : "49",
"$carrier" : "Vodafone India",
"$radio" : "HSDPA",
"$region" : "Gujarat",
"$screen_width" : 375,
"$wifi" : false,
"mp_lib" : "iphone",
"product_unlike_flag" : false,
"mp_device_model" : "iPhone7,2",
"user_id" : "4",
"$city" : "Ahmedabad",
"$manufacturer" : "Apple",
"$os" : "iPhone OS",
"brand" : "AO",
"gender" : "Men",
"mp_country_code" : "IN",
"time" : 1445376786,
"$app_release" : "0.8.0",
"$lib_version" : "2.8.2",
"$model" : "iPhone7,2",
"$screen_height" : 667,
"category" : "48",
"$app_version" : "0.8.0",
"$os_version" : "9.0.2",
"itemcode" : "174",
"source" : "Product"
}
}
I want to extract user_id and city from this data.
I tried these Command :
Command1 :
db.raw_data.aggregate([{$group : {_id :{'user_id': "$properties.user_id","cat":"$cat","brand":"$properties.brand" } ,"num_tutorial" :{$sum:1}}} ,{ $project : {properties.$city : 1 } } ])
Command2:
db.raw_data.find({'cat':'like'},{'properties.$city':1})
which gave me an error:
Error: error: {
"$err" : "Can't canonicalize query: BadValue Positional projection 'properties.$city' does not match the query document.",
"code" : 17287
}
Is there any way/query so I can extract user-city relations from this data.

Answer 1

In suck cases mongodb recommends to use unicode equivalent of '$'. This method is called $ sign escaping.
So your find query will look like following:
db.raw_data.find({'cat':'like'},{'properties.\uff04city':1})
Hope this helps :)
Result:
> db.collection.find({},{'properties.\uff04city':1})
{ "_id" : ObjectId("5656e09ccb0a925b3d5d16f2"), "properties" : { "＄city" : "value" } }
{ "_id" : ObjectId("5656e502cb0a925b3d5d16f3"), "properties" : { "＄city" : "value" } }