OrderBy in mongo db to get some thing like this - mongodb

Question

I am very new to mongodb. Please help me get a solution for this.
The below are the documents in mongo db :
{name : sass, city : banga},
{name : pass, city : banga},
{name : kanga, city : runga},
{name : jass, city : canga},
{name : dass, city : tunga}
I want to query with city as 'banga', so that the result array should contain documents with city 'banga' first and then all other documents.
Thanks.

Answer 1

Please try this. The below query will give you "banga" at the top and all other documents after that.
Note:-
Please note that the query doesn't sort the data by city. It just return the "banga" documents at the top. The documents where "city" not equals to "banga" could be in any order after that.
Query:-
You may need to change the collection name accordingly in the below query.
db.collection.aggregate([
{ $project: {_id : 1, "name": 1, "city" : 1,
isRequiredCity: { $cond: { if: { $eq: [ "$city", "banga" ] }, then: 0, else: 1 } }} },
{$sort : {"isRequiredCity" : 1} }
]);
Output:-
/* 1 */
{
"_id" : ObjectId("58342ccaba41f1f22e600c67"),
"name" : "sass",
"city" : "banga",
"isRequiredCity" : 0
}
/* 2 */
{
"_id" : ObjectId("58342ccaba41f1f22e600c6a"),
"name" : "pass",
"city" : "banga",
"isRequiredCity" : 0
}
/* 3 */
{
"_id" : ObjectId("58342ccaba41f1f22e600c68"),
"name" : "kanga",
"city" : "runga",
"isRequiredCity" : 1
}
/* 4 */
{
"_id" : ObjectId("58342ccaba41f1f22e600c69"),
"name" : "jass",
"city" : "canga",
"isRequiredCity" : 1
}
/* 5 */
{
"_id" : ObjectId("58342ccaba41f1f22e600c6b"),
"name" : "dass",
"city" : "tunga",
"isRequiredCity" : 1
}

Answer 2

An option for that is to add a specific field to all the documents. The new field will identify by a unique value all the documents contains city : banga.
Than you can order all the collection based on that values.
For example, lets add to all the collection the unique field and a unique value:
db.collection.update({'city':'banga'}, {$set:{"value":10}}, {upsert:false, multi:true});
db.collection.update({'city': { $ne: 'banga'}}, {$set:{"value":1}}, {upsert:false, multi:true});
Now, each document with field 'city'=='banga' has value = 10, and each document with field 'city'!='banga' has value = 1.
And now you can sort the documents in the collection using the value field.
db.collection.find({}).sort({'value': -1})