Mongo DB Sorting issue with Ascending order - mongodb

Question

I have a collection named "formTest123" with following records:
/* 0 */
{
"_id" : ObjectId("5784f5aeef31a98294231459"),
"data" : [
{
"name" : "Amir",
"Other" : [
{
"data" : {
"city" : {
"address" : "pncjj"
}
}
},
{
"data" : {
"state" : {
"address" : "xyz"
}
}
}
]
}
]
}
/* 1 */
{
"_id" : ObjectId("5784f62cef31a9829423145a"),
"data" : [
{
"name" : "Zssmir",
"Other" : [
{
"data" : {
"city" : {
"address" : "bncd"
}
}
},
{
"data" : {
"state" : {
"address" : "gyk"
}
}
}
]
}
]
}
/* 2 */
{
"_id" : ObjectId("5784f636ef31a9829423145b"),
"data" : [
{
"name" : "Cmir",
"Other" : [
{
"data" : {
"city" : {
"address" : "tuhn"
}
}
},
{
"data" : {
"state" : {
"address" : "knm"
}
}
}
]
}
]
}
When I query this collection with:
db.formTest123.find().sort( { "data.Other.data.city.address" : -1})
means in descending order it gives correct output that is :
/* 0 */
{
"_id" : ObjectId("5784f636ef31a9829423145b"),
"data" : [
{
"name" : "Cmir",
"Other" : [
{
"data" : {
"city" : {
"address" : "tuhn"
}
}
},
{
"data" : {
"state" : {
"address" : "knm"
}
}
}
]
}
]
}
/* 1 */
{
"_id" : ObjectId("5784f5aeef31a98294231459"),
"data" : [
{
"name" : "Amir",
"Other" : [
{
"data" : {
"city" : {
"address" : "pncjj"
}
}
},
{
"data" : {
"state" : {
"address" : "xyz"
}
}
}
]
}
]
}
/* 2 */
{
"_id" : ObjectId("5784f62cef31a9829423145a"),
"data" : [
{
"name" : "Zssmir",
"Other" : [
{
"data" : {
"city" : {
"address" : "bncd"
}
}
},
{
"data" : {
"state" : {
"address" : "gyk"
}
}
}
]
}
]
}
But When I query with:
db.formTest123.find().sort( { "data.Other.data.city.address" : 1})
to get in ascending order of "city.address" it gives:
/* 0 */
{
"_id" : ObjectId("5784f5aeef31a98294231459"),
"data" : [
{
"name" : "Amir",
"Other" : [
{
"data" : {
"city" : {
"address" : "pncjj"
}
}
},
{
"data" : {
"state" : {
"address" : "xyz"
}
}
}
]
}
]
}
/* 1 */
{
"_id" : ObjectId("5784f62cef31a9829423145a"),
"data" : [
{
"name" : "Zssmir",
"Other" : [
{
"data" : {
"city" : {
"address" : "bncd"
}
}
},
{
"data" : {
"state" : {
"address" : "gyk"
}
}
}
]
}
]
}
/* 2 */
{
"_id" : ObjectId("5784f636ef31a9829423145b"),
"data" : [
{
"name" : "Cmir",
"Other" : [
{
"data" : {
"city" : {
"address" : "tuhn"
}
}
},
{
"data" : {
"state" : {
"address" : "knm"
}
}
}
]
}
]
}
That is clearly wrong as now records are not sorted in ascending order by "city.address"
Can any one guess what is problem with ascending order?

Answer 1

It is because it is ordering from the "lowest" to the "highest".
Problem is, that undefined and/or null has "lower" value than any existing number.
When you ordering by array, it looks to all documents in that array and takes the minimum value, which is "undefined" in the document, which contains state.
If you for example add "city.address" with high-enough value to all your documents, it will work for ascending. Like this one :
"Other" : [
{
"data" : {
"city" : {
"address" : "tuhn"
}
}
},
{
"data" : {
"state" : {
"address" : "knm"
}
"city" : {
"address" : "zzzzzzzzzzzzzzzzz"
}
}
}
]
Note : For descending, it takes maximum value, therefore it "works", because any city.address override all the null and undefined.
If you need ascending order and dont want to change the data structure, I would suggest to order it descending and then reverse order programically, if it is possible.

Answer 2

Since field "Other" is an array of (sub)documents, we must specify the index of the sub-document.
Based on the schema specified in question, following works.
db.formTest123.find().sort( { "data.Other.0.data.city.address" : 1});
and
db.formTest123.find().sort( { "data.Other.0.data.city.address" : -1});