how to match the last value of array in mongo db? [duplicate] - mongodb

Question

I have a sample document like shown below
{
"_id" : "docID",
"ARRAY" : [
{
"k" : "value",
"T" : "20:15:35",
"I" : "Hai"
},
{
"K" : "some value",
"T" : "20:16:35",
"I" : "Hello"
},
{
"K" : "some other value",
"T" : "20:15:35",
"I" : "Update"
}
]
}
I am trying to update the last element in the "ARRAY" based on field "ARRAY.T"(which is only field i know at the point of update), but what my problem is first element in the array matches the query and its ARRAY.I field is updated.
Query used to update:
db.collection.update( { _id: "docID","ARRAY.T" : "20:15:35"},
{ $set: { "ARRAY.$.I": "Updated value" }
})
Actually i don't know index of the array where to update so i have to use ARRAY.I in the query, is there any way to to tell Mongodb to update the first element matched the query from last of the array.

Answer 1

I understand what you are saying in that you want to match the last element in this case or in fact process the match in reverse order. There is no way to modify this and the index stored in the positional $ operator will always be the "first" match.
But you can change your approach to this, as the default behavior of $push is to "append" to the end of the array. But MongoDB 2.6 introduced a $position modifier so you can in fact always "pre-pend" to the array meaning your "oldest" item is at the end.
Take this for example:
db.artest.update(
{ "array": { "$in": [5] } },
{ "$push": { "array": { "$each": [5], "$position": 0 } }},
{ "upsert": true }
)
db.artest.update(
{ "array": { "$in": [5] } },
{ "$push": { "array": { "$each": [6], "$position": 0 } }},
{ "upsert": true }
)
This results in a document that is the "reverse" of the normal $push behavior:
{ "_id" : ObjectId("53eaf4517d0dc314962c93f4"), "array" : [ 6, 5 ] }
Alternately you could apply the $sort modifier when updating your documents in order to "order" the elements so they were reversed. But that may not be the best option if duplicate values are stored.
So look into storing your arrays in "reverse" if you intend to match the "newest" items "first". Currently that is your only way of getting your "match from last" behavior.