Ensure Unique indexes in embedded doc in mongodb - mongodb

Question

Is there a way to make a subdocument within a list have a unique field in mongodb?
document structure:
{
"_id" : "2013-08-13",
"hours" : [
{
"hour" : "23",
"file" : [
{
"date_added" : ISODate("2014-04-03T18:54:36.400Z"),
"name" : "1376434800_file_output_2014-03-10-09-27_44.csv"
},
{
"date_added" : ISODate("2014-04-03T18:54:36.410Z"),
"name" : "1376434800_file_output_2014-03-10-09-27_44.csv"
},
{
"date_added" : ISODate("2014-04-03T18:54:36.402Z"),
"name" : "1376434800_file_output_2014-03-10-09-27_44.csv"
},
{
"date_added" : ISODate("2014-04-03T18:54:36.671Z"),
"name" : "1376434800_file_output_2014-03-10-09-27_44.csv"
}
]
}
]
}
I want to make sure that the document's hours.hour value has a unique item when inserted. The issue is hours is a list. Can you ensureIndex in this way?

Answer 1

Indexes are not the tool for ensuring uniqueness in an embedded array, rather they are used across documents to ensure that certain fields do not repeat there.
As long as you can be certain that the content you are adding does not differ from any other value in any way then you can use the $addToSet operator with update:
db.collection.update(
{ "_id": "2013-08-13", "hour": 23 },
{ "$addToSet": {
"hours.$.file": {
"date_added" : ISODate("2014-04-03T18:54:36.671Z"),
"name" : "1376434800_file_output_2014-03-10-09-27_44.csv"
}
}}
)
So that document would not be added as there is already an element matching those exact values within the target array. If the content was different (and that means any part of the content, then a new item would be added.
For anything else you would need to maintain that manually by loading up the document and inspecting the elements of the array. Say for a different "filename" with exactly the same timestamp.
Problems with your Schema
Now the question is answered I want to point out the problems with your schema design.
Dates as strings are "horrible". You may think you need them but you do not. See the aggregation framework date operators for more on this.
You have nested arrays, which generally should be avoided. The general problems are shown in the documentation for the positional $ operator. That says you only get one match on position, and that is always the "top" level array. So updating beyond adding things as shown above is going to be difficult.
A better schema pattern for you is to simply do this:
{
"date_added" : ISODate("2014-04-03T18:54:36.400Z"),
"name" : "1376434800_file_output_2014-03-10-09-27_44.csv"
},
{
"date_added" : ISODate("2014-04-03T18:54:36.410Z"),
"name" : "1376434800_file_output_2014-03-10-09-27_44.csv"
},
{
"date_added" : ISODate("2014-04-03T18:54:36.402Z"),
"name" : "1376434800_file_output_2014-03-10-09-27_44.csv"
},
{
"date_added" : ISODate("2014-04-03T18:54:36.671Z"),
"name" : "1376434800_file_output_2014-03-10-09-27_44.csv"
}
If that is in it's own collection then you can always actually use indexes to ensure uniqueness. The aggregation framework can break down the date parts and hours where needed.
Where you must have that as part of another document then try at least to avoid the nested arrays. This would be acceptable but not as flexible as separating the entries:
{
"_id" : "2013-08-13",
"hours" : {
"23": [
{
"date_added" : ISODate("2014-04-03T18:54:36.400Z"),
"name" : "1376434800_file_output_2014-03-10-09-27_44.csv"
},
{
"date_added" : ISODate("2014-04-03T18:54:36.410Z"),
"name" : "1376434800_file_output_2014-03-10-09-27_44.csv"
},
{
"date_added" : ISODate("2014-04-03T18:54:36.402Z"),
"name" : "1376434800_file_output_2014-03-10-09-27_44.csv"
},
{
"date_added" : ISODate("2014-04-03T18:54:36.671Z"),
"name" : "1376434800_file_output_2014-03-10-09-27_44.csv"
}
]
}
}
It depends on your intended usage, the last would not allow you to do any type of aggregation comparison across hours within a day. Not in any simple way. The former does this easily and you can still break down selections by day and hour with ease.
Then again, if you are only ever appending information then your existing schema should be find. But be aware of the possible issues and alternatives.