split string expression in aggregation framework ?

[ddorian]

Basically, is there an expression to create some fields by splitting an existing field in the aggregation framework ? I didn't find anything from searching.

[Will Berkeley]

What do you have in mind as a "split", specifically? The string expression operators for the $project stage include a $substr operator that extracts a substring of a given string.

-Will

[Dorian Hoxha]

In python:

"0000:11111".split(":")[1]

Split the string by a character, returning an array of strings and choose one of them (by position).

On Thu, Sep 4, 2014 at 5:00 PM, Will Berkeley <william....@mongodb.com> wrote:

What do you have in mind as a "split", specifically? The string expression operators for the $project stage include a $substr operator that extracts a substring of a given string.

-Will

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[Will Berkeley]

Indeed, I know what a split function does, but what is your use case for a split? There's clearly no split function available in the aggregation framework but that doesn't mean a suitable substitute or workaround can't be found, depending on exactly what you're trying to do.

-Will

[Dorian Hoxha]

I have documents {"_id": "site_id:yymmdd:article_id", "views": 24"}

And i want to group top articles, and getting the 'article_id' by splitting the "_id" of the document.

On Thu, Sep 4, 2014 at 6:24 PM, Will Berkeley <william....@mongodb.com> wrote:

Indeed, I know what a split function does, but what is your use case for a split? There's clearly no split function available in the aggregation framework but that doesn't mean a suitable substitute or workaround can't be found, depending on exactly what you're trying to do.

-Will

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[Will Berkeley]

Two methods: first, you could split the _id before you insert and store it as an embedded document like

{ "_id": { "site_id" : site_id, "date" : yymmdd, "article_id" : article_id }, "views": 24 }

assuming you want to keep the combination of site_id, date, and article_id as the unique key. This seems best to me. It's upgrading the three conceptually distinct components of an _id value to three separate fields of the document.

Second, if the site_id has a fixed length, say 7 characters like the string "site_id", you can effectively split the current value of _id with the $substr operator:

> db.test.aggregate([{ "$project" : { "site_id" : { "$substr" : ["$_id", 0, 7 ] }, "date" : { "$substr" : [ "$_id", 8, 6 ] }, "article_id" : { "$substr" : ["$_id", 15, -1] } } }])

{ "_id" : "site_id:yymmdd:article_id", "site_id" : "site_id", "date" : "yymmdd", "article_id" : "article_id" }

-Will

[Dorian Hoxha]

The site_id is not a fixed length, but I can zero-pad it.

Thanks

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[Asya Kamsky]

I have a "stupid aggregation trick" that shows how you can get a first
location of a particular character in a string (which you can then use
as input to $substr. It's not pretty though.

http://www.kamsky.org/stupid-tricks-with-mongodb/ugly-way-to-parse-a-string-with-aggregation-framework

Asya

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