I ran into an interesting ObjectScript use case today with a general solution that I wanted to share.
Use case:
I have a JSON array (specifically, in my case, an array of issues from Jira) that I want to aggregate over a few fields - say, category, priority, and issue type. I then want to flatten the aggregates into a simple list with the total for each of the groups. Of course, for the aggregation, it makes sense to use a local array in the form:
agg(category, priority, type) = total
Such that for each record in the input array I can just:
Do $increment(agg(category, priority, type))
But once I've done the aggregation, I want to get that into an easier form to iterate over, like an integer-subscripted array:
summary = n summary(1) = $listbuild(total1, category1, priority1, type1) ... summary(n) = $listbuild(totalN, categoryN, priorityN, typeN)
Basic Solution:
The simple approach is just to have three nested For loops with $Order - for example:
Set category = ""
For {
Set category = $Order(agg(category))
Quit:category=""
Set priority = ""
For {
Set priority = $Order(agg(category,priority))
Quit:priority=""
Set type = ""
For {
Set type = $Order(agg(category,priority,type),1,total)
Quit:type=""
Set summary($i(summary)) = $listbuild(total,category,priority,type)
}
}
}
That's what I started out with, but it's a lot of code, and if I had more dimensions to aggregate over it'd get unwieldy quickly. It made me wonder - is there a general solution to accomplish the same thing? It turns out, there is!
Better Solution with $Query:
I decided that using $query would help. Note that this solution assumes uniform depth of subscripts/values across the whole local array; weird things would happen if this assumption was violated.
ClassMethod Flatten(ByRef deep, Output flat) [ PublicList = deep ]
{
Set reference = "deep"
For {
Set reference = $query(@reference)
Quit:reference=""
Set value = $listbuild(@reference)
For i=1:1:$qlength(reference) {
Set value = value_$listbuild($qsubscript(reference,i))
}
Set flat($i(flat)) = value
}
}
So the above snippet is replaced with:
Do ..Flatten(.agg,.summary)
A few things to note about this solution:
- deep needs to be in the PublicList for $query to be able to operate on it
- in each iteration, reference is changed to reference the next set of subscripts in deep that has a value - e.g., the value might be: deep("foo","bar")
- $qlength returns the number of subscripts in reference
- $qsubscript returns the value of the i'th subscript of reference
- When $listbuild lists are concatenated, a valid $listbuild list with the combined lists is the result (this is way better than using any other delimiter!)
Summary
$query, $qlength, and $qsubscript are handy for dealing with globals/local arrays of arbitrary depth.
Further Reading
$Query: https://docs.intersystems.com/irisforhealthlatest/csp/docbook/DocBook.UI.Page.cls?KEY=RCOS_FQUERY
$QSubscript: https://docs.intersystems.com/irisforhealthlatest/csp/docbook/Doc.View.cls?KEY=RCOS_fqsubscript
$QLength: https://docs.intersystems.com/irisforhealthlatest/csp/docbook/Doc.View.cls?KEY=RCOS_fqlength
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