· Apr 19, 2019

SQL joining tables on collection list

Class ICT.Experiments.A Extends %Persistent
Property Name As %String;
Property Collection As list Of ICT.Experiments.B;
Property Collection2 As array Of ICT.Experiments.C;

Class ICT.Experiments.B Extends %Persistent
Property Name As %String;

Class ICT.Experiments.C Extends %Persistent
Property Name As %String;

I have the classes above and I can select columns from the array collection by using two joins as follows:

mainTable.ID, mainTable.Name, c.Name
FROM ICT_Experiments.A mainTable
LEFT JOIN ICT_Experiments.A_Collection2 arrayTable ON arrayTable.A = mainTable.%ID
LEFT JOIN ICT_Experiments.C c ON c.%ID = arrayTable.Collection2

This is quite straightforward since array collections map onto a table of their own. But I am having a hard time selecting similar information for the list collection. In the documentaion all I could find was an example, which used a list of string instead of list of objects.

I want to iterate through the resultset of the query in code. But when I use "ResultSet.%Get("Collection")", all I get is a list of the primary keys of the objects. Any idea on how to go about this in a single query?

Discussion (9)0
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I may misunderstand your intentions but

when I use "ResultSet.%Get("Collection")", all I get is a list of the primary keys of the objects

now you all you miss for each PrimaryKey is 

Set collB=##class(ICT.Experiments.B).%OpenId(primaryKey) 


Set collC=##class(ICT.Experiments.C).%OpenId(primaryKey)

and the object is yours.

With your class definition,  PrimaryKey is the Idkey of the Object.

You can also write a stored procedure which accepts id and returns the field you need.

Here's an example:

ClassMethod ListProp(id, prop, listprop) As %String [ SqlProc ]
    set ids = $classmethod(, prop _ "GetStored", id)
    quit:'$lv(ids) ""
    set ptr = 0
    set result = ""
    if listprop="id" {
        while $listnext(ids, ptr, value) {
            set result = result _ value
    } else {
        set propClass = $$$defMemberKeyGet($classname(),$$$cCLASSproperty,prop,$$$cPROPtype)
        while $listnext(ids, ptr, value) {
            set result = result _ $lb($classmethod(propClass, listprop _ "GetStored", $lg(value)))
    quit $lts(result)


  • id - root object id
  • prop - name of "List Of" property (in your case - Collection)
  • listprop - name of element property (in your case  name of any property from ICT.Experiments.B class)

Depends a bit on what you want. If you want to use them in the WHERE clause, you can leave the "list of" structure as-is and use our %FOR SOME ELEMENT syntax. In your case, for retrieving them in the SELECT list, changing the projection or your data model is probably the pragmatic choice. Note that collections are a really powerful feature when working in the Object paradigm, but somewhat constrained by SQL standard operations when accessing through the relational paradigm.