Aggregation not eliminating invalid groupings based on common ancestor

[johngrimes]

This change relates to removing invalid combinations of grouping values when aggregations are executed over paths that share a common ancestor element.

This is show by the following example. Given a set of Observation resources:

id	name.given	name.family
1	Benjamin	Franklin
1	Silence	Dogood
2	Isaac	Asimov
2	Paul	French

And the following query:

{
  "resourceType": "Parameters",
  "parameter": [
    {
      "name": "aggregation",
      "valueString": "count()"
    },
    {
      "name": "grouping",
      "valueString": "name.given"
    },
    {
      "name": "grouping",
      "valueString": "name.family"
    }
  ]
}

We should get the following result:

Given name	Family name	Number of patients
Benjamin	Franklin	1
Silence	Dogood	1
Isaac	Asimov	1
Paul	French	1

Not this:

Given name	Family name	Number of patients
Benjamin	Franklin	1
Benjamin	Dogood	1
Silence	Franklin	1
Silence	Dogood	1
Isaac	Asimov	1
Isaac	French	1
Paul	Asimov	1
Paul	Frence	1

This is not technically a bug, but it is a refinement of the way that the Aggregate operation works, and it brings it in line with the Extract column joining logic.

[johngrimes]

Possible fixes for #387 may need to take the requirements of this issue into account.

[johngrimes]

I've added a couple of failing tests to the new branch issue/542.

[piotrszul]

@johngrimes I think that we discussed was that

{
  "resourceType": "Parameters",
  "parameter": [
    {
      "name": "aggregation",
      "valueString": "count()"
    },
    {
      "name": "grouping",
      "valueString": "name.given"
    },
    {
      "name": "grouping",
      "valueString": "name.family"
    }
  ]
}

would actually produce the second results (with 8 rows).

But:

{
  "resourceType": "Parameters",
  "parameter": [
    {
      "name": "aggregation",
      "valueString": "name.count()"
    },
    {
      "name": "grouping",
      "valueString": "name.given"
    },
    {
      "name": "grouping",
      "valueString": "name.family"
    }
  ]
}

would produce the first one (with 4 rows).

And something like this:

{
  "resourceType": "Parameters",
  "parameter": [
    {
      "name": "aggregation",
      "valueString": "name.where($this.given = 'Paul).count()"
    },
    {
      "name": "grouping",
      "valueString": "name.given"
    },
    {
      "name": "grouping",
      "valueString": "name.family"
    }
  ]
}

Should produce:

Given name	Family name	Number of patients
Paul	French	1

Although maybe it should be:

Given name	Family name	Number of patients
Benjamin	Franklin	0
Silence	Dogood	0
Isaac	Asimov	0
Paul	French	1

????