(core) Allow linking to summary table via reference to source table

Summary:
Fixes a bug introduced in https://phab.getgrist.com/D3416 which exposed a new type of linking option that didn't actually work. Specifically it allowed selecting by a summary table when the target widget has a reference column to the source table of the summary. This diff correctly implements this linking by linking the reference column against the 'group' column of the summary table, so the source table data (which the client may not have access to) isn't involved. But the 'group' column name is hidden from the 'select by' option label to avoid confusion for users, so it just looks like another kind of summary table linking, and indeed it can be thought of purely in terms of matching groupby columns etc.

Discussion here: https://grist.slack.com/archives/C02EGJ1FUCV/p1654039063055499

Also fixes a related old bug that offered similar meaningless options involving summary tables, like selecting the summary table by the source table or linking summary tables with disjoint groupby columns.

Test Plan: Updated SelectBySummaryRef test and fixture doc for the new behaviour. Also updated a couple of older tests which were incorrectly asserting the buggy option to select a summary table by the source table.

Reviewers: dsagal

Reviewed By: dsagal

Subscribers: dsagal

Differential Revision: https://phab.getgrist.com/D3464

app/client/ui/selectBy.ts

@@ -3,6 +3,7 @@ import { IPageWidget } from 'app/client/ui/PageWidgetPicker';
 import { getReferencedTableId } from 'app/common/gristTypes';
 import { IOptionFull } from 'grainjs';
 import * as assert from 'assert';
+import * as gutil from "app/common/gutil";
 
 // some unicode characters
 const BLACK_CIRCLE = '\u2022';
@@ -40,6 +41,9 @@ interface LinkNode {
   // is the table a summary table
   isSummary: boolean;
 
+  // For a summary table, the set of col refs of the groupby columns of the underlying table
+  groupbyColumns?: Set<number>;
+
   // list of ids of the sections that are ancestors to this section according to the linked section
   // relationship
   ancestors: Set<number>;
@@ -56,6 +60,12 @@ interface LinkNode {
 }
 
 
+// Returns true if this node corresponds to the special 'group' reflist column of a summary table
+function isSummaryGroup(node: LinkNode): boolean {
+  return node.isSummary && node.column?.colId.peek() === "group";
+}
+
+
 // Returns true is the link from `source` to `target` is valid, false otherwise.
 function isValidLink(source: LinkNode, target: LinkNode) {
 
@@ -69,12 +79,36 @@ function isValidLink(source: LinkNode, target: LinkNode) {
     return false;
   }
 
-  // cannot link to the somewhat special 'group' reflist column of summary tables
-  // because in most cases it's equivalent to the usual summary table linking but potentially slower,
-  // and in other cases it may cause confusion with overwhelming options.
+  // Can only link to the somewhat special 'group' reflist column of summary tables
+  // with another ref/reflist column that isn't also a group column
+  // because otherwise it's equivalent to the usual summary table linking but potentially slower
   if (
-    (source.isSummary && source.column?.colId.peek() === "group") ||
-    (target.isSummary && target.column?.colId.peek() === "group")
+    isSummaryGroup(source) && (!target.column || isSummaryGroup(target)) ||
+    isSummaryGroup(target)
+  ) {
+    return false;
+  }
+
+  // Cannot directly link a summary table to a column referencing the source table.
+  // Instead the ref column must link against the group column of the summary table, which is allowed above.
+  // The 'group' column name will be hidden from the options so it feels like linking using summaryness.
+  if (
+    (source.groupbyColumns && !source.column && target.column) ||
+    (target.groupbyColumns && !target.column && source.column)
+  ) {
+    return false;
+  }
+
+  // If the target is a summary table and we're linking based on 'summaryness' (i.e. there are no ref columns)
+  // then the source must be a less detailed summary table, i.e. having a subset of the groupby columns.
+  // (or they should be the same summary table for same-record linking, which this check allows through)
+  if (
+    !source.column &&
+    !target.column &&
+    target.groupbyColumns && !(
+      source.groupbyColumns &&
+      gutil.isSubset(source.groupbyColumns, target.groupbyColumns)
+    )
   ) {
     return false;
   }
@@ -136,11 +170,15 @@ export function selectBy(docModel: DocModel, sources: ViewSectionRec[],
       // a human readable description
       let label = srcNode.section.titleDef();
 
-      // add the source node col name or nothing for table node
-      label += srcNode.column ? ` ${BLACK_CIRCLE} ${srcNode.column.label.peek()}` : '';
+      // add the source node col name (except for 'group') or nothing for table node
+      if (srcNode.column && !isSummaryGroup(srcNode)) {
+        label += ` ${BLACK_CIRCLE} ${srcNode.column.label.peek()}`;
+      }
 
-      // add the target column name only if target has multiple valid nodes
-      label += hasMany && tgtNode.column ? ` ${RIGHT_ARROW} ${tgtNode.column.label.peek()}` : '';
+      // add the target column name (except for 'group') only if target has multiple valid nodes
+      if (hasMany && tgtNode.column && !isSummaryGroup(tgtNode)) {
+        label += ` ${RIGHT_ARROW} ${tgtNode.column.label.peek()}`;
+      }
 
       // add the new option
       options.push({ label, value });
@@ -183,9 +221,11 @@ function fromViewSectionRec(section: ViewSectionRec): LinkNode[] {
     ancestors.add(sec.getRowId());
   }
 
-  const mainNode = {
+  const isSummary = table.primaryTableId.peek() !== table.tableId.peek();
+  const mainNode: LinkNode = {
     tableId: table.primaryTableId.peek(),
-    isSummary: table.primaryTableId.peek() !== table.tableId.peek(),
+    isSummary,
+    groupbyColumns: isSummary ? table.summarySourceColRefs.peek() : undefined,
     widgetType: section.parentKey.peek(),
     ancestors,
     section,
@@ -200,10 +240,11 @@ function fromPageWidget(docModel: DocModel, pageWidget: IPageWidget): LinkNode[]
   if (typeof pageWidget.table !== 'number') { return []; }
 
   const table = docModel.tables.getRowModel(pageWidget.table);
-
+  const isSummary = pageWidget.summarize;
   const mainNode: LinkNode = {
     tableId: table.primaryTableId.peek(),
-    isSummary: pageWidget.summarize,
+    isSummary,
+    groupbyColumns: isSummary ? new Set(pageWidget.columns) : undefined,
     widgetType: pageWidget.type,
     ancestors: new Set(),
     section: docModel.viewSections.getRowModel(pageWidget.section),