fix(pages): expand multi-hop association XPath paths to avoid CE1613

A datasource constrained two associations away with the natural shorthand
  database from Travel.Activity where [Travel.Day_Activity/Travel.Trip_Day = $Trip]
failed with CE1613 'The selected entity Travel.Trip_Day no longer exists': Mendix
XPath requires the intermediate entity between two association hops
(Assoc1/Entity/Assoc2), and mxcli stored the shorthand verbatim so mxbuild read
the second association as an entity.

buildDataSourceV3 now expands consecutive association segments, inserting the
intermediate entity resolved from each association's target
(resolveAssociationDestination). Safe by construction: only runs of slash-joined
qualified names anchored at an association from the datasource entity are touched,
and an entity is inserted only between two segments that both resolve as
associations — so the already-correct full form, single-hop constraints, and
attribute paths are returned verbatim. Verified against the reporter's Travel
schema: the two-hop Timeline datasource resolves, mx check 0 errors.

Known related (separate): attribute-over-association shorthand
([Assoc/Attribute = x], needs [Assoc/Entity/Attribute = x]) is not yet expanded.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01JXnEgoc2NQP1Y2TWMCMXC4

Author: claude

mdl-examples/bug-tests/xpath-assoc-path-expansion.mdl

@@ -0,0 +1,56 @@
+-- ============================================================================
+-- Bug: multi-hop association XPath in a datasource trips CE1613
+-- ============================================================================
+--
+-- Symptom (from a Travel itinerary project):
+--   A Timeline over Activities constrained to a Trip two associations away:
+--     datasource: database from Travel.Activity
+--                 where [Travel.Day_Activity/Travel.Trip_Day = $Trip]
+--   failed `mx check` / the deploy build with:
+--     [CE1613] "The selected entity 'Travel.Trip_Day' no longer exists."
+--   mxbuild read the second association segment as an ENTITY.
+--
+-- Root cause:
+--   Mendix XPath requires the intermediate entity between two association hops
+--   — `[Assoc1/Entity/Assoc2 = $x]`. The MDL shorthand `[Assoc1/Assoc2 = $x]`
+--   (natural, and what an LLM emits) omits it, and mxcli stored the constraint
+--   verbatim, so mxbuild couldn't resolve the second hop.
+--
+-- Fix:
+--   buildDataSourceV3 now expands consecutive association segments, inserting the
+--   intermediate entity (resolved from each association's target). The full form,
+--   single-hop constraints, and attribute paths are left verbatim, so it never
+--   corrupts a valid constraint.
+--
+-- Verify:
+--   mxcli exec mdl-examples/bug-tests/xpath-assoc-path-expansion.mdl -p app.mpr
+--   mx check app.mpr    # 0 errors — the two-hop path resolves
+-- ============================================================================
+
+create module Travel;
+/
+create entity Travel.Trip ( Title: String );
+create entity Travel.Day ( DayNumber: Integer );
+create entity Travel.Activity ( Name: String, StartTime: DateTime );
+/
+-- Day belongs to Trip; Activity belongs to Day.
+create association Travel.Trip_Day from Travel.Day to Travel.Trip;
+create association Travel.Day_Activity from Travel.Activity to Travel.Day;
+/
+create or replace page Travel.Trip_Detail (
+  params: { $Trip: Travel.Trip },
+  layout: Atlas_Core.Atlas_Default
+) {
+  -- one-hop reverse association (already worked; must stay working)
+  pluggablewidget 'com.mendix.widget.web.treenode.TreeNode' tnDays (
+    datasource: database from Travel.Day where [Travel.Trip_Day = $Trip] sort by DayNumber asc,
+    headerType: 'text', headerCaption: 'Day {DayNumber}', hasChildren: false, showIcon: 'left'
+  )
+  -- two-hop reverse association shorthand — expanded to Assoc/Entity/Assoc.
+  pluggablewidget 'com.mendix.widget.web.timeline.Timeline' tlTrip (
+    datasource: database from Travel.Activity where [Travel.Day_Activity/Travel.Trip_Day = $Trip] sort by StartTime asc,
+    title: '{Name}',
+    groupEvents: false,
+    customVisualization: false
+  )
+}

mdl/executor/cmd_pages_builder_v3.go

@@ -5,6 +5,7 @@ package executor
 import (
 	"fmt"
 	"log"
+	"regexp"
 	"strings"
 
 	"github.com/mendixlabs/mxcli/mdl/ast"
@@ -663,9 +664,11 @@ func (pb *pageBuilder) buildDataSourceV3(ds *ast.DataSourceV3) (pages.DataSource
 			EntityName: ds.Reference,
 		}
 
-		// Handle WHERE clause
+		// Handle WHERE clause. Expand association-only paths to the Assoc/Entity/Assoc
+		// form Mendix requires (see expandXPathAssociationPath), so the shorthand
+		// `[Mod.Assoc1/Mod.Assoc2 = $x]` doesn't trip CE1613 at build time.
 		if ds.Where != "" {
-			dbSource.XPathConstraint = ds.Where
+			dbSource.XPathConstraint = pb.expandXPathAssociationPath(ds.Where, ds.Reference)
 		}
 
 		// Handle ORDER BY
@@ -787,6 +790,59 @@ func (pb *pageBuilder) buildDataSourceV3(ds *ast.DataSourceV3) (pages.DataSource
 //
 // Convention (per CLAUDE.md): ParentID = FROM entity, ChildID = TO entity.
 // For `Module.OrderLine_Order` (`FROM OrderLine TO Order`), context=Order → dest=OrderLine (parent side).
+// xpathAssocRunRe matches a run of two or more slash-joined qualified names
+// (Module.Name/Module.Name[/...]) — an association/entity path. Single qualified
+// names (one-hop `[Mod.Assoc = $x]`) and unqualified attribute steps (`Mod.Assoc/Attr`)
+// don't match, so they're never rewritten.
+var xpathAssocRunRe = regexp.MustCompile(`[A-Za-z_]\w*\.[A-Za-z_]\w*(?:/[A-Za-z_]\w*\.[A-Za-z_]\w*)+`)
+
+// expandXPathAssociationPath rewrites an XPath constraint so that consecutive
+// association segments get the intermediate entity Mendix requires between them:
+// `[Mod.Assoc1/Mod.Assoc2 = $x]` → `[Mod.Assoc1/Mod.Entity/Mod.Assoc2 = $x]`. Without
+// this, mxbuild reads the second association as an entity and fails with CE1613
+// ("selected entity ... no longer exists").
+//
+// The rewrite is safe by construction: it only touches runs of slash-joined qualified
+// names anchored at an association from the datasource entity, and inserts an entity
+// only between two segments that both resolve as associations. The already-correct
+// full form (where the middle segment is an entity, not an association), single-hop
+// constraints, attribute paths, and anything unresolvable are returned verbatim.
+func (pb *pageBuilder) expandXPathAssociationPath(constraint, contextEntity string) string {
+	if contextEntity == "" || !strings.Contains(constraint, "/") {
+		return constraint
+	}
+	return xpathAssocRunRe.ReplaceAllStringFunc(constraint, func(run string) string {
+		segs := strings.Split(run, "/")
+		// Only expand a run that starts with an association reachable from the
+		// datasource entity; otherwise leave it untouched (it may be an unrelated
+		// qualified path we don't understand).
+		if pb.resolveAssociationDestination(segs[0], contextEntity) == "" {
+			return run
+		}
+		out := make([]string, 0, len(segs)*2)
+		ctx := contextEntity
+		for i, seg := range segs {
+			dest := pb.resolveAssociationDestination(seg, ctx)
+			out = append(out, seg)
+			if dest == "" {
+				// Not an association from here — treat as an entity segment.
+				ctx = seg
+				continue
+			}
+			// Association: if the next segment is also an association (i.e. the
+			// intermediate entity was omitted), insert the destination entity.
+			if i+1 < len(segs) {
+				next := segs[i+1]
+				if next != dest && pb.resolveAssociationDestination(next, dest) != "" {
+					out = append(out, dest)
+				}
+			}
+			ctx = dest
+		}
+		return strings.Join(out, "/")
+	})
+}
+
 func (pb *pageBuilder) resolveAssociationDestination(assocQN, contextEntity string) string {
 	if assocQN == "" {
 		return ""