fix: serialize ByAssociation DataSource for DataGrid 2 (mendixlabs#198 follow-up)

Two related issues, fixed together:

1. Grammar rejected qualified association names. The previous
   \`attributePathV3\` rule didn't allow dots, so
   \`ASSOCIATION Module.Order_OrderLine\` and
   \`\$currentObject/Module.Order_OrderLine\` both failed to parse.
   Added a new \`associationPathV3\` rule that accepts qualified names
   separated by /, supporting both single-step
   (\`Module.Assoc\`) and multi-step
   (\`Module.Assoc/Module.DestEntity\`) paths.

2. The writer never emitted Forms\$AssociationSource BSON. The executor
   built a pages.AssociationSource model object but
   SerializeCustomWidgetDataSource (used by pluggable widgets like
   DataGrid 2) had no case for it, falling through to nil. Added the
   case with a serializeAssociationSource helper that emits the proper
   DomainModels\$IndirectEntityRef + Steps + optional SourceVariable
   structure matching what Studio Pro produces.

Verified end-to-end against test3 (Mendix 11.9): a DataGrid 2 with
\`DataSource: \$currentObject/AssocGrid.OrderLine_Order/AssocGrid.OrderLine\`
now generates valid BSON. \`mx check\` passes (the remaining CE7007 is
on the parent DataView, a separate pre-existing issue).

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: ako

mdl/ast/ast_page_v3.go

@@ -60,11 +60,12 @@ type WidgetV3 struct {
 
 // DataSourceV3 represents a V3 datasource expression.
 type DataSourceV3 struct {
-	Type      string          // "parameter", "database", "microflow", "nanoflow", "association", "selection"
-	Reference string          // Entity name, flow name, widget name, or parameter name
-	Args      []FlowArgV3     // Arguments for microflow/nanoflow calls
-	Where     string          // XPath constraint (for database source)
-	OrderBy   []OrderByItemV3 // Sort order (for database source)
+	Type            string          // "parameter", "database", "microflow", "nanoflow", "association", "selection"
+	Reference       string          // Entity name, flow name, widget name, or parameter name
+	ContextVariable string          // Context variable name (for association source: $currentObject → "currentObject")
+	Args            []FlowArgV3     // Arguments for microflow/nanoflow calls
+	Where           string          // XPath constraint (for database source)
+	OrderBy         []OrderByItemV3 // Sort order (for database source)
 }
 
 // FlowArgV3 represents an argument for microflow/nanoflow/page calls.

mdl/executor/cmd_pages_builder_v3.go

@@ -586,13 +586,20 @@ func (pb *pageBuilder) buildDataSourceV3(ds *ast.DataSourceV3) (pages.DataSource
 		}, entityName, nil
 
 	case "association":
-		// Association path source
+		// Association path source — emits Forms$AssociationSource BSON.
+		// EntityPath is the qualified association name (and optional /DestinationEntity).
+		// ContextVariable is the page parameter name (empty when source is $currentObject of parent dataview).
+		ctxVar := ds.ContextVariable
+		if ctxVar == "currentObject" {
+			ctxVar = "" // implicit context — no SourceVariable in BSON
+		}
 		return &pages.AssociationSource{
 			BaseElement: model.BaseElement{
 				ID:       model.ID(mpr.GenerateID()),
 				TypeName: "Forms$AssociationSource",
 			},
-			EntityPath: ds.Reference,
+			EntityPath:      ds.Reference,
+			ContextVariable: ctxVar,
 		}, "", nil
 
 	case "selection":

mdl/grammar/MDLParser.g4

@@ -2168,17 +2168,22 @@ attributeListV3
 
 // V3 DataSource expressions
 dataSourceExprV3
-    : VARIABLE SLASH attributePathV3                  // $currentObject/Module.Assoc (ByAssociation — sugar for ASSOCIATION)
+    : VARIABLE SLASH associationPathV3                // $currentObject/Module.Assoc (ByAssociation — sugar for ASSOCIATION)
     | VARIABLE                                        // $ParamName
     | DATABASE FROM? qualifiedName                    // DATABASE [FROM] Entity [WHERE ...] [SORT BY ...]
       (WHERE (xpathConstraint (andOrXpath? xpathConstraint)* | expression))?
       (SORT_BY sortColumn (COMMA sortColumn)*)?
     | MICROFLOW qualifiedName microflowArgsV3?        // MICROFLOW Module.Flow
     | NANOFLOW qualifiedName microflowArgsV3?         // NANOFLOW Module.Flow
-    | ASSOCIATION attributePathV3                     // ASSOCIATION Path (explicit form)
+    | ASSOCIATION associationPathV3                   // ASSOCIATION Module.Assoc (explicit form)
     | SELECTION IDENTIFIER                            // SELECTION widgetName
     ;
 
+// Association path: Module.Assoc or Module.Assoc/Module.Entity or multi-step
+associationPathV3
+    : qualifiedName (SLASH qualifiedName)*
+    ;
+
 // V3 Action expressions
 actionExprV3
     : SAVE_CHANGES (CLOSE_PAGE)?                      // SAVE_CHANGES or SAVE_CHANGES CLOSE_PAGE

mdl/grammar/parser/MDLParser.interp

@@ -618,8 +618,9 @@ func buildDataSourceV3(ctx parser.IDataSourceExprV3Context) *ast.DataSourceV3 {
 	if v := dsCtx.VARIABLE(); v != nil && dsCtx.SLASH() != nil {
 		// $currentObject/Module.Assoc — ByAssociation data source (sugar for ASSOCIATION Path)
 		ds.Type = "association"
-		if pathCtx := dsCtx.AttributePathV3(); pathCtx != nil {
-			ds.Reference = buildAttributePathV3(pathCtx)
+		ds.ContextVariable = strings.TrimPrefix(v.GetText(), "$")
+		if pathCtx := dsCtx.AssociationPathV3(); pathCtx != nil {
+			ds.Reference = buildAssociationPathV3(pathCtx)
 		}
 	} else if v := dsCtx.VARIABLE(); v != nil {
 		// $ParamName
@@ -669,8 +670,8 @@ func buildDataSourceV3(ctx parser.IDataSourceExprV3Context) *ast.DataSourceV3 {
 	} else if dsCtx.ASSOCIATION() != nil {
 		// ASSOCIATION Path
 		ds.Type = "association"
-		if pathCtx := dsCtx.AttributePathV3(); pathCtx != nil {
-			ds.Reference = buildAttributePathV3(pathCtx)
+		if pathCtx := dsCtx.AssociationPathV3(); pathCtx != nil {
+			ds.Reference = buildAssociationPathV3(pathCtx)
 		}
 	} else if dsCtx.SELECTION() != nil {
 		// SELECTION widgetName
@@ -804,6 +805,20 @@ func buildAttributeListV3(ctx parser.IAttributeListV3Context) []string {
 	return attrs
 }
 
+// buildAssociationPathV3 builds an association path string from a parser context.
+// Format: "Module.Assoc" or "Module.Assoc/Module.Entity" — qualified names separated by /.
+func buildAssociationPathV3(ctx parser.IAssociationPathV3Context) string {
+	if ctx == nil {
+		return ""
+	}
+	apc := ctx.(*parser.AssociationPathV3Context)
+	var parts []string
+	for _, qn := range apc.AllQualifiedName() {
+		parts = append(parts, getQualifiedNameText(qn))
+	}
+	return strings.Join(parts, "/")
+}
+
 // buildAttributePathV3 builds an attribute path string.
 // Handles quoted identifiers (e.g., "Order") by stripping quotes.
 func buildAttributePathV3(ctx parser.IAttributePathV3Context) string {

mdl/grammar/parser/mdl_parser.go

@@ -295,11 +295,64 @@ func SerializeCustomWidgetDataSource(ds pages.DataSource) bson.D {
 				{Key: "ParameterMappings", Value: bson.A{int32(3)}},
 			}},
 		}
+	case *pages.AssociationSource:
+		return serializeAssociationSource(d)
 	default:
 		return nil
 	}
 }
 
+// serializeAssociationSource builds a Forms$AssociationSource BSON document.
+// EntityPath is "Module.Assoc" or "Module.Assoc/Module.DestEntity".
+// When DestinationEntity is omitted, it's left empty — Studio Pro will resolve it.
+func serializeAssociationSource(d *pages.AssociationSource) bson.D {
+	parts := strings.Split(d.EntityPath, "/")
+	association := parts[0]
+	destEntity := ""
+	if len(parts) >= 2 {
+		destEntity = parts[1]
+	}
+
+	step := bson.D{
+		{Key: "$ID", Value: idToBsonBinary(generateUUID())},
+		{Key: "$Type", Value: "DomainModels$EntityRefStep"},
+		{Key: "Association", Value: association},
+		{Key: "DestinationEntity", Value: destEntity},
+	}
+
+	entityRef := bson.D{
+		{Key: "$ID", Value: idToBsonBinary(generateUUID())},
+		{Key: "$Type", Value: "DomainModels$IndirectEntityRef"},
+		{Key: "Steps", Value: bson.A{int32(2), step}},
+	}
+
+	var sourceVar any
+	if d.ContextVariable != "" {
+		sourceVar = bson.D{
+			{Key: "$ID", Value: idToBsonBinary(generateUUID())},
+			{Key: "$Type", Value: "Forms$PageVariable"},
+			{Key: "LocalVariable", Value: ""},
+			{Key: "PageParameter", Value: d.ContextVariable},
+			{Key: "SnippetParameter", Value: ""},
+			{Key: "SubKey", Value: ""},
+			{Key: "UseAllPages", Value: false},
+			{Key: "Widget", Value: ""},
+		}
+	}
+
+	id := string(d.ID)
+	if id == "" {
+		id = generateUUID()
+	}
+	return bson.D{
+		{Key: "$ID", Value: idToBsonBinary(id)},
+		{Key: "$Type", Value: "Forms$AssociationSource"},
+		{Key: "EntityRef", Value: entityRef},
+		{Key: "ForceFullObjects", Value: false},
+		{Key: "SourceVariable", Value: sourceVar},
+	}
+}
+
 // ============================================================================
 // Reference Serialization
 // ============================================================================

mdl/grammar/parser/mdlparser_base_listener.go

@@ -78,7 +78,8 @@ func (ListenToWidgetSource) isDataSource() {}
 // AssociationSource retrieves data via association.
 type AssociationSource struct {
 	model.BaseElement
-	EntityPath string `json:"entityPath"`
+	EntityPath      string `json:"entityPath"`               // "Module.Assoc" or "Module.Assoc/Module.DestEntity"
+	ContextVariable string `json:"contextVariable,omitempty"` // page parameter name (without $) — empty for $currentObject
 }
 
 func (AssociationSource) isDataSource() {}