fix: remove invalid ParentConnection/ChildConnection from CrossAssociation BSON (#50)

.claude/skills/debug-bson.md

@@ -5,6 +5,8 @@ This skill provides a systematic workflow for debugging BSON serialization error
 ## When to Use This Skill
 
 Use when encountering:
+- **Studio Pro crash** `System.InvalidOperationException: Sequence contains no matching element` at `MprProperty..ctor`
+- **CE1613** "The selected attribute/enumeration no longer exists"
 - **CE0463** "The definition of this widget has changed"
 - **CE0642** "Property X is required"
 - **CE0091** validation errors on widget properties
@@ -135,6 +137,67 @@ Templates must include both `type` (PropertyTypes schema) AND `object` (default
 
 ## Common Error Patterns
 
+### Studio Pro Crash: InvalidOperationException in MprProperty..ctor
+
+**Symptom**: Studio Pro crashes when opening a project with `System.InvalidOperationException: Sequence contains no matching element` at `Mendix.Modeler.Storage.Mpr.MprProperty..ctor`.
+
+**Root cause**: A BSON document contains a property (field name) that does not exist in the Mendix type definition for its `$Type`. Studio Pro's `MprProperty` constructor uses `First()` to look up each BSON field in the type cache, and crashes on unrecognized fields.
+
+**Diagnosis workflow**:
+
+1. **Collect all (type, property) pairs from the crash project** (requires `pip install pymongo`):
+```python
+import bson, os
+from collections import defaultdict
+
+type_props = defaultdict(set)
+
+def walk_bson(obj, tp):
+    if isinstance(obj, dict):
+        t = obj.get("$Type", "")
+        if t:
+            for k in obj.keys():
+                if k not in ("$Type", "$ID"):
+                    tp[t].add(k)
+        for v in obj.values():
+            walk_bson(v, tp)
+    elif isinstance(obj, list):
+        for item in obj:
+            walk_bson(item, tp)
+
+for root, dirs, files in os.walk("mprcontents"):
+    for f in files:
+        if f.endswith(".mxunit"):
+            with open(os.path.join(root, f), "rb") as fh:
+                walk_bson(bson.decode(fh.read()), type_props)
+```
+
+2. **Compare against a known-good baseline project** (e.g., GenAIDemo):
+```python
+# Collect baseline_props the same way, then:
+for t, props in crash_props.items():
+    if t in baseline_props:
+        extra = props - baseline_props[t]
+        if extra:
+            print(f"{t}: EXTRA props = {sorted(extra)}")
+```
+
+3. **Extra properties = the crash cause**. The fix is to remove those fields from the writer function.
+
+**Example**: `DomainModels$CrossAssociation` had `ParentConnection` and `ChildConnection` copied from `DomainModels$Association`, but these fields don't exist on `CrossAssociation`. Removing them fixed the crash.
+
+**Key principle**: When copying serialization code between similar types (e.g., Association → CrossAssociation), always verify which fields belong to each type by checking a baseline project's BSON.
+
+### CE1613: Selected Attribute/Enumeration No Longer Exists
+
+**Symptom**: `mx check` reports `[CE1613] "The selected attribute 'Module.Entity.AssocName' no longer exists."` or `"The selected enumeration 'Module.Entity' no longer exists."`
+
+**Root cause**: Two variants:
+
+1. **Association stored as Attribute**: In `ChangeActionItem` BSON, an association name was written to the `Attribute` field instead of the `Association` field. Check the executor code that builds `MemberChange` — it must query the domain model to distinguish associations from attributes.
+
+2. **Entity treated as Enumeration**: In `CreateVariableAction` BSON, an entity qualified name was used as `DataTypes$EnumerationType` instead of `DataTypes$ObjectType`. Check `buildDataType()` in the visitor — bare qualified names default to `TypeEnumeration` and need catalog-based disambiguation.
+
 ### CE0463: Widget Definition Changed
 
 **Root cause**: Object property values inconsistent with mode-dependent visibility rules.

mdl/executor/bugfix_regression_test.go

@@ -390,3 +390,36 @@ func TestDeriveColumnName_CaptionSpecialChars(t *testing.T) {
 		t.Errorf("deriveColumnName(special chars) = %q, want %q", got, "Order__ID__main")
 	}
 }
+
+// =============================================================================
+// Issue #50: Association misidentified as Attribute (fallback without reader)
+// =============================================================================
+
+// TestResolveMemberChange_FallbackWithoutReader verifies that resolveMemberChange
+// falls back to dot-contains heuristic when no reader is available.
+// Regression: https://github.com/mendixlabs/mxcli/issues/50
+func TestResolveMemberChange_FallbackWithoutReader(t *testing.T) {
+	fb := &flowBuilder{
+		// reader is nil — simulates no project context
+	}
+
+	// Without reader: a name without dot should default to attribute
+	mc := &microflows.MemberChange{}
+	fb.resolveMemberChange(mc, "Label", "Demo.Child")
+	if mc.AttributeQualifiedName != "Demo.Child.Label" {
+		t.Errorf("expected attribute 'Demo.Child.Label', got %q", mc.AttributeQualifiedName)
+	}
+	if mc.AssociationQualifiedName != "" {
+		t.Errorf("expected empty association, got %q", mc.AssociationQualifiedName)
+	}
+
+	// With a dot in the name: should be treated as fully-qualified association (fallback)
+	mc2 := &microflows.MemberChange{}
+	fb.resolveMemberChange(mc2, "Demo.Child_Parent", "Demo.Child")
+	if mc2.AssociationQualifiedName != "Demo.Child_Parent" {
+		t.Errorf("expected association 'Demo.Child_Parent', got %q", mc2.AssociationQualifiedName)
+	}
+	if mc2.AttributeQualifiedName != "" {
+		t.Errorf("expected empty attribute, got %q", mc2.AttributeQualifiedName)
+	}
+}

sdk/mpr/writer_domainmodel.go

@@ -921,22 +921,23 @@ func serializeCrossAssociation(ca *domainmodel.CrossModuleAssociation) bson.M {
 	if storageFormat == "" {
 		storageFormat = "Column"
 	}
+	// CrossAssociation does NOT have ParentConnection/ChildConnection properties
+	// (unlike Association). Writing them causes Studio Pro to crash with
+	// InvalidOperationException in MprProperty..ctor.
 	return bson.M{
-		"$ID":              idToBsonBinary(string(ca.ID)),
-		"$Type":            "DomainModels$CrossAssociation",
-		"Name":             ca.Name,
-		"Documentation":    ca.Documentation,
-		"ExportLevel":      "Hidden",
-		"GUID":             idToBsonBinary(string(ca.ID)),
-		"ParentPointer":    idToBsonBinary(string(ca.ParentID)),
-		"Child":            ca.ChildRef,
-		"Type":             string(ca.Type),
-		"Owner":            string(ca.Owner),
-		"ParentConnection": "0;50",
-		"ChildConnection":  "100;50",
-		"StorageFormat":    storageFormat,
-		"Source":           nil,
-		"DeleteBehavior":   serializeDeleteBehavior(ca.ParentDeleteBehavior, ca.ChildDeleteBehavior),
+		"$ID":            idToBsonBinary(string(ca.ID)),
+		"$Type":          "DomainModels$CrossAssociation",
+		"Name":           ca.Name,
+		"Documentation":  ca.Documentation,
+		"ExportLevel":    "Hidden",
+		"GUID":           idToBsonBinary(string(ca.ID)),
+		"ParentPointer":  idToBsonBinary(string(ca.ParentID)),
+		"Child":          ca.ChildRef,
+		"Type":           string(ca.Type),
+		"Owner":          string(ca.Owner),
+		"StorageFormat":  storageFormat,
+		"Source":         nil,
+		"DeleteBehavior": serializeDeleteBehavior(ca.ParentDeleteBehavior, ca.ChildDeleteBehavior),
 	}
 }
 

sdk/mpr/writer_domainmodel_test.go

@@ -0,0 +1,89 @@
+// SPDX-License-Identifier: Apache-2.0
+
+package mpr
+
+import (
+	"testing"
+
+	"github.com/mendixlabs/mxcli/sdk/domainmodel"
+)
+
+// =============================================================================
+// Issue #50: CrossAssociation must NOT include ParentConnection/ChildConnection
+// =============================================================================
+
+// TestSerializeCrossAssociation_NoConnectionFields verifies that
+// serializeCrossAssociation does NOT emit ParentConnection or ChildConnection.
+// These properties only exist on DomainModels$Association, not on
+// DomainModels$CrossAssociation. Writing them causes Studio Pro to crash with
+// System.InvalidOperationException: Sequence contains no matching element.
+func TestSerializeCrossAssociation_NoConnectionFields(t *testing.T) {
+	ca := &domainmodel.CrossModuleAssociation{
+		Name:     "Child_Parent",
+		ParentID: "parent-entity-id",
+		ChildRef: "OtherModule.Parent",
+		Type:     domainmodel.AssociationTypeReference,
+		Owner:    domainmodel.AssociationOwnerDefault,
+	}
+	ca.ID = "test-cross-assoc-id"
+
+	result := serializeCrossAssociation(ca)
+
+	// Must NOT contain these fields
+	for key := range result {
+		if key == "ParentConnection" {
+			t.Error("serializeCrossAssociation must NOT include ParentConnection (only valid for Association)")
+		}
+		if key == "ChildConnection" {
+			t.Error("serializeCrossAssociation must NOT include ChildConnection (only valid for Association)")
+		}
+	}
+
+	// Must contain all expected fields (exhaustive structural contract)
+	expectedKeys := []string{"$ID", "$Type", "Name", "Child", "ParentPointer", "Type", "Owner",
+		"Documentation", "ExportLevel", "GUID", "StorageFormat", "Source", "DeleteBehavior"}
+	for _, key := range expectedKeys {
+		if _, ok := result[key]; !ok {
+			t.Errorf("serializeCrossAssociation missing expected field %q", key)
+		}
+	}
+
+	// $Type must be CrossAssociation
+	if got := result["$Type"]; got != "DomainModels$CrossAssociation" {
+		t.Errorf("$Type = %q, want %q", got, "DomainModels$CrossAssociation")
+	}
+}
+
+// TestSerializeAssociation_HasConnectionFields verifies that the regular
+// serializeAssociation DOES include ParentConnection and ChildConnection
+// (to ensure we didn't accidentally remove them from the wrong function).
+func TestSerializeAssociation_HasConnectionFields(t *testing.T) {
+	a := &domainmodel.Association{
+		Name:     "Child_Parent",
+		ParentID: "parent-entity-id",
+		ChildID:  "child-entity-id",
+		Type:     domainmodel.AssociationTypeReference,
+		Owner:    domainmodel.AssociationOwnerDefault,
+	}
+	a.ID = "test-assoc-id"
+
+	result := serializeAssociation(a)
+
+	hasParentConn := false
+	hasChildConn := false
+	for key := range result {
+		if key == "ParentConnection" {
+			hasParentConn = true
+		}
+		if key == "ChildConnection" {
+			hasChildConn = true
+		}
+	}
+
+	if !hasParentConn {
+		t.Error("serializeAssociation must include ParentConnection")
+	}
+	if !hasChildConn {
+		t.Error("serializeAssociation must include ChildConnection")
+	}
+}