Extend relationship with attributes between parent and child entity

examples/mapping-example/ExampleCRM/relationships/Address_Customer.relationship.cm

@@ -2,4 +2,7 @@ relationship:
     id: Address_Customer
     parent: Customer
     child: ExampleCRM.Address
-    type: "1:1"
+    type: "1:1"
+    attributes:
+        - parent: Customer.Id
+          child: ExampleCRM.Address.CustomerID

examples/mapping-example/ExampleCRM/relationships/Order_Customer.relationship.cm

@@ -2,4 +2,7 @@ relationship:
     id: Order_Customer
     parent: Customer
     child: Order
-    type: "1:1"
+    type: "1:1"
+    attributes:
+        - parent: Customer.Id
+          child: Order.CustomerId

extensions/crossmodel-lang/src/language-server/cross-model-completion-provider.ts

@@ -18,6 +18,7 @@ import { CompletionItemKind, InsertTextFormat } from 'vscode-languageserver-prot
 import type { Range } from 'vscode-languageserver-types';
 import { CrossModelServices } from './cross-model-module.js';
 import { isExternalDescriptionForLocalPackage } from './cross-model-scope.js';
+import { RelationshipAttribute, isRelationshipAttribute } from './generated/ast.js';
 import { fixDocument } from './util/ast-util.js';
 
 /**
@@ -182,8 +183,25 @@ export class CrossModelCompletionProvider extends DefaultCompletionProvider {
    }
 
    protected override filterCrossReference(context: CompletionContext, description: AstNodeDescription): boolean {
-      // we want to keep fully qualified names in the scope so we can do proper linking
-      // but want to hide it from the user for local options, i.e., if we are in the same project we can skip the project name
-      return !isExternalDescriptionForLocalPackage(description, this.packageId) && super.filterCrossReference(context, description);
+      if (isRelationshipAttribute(context.node)) {
+         return this.filterRelationshipAttribute(context.node, context, description);
+      }
+      if (isExternalDescriptionForLocalPackage(description, this.packageId)) {
+         // we want to keep fully qualified names in the scope so we can do proper linking
+         // but want to hide it from the user for local options, i.e., if we are in the same project we can skip the project name
+         return false;
+      }
+      return super.filterCrossReference(context, description);
+   }
+
+   protected filterRelationshipAttribute(node: RelationshipAttribute, context: CompletionContext, desc: AstNodeDescription): boolean {
+      // only show relevant attributes depending on the parent or child context
+      if (context.features.find(feature => feature.property === 'child')) {
+         return desc.name.startsWith(node.$container.child?.$refText + '.');
+      }
+      if (context.features.find(feature => feature.property === 'parent')) {
+         return desc.name.startsWith(node.$container.parent?.$refText + '.');
+      }
+      return true;
    }
 }

extensions/crossmodel-lang/src/language-server/cross-model-validator.ts

@@ -5,7 +5,9 @@ import { AstNode, ValidationAcceptor, ValidationChecks } from 'langium';
 import type { CrossModelServices } from './cross-model-module.js';
 import { ID_PROPERTY, IdentifiableAstNode } from './cross-model-naming.js';
 import {
+   Attribute,
    CrossModelAstType,
+   Relationship,
    RelationshipEdge,
    SourceObject,
    isEntity,
@@ -25,7 +27,8 @@ export function registerValidationChecks(services: CrossModelServices): void {
    const checks: ValidationChecks<CrossModelAstType> = {
       AstNode: validator.checkNode,
       RelationshipEdge: validator.checkRelationshipEdge,
-      SourceObject: validator.checkSourceObject
+      SourceObject: validator.checkSourceObject,
+      Relationship: validator.checkRelationship
    };
    registry.register(checks, validator);
 }
@@ -83,6 +86,37 @@ export class CrossModelValidator {
       }
    }
 
+   checkRelationship(relationship: Relationship, accept: ValidationAcceptor): void {
+      // we check that each attribute actually belongs to their respective entity (parent, child)
+      // and that each attribute is only used once
+      const usedParentAttributes: Attribute[] = [];
+      const usedChildAttributes: Attribute[] = [];
+      for (const attribute of relationship.attributes) {
+         if (attribute.parent.ref) {
+            if (attribute.parent?.ref?.$container !== relationship.parent?.ref) {
+               accept('error', 'Not a valid parent attribute.', { node: attribute, property: 'parent' });
+            }
+            if (usedParentAttributes.includes(attribute.parent.ref)) {
+               // duplicate detected
+               accept('error', 'Each parent attribute can only be referenced once.', { node: attribute, property: 'parent' });
+            } else {
+               usedParentAttributes.push(attribute.parent.ref);
+            }
+         }
+         if (attribute.child.ref) {
+            if (attribute.child?.ref?.$container !== relationship.child?.ref) {
+               accept('error', 'Not a valid child attribute.', { node: attribute, property: 'child' });
+            }
+            if (usedChildAttributes.includes(attribute.child.ref)) {
+               // duplicate detected
+               accept('error', 'Each child attribute can only be referenced once.', { node: attribute, property: 'child' });
+            } else {
+               usedChildAttributes.push(attribute.child.ref);
+            }
+         }
+      }
+   }
+
    checkRelationshipEdge(edge: RelationshipEdge, accept: ValidationAcceptor): void {
       if (edge.sourceNode?.ref?.entity?.ref?.$type !== edge.relationship?.ref?.parent?.ref?.$type) {
          accept('error', 'Source must match type of parent.', { node: edge, property: 'sourceNode' });

extensions/crossmodel-lang/src/language-server/generated/ast.ts

@@ -200,6 +200,7 @@ export function isReferenceSource(item: unknown): item is ReferenceSource {
 export interface Relationship extends AstNode {
     readonly $container: CrossModelRoot;
     readonly $type: 'Relationship';
+    attributes: Array<RelationshipAttribute>
     child?: Reference<Entity>
     description?: string
     id?: string
@@ -214,6 +215,20 @@ export function isRelationship(item: unknown): item is Relationship {
     return reflection.isInstance(item, Relationship);
 }
 
+export interface RelationshipAttribute extends AstNode {
+    readonly $container: Relationship;
+    readonly $type: 'RelationshipAttribute';
+    child: Reference<Attribute>
+    description?: string
+    parent: Reference<Attribute>
+}
+
+export const RelationshipAttribute = 'RelationshipAttribute';
+
+export function isRelationshipAttribute(item: unknown): item is RelationshipAttribute {
+    return reflection.isInstance(item, RelationshipAttribute);
+}
+
 export interface RelationshipCondition extends AstNode {
     readonly $container: SourceObjectRelations;
     readonly $type: 'RelationshipCondition';
@@ -366,6 +381,7 @@ export type CrossModelAstType = {
     NumberLiteral: NumberLiteral
     ReferenceSource: ReferenceSource
     Relationship: Relationship
+    RelationshipAttribute: RelationshipAttribute
     RelationshipCondition: RelationshipCondition
     RelationshipEdge: RelationshipEdge
     SourceObject: SourceObject
@@ -381,7 +397,7 @@ export type CrossModelAstType = {
 export class CrossModelAstReflection extends AbstractAstReflection {
 
     getAllTypes(): string[] {
-        return ['Attribute', 'AttributeMapping', 'AttributeMappingSource', 'AttributeMappingTarget', 'CrossModelRoot', 'Entity', 'EntityAttribute', 'EntityNode', 'EntityNodeAttribute', 'JoinCondition', 'JoinExpression', 'Mapping', 'NumberLiteral', 'ReferenceSource', 'Relationship', 'RelationshipCondition', 'RelationshipEdge', 'SourceObject', 'SourceObjectAttribute', 'SourceObjectCondition', 'SourceObjectRelations', 'StringLiteral', 'SystemDiagram', 'TargetObject', 'TargetObjectAttribute'];
+        return ['Attribute', 'AttributeMapping', 'AttributeMappingSource', 'AttributeMappingTarget', 'CrossModelRoot', 'Entity', 'EntityAttribute', 'EntityNode', 'EntityNodeAttribute', 'JoinCondition', 'JoinExpression', 'Mapping', 'NumberLiteral', 'ReferenceSource', 'Relationship', 'RelationshipAttribute', 'RelationshipCondition', 'RelationshipEdge', 'SourceObject', 'SourceObjectAttribute', 'SourceObjectCondition', 'SourceObjectRelations', 'StringLiteral', 'SystemDiagram', 'TargetObject', 'TargetObjectAttribute'];
     }
 
     protected override computeIsSubtype(subtype: string, supertype: string): boolean {
@@ -427,6 +443,10 @@ export class CrossModelAstReflection extends AbstractAstReflection {
             case 'ReferenceSource:value': {
                 return SourceObjectAttribute;
             }
+            case 'RelationshipAttribute:child':
+            case 'RelationshipAttribute:parent': {
+                return Attribute;
+            }
             case 'RelationshipCondition:relationship':
             case 'RelationshipEdge:relationship': {
                 return Relationship;
@@ -462,6 +482,14 @@ export class CrossModelAstReflection extends AbstractAstReflection {
                     ]
                 };
             }
+            case 'Relationship': {
+                return {
+                    name: 'Relationship',
+                    mandatory: [
+                        { name: 'attributes', type: 'array' }
+                    ]
+                };
+            }
             case 'SourceObject': {
                 return {
                     name: 'SourceObject',