Add registering of client-side relationship properties (#5936)

app/models/decorators/Relation.ts

@@ -4,6 +4,47 @@ import type Model from "../base/Model";
 type RelationOptions = {
   /** Whether this relation is required */
   required?: boolean;
+  /** Behavior of relationship on deletion */
+  onDelete: "cascade" | "null" | "ignore";
+};
+
+type RelationProperties = {
+  /** The name of the property on the model that stores the ID of the relation */
+  idKey: string;
+  /** A function that returns the class of the relation */
+  relationClassResolver: () => typeof Model;
+  /** Options for the relation */
+  options: RelationOptions;
+};
+
+type InverseRelationProperties = RelationProperties & {
+  /** The name of the model class that owns this relation */
+  modelName: string;
+};
+
+const relations = new Map<string, Map<string, RelationProperties>>(new Map());
+
+/**
+ * Returns the inverse relation properties for the given model class.
+ *
+ * @param targetClass The model class to get inverse relations for.
+ * @returns A map of inverse relation properties keyed by the property name.
+ */
+export const getInverseRelationsForModelClass = (targetClass: typeof Model) => {
+  const inverseRelations = new Map<string, InverseRelationProperties>();
+
+  relations.forEach((relation, modelName) => {
+    relation.forEach((properties, propertyName) => {
+      if (properties.relationClassResolver().name === targetClass.name) {
+        inverseRelations.set(propertyName, {
+          ...properties,
+          modelName,
+        });
+      }
+    });
+  });
+
+  return inverseRelations;
 };
 
 /**
@@ -20,8 +61,19 @@ export default function Relation<T extends typeof Model>(
 ) {
   return function (target: any, propertyKey: string) {
     const idKey = `${String(propertyKey)}Id`;
-    const relationClass = classResolver();
-    const relationClassName = relationClass.name;
+
+    // If the relation has options provided then register them in a map for later lookup. We can use
+    // this to determine how to update relations when a model is deleted.
+    if (options) {
+      const configForClass =
+        relations.get(target.constructor.name) || new Map();
+      configForClass.set(propertyKey, {
+        options,
+        relationClassResolver: classResolver,
+        idKey,
+      });
+      relations.set(target.constructor.name, configForClass);
+    }
 
     Object.defineProperty(target, propertyKey, {
       get() {
@@ -31,6 +83,7 @@ export default function Relation<T extends typeof Model>(
           return undefined;
         }
 
+        const relationClassName = classResolver().name;
         const store =
           this.store.rootStore[`${relationClassName.toLowerCase()}s`];
         invariant(store, `Store for ${relationClassName} not found`);
@@ -41,6 +94,7 @@ export default function Relation<T extends typeof Model>(
         this[idKey] = newValue ? newValue.id : undefined;
 
         if (newValue) {
+          const relationClassName = classResolver().name;
           const store =
             this.store.rootStore[`${relationClassName.toLowerCase()}s`];
           invariant(store, `Store for ${relationClassName} not found`);