RCOCOA-2238: Allow defining models with only computed properties (#8484)

* Add an empty changelog section

* Lift restriction on objects with computed properties

* Fix lint violations, rename class

* Address CR comments

CHANGELOG.md

@@ -1,7 +1,7 @@
 x.y.z Release notes (yyyy-MM-dd)
 =============================================================
 ### Enhancements
-* None.
+* Lifted a limitation that would prevent declaring a model with only computed properties. ([#8414](https://github.com/realm/realm-swift/issues/8414))
 
 ### Fixed
 * <How to hit and notice issue? what was the impact?> ([#????](https://github.com/realm/realm-swift/issues/????), since v?.?.?)

Realm/RLMObjectSchema.mm

@@ -211,8 +211,9 @@ + (instancetype)schemaForObjectClass:(Class)objectClass {
 
     if ([objectClass shouldIncludeInDefaultSchema]
         && schema.isSwiftClass
-        && schema.properties.count == 0) {
-        @throw RLMException(@"No properties are defined for '%@'. Did you remember to mark them with '@objc' in your model?", schema.className);
+        && schema.properties.count == 0
+        && schema.computedProperties.count == 0) {
+        @throw RLMException(@"No properties are defined for '%@'. Did you remember to mark them with '@objc' or '@Persisted' in your model?", schema.className);
     }
     return schema;
 }

Realm/Tests/Swift/SwiftSchemaTests.swift

@@ -132,6 +132,24 @@ class NoProps: FakeObject {
     // no @objc properties
 }
 
+class OnlyComputedSource: RLMObject {
+    @objc dynamic var link: OnlyComputedTarget?
+}
+
+class OnlyComputedTarget: RLMObject {
+    @objc dynamic var backlinks: RLMLinkingObjects<OnlyComputedSource>?
+
+    override class func linkingObjectsProperties() -> [String : RLMPropertyDescriptor] {
+        return ["backlinks": RLMPropertyDescriptor(with: OnlyComputedSource.self, propertyName: "link")]
+    }
+}
+
+class OnlyComputedNoBacklinksProps: FakeObject {
+    var computedProperty: String {
+        return "Test_String"
+    }
+}
+
 @MainActor
 class RequiresObjcName: RLMObject {
     static var enable = false
@@ -170,7 +188,26 @@ class SwiftRLMSchemaTests: RLMMultiProcessTestCase {
 
     func testShouldRaiseObjectWithoutProperties() {
         assertThrowsWithReasonMatching(RLMObjectSchema(forObjectClass: NoProps.self),
-                                       "No properties are defined for 'NoProps'. Did you remember to mark them with '@objc' in your model?")
+                                       "No properties are defined for 'NoProps'. Did you remember to mark them with '@objc' or '@Persisted' in your model?")
+    }
+
+    func testShouldNotThrowForObjectWithOnlyBacklinksProps() {
+        let config = RLMRealmConfiguration.default()
+        config.objectClasses = [OnlyComputedTarget.self, OnlyComputedSource.self]
+        config.inMemoryIdentifier = #function
+        let r = try! RLMRealm(configuration: config)
+        try! r.transaction {
+            _ = OnlyComputedTarget.create(in: r, withValue: [])
+        }
+
+        let schema = OnlyComputedTarget().objectSchema
+        XCTAssertEqual(schema.computedProperties.count, 1)
+        XCTAssertEqual(schema.properties.count, 0)
+    }
+
+    func testShouldThrowForObjectWithOnlyComputedNoBacklinksProps() {
+        assertThrowsWithReasonMatching(RLMObjectSchema(forObjectClass: OnlyComputedNoBacklinksProps.self),
+                                       "No properties are defined for 'OnlyComputedNoBacklinksProps'. Did you remember to mark them with '@objc' or '@Persisted' in your model?")
     }
 
     func testSchemaInitWithLinkedToObjectUsingInitWithValue() {

RealmSwift/Tests/SwiftLinkTests.swift

@@ -111,4 +111,44 @@ class SwiftLinkTests: TestCase {
 
         XCTAssertEqual(0, owners.count)
     }
+
+    func testLinkingObjectsWithNoPersistedProps() {
+        let realm = realmWithTestPath()
+
+        let target = OnlyComputedProps()
+
+        let source1 = LinkToOnlyComputed()
+        source1.value = 1
+        source1.link = target
+
+        XCTAssertEqual(target.backlinks.count, 0, "Linking objects are not available until the object is persisted")
+
+        try! realm.write {
+            realm.add(source1)
+        }
+
+        XCTAssertEqual(target.backlinks.count, 1)
+        XCTAssertEqual(target.backlinks.first!.value, source1.value)
+
+        let source2 = LinkToOnlyComputed()
+        source2.value = 2
+        source2.link = target
+
+        XCTAssertEqual(target.backlinks.count, 1, "Linking objects to an unpersisted object are not available")
+        try! realm.write {
+            realm.add(source2)
+        }
+
+        XCTAssertEqual(target.backlinks.count, 2)
+        XCTAssertTrue(target.backlinks.contains(where: { $0.value == 2 }))
+
+        let targetWithNoLinks = OnlyComputedProps()
+        try! realm.write {
+            // Implicitly verify we can persist a RealmObject with no persisted properties and
+            // no objects linking to it
+            realm.add(targetWithNoLinks)
+        }
+
+        XCTAssertEqual(targetWithNoLinks.backlinks.count, 0, "No object is linking to targetWithNoLinks")
+    }
 }

RealmSwift/Tests/SwiftTestObjects.swift

@@ -868,3 +868,12 @@ class ObjectWithNestedEmbeddedObject: Object {
 private class PrivateObjectSubclass: Object {
     @objc dynamic var value = 0
 }
+
+class LinkToOnlyComputed: Object {
+    @Persisted var value: Int = 0
+    @Persisted var link: OnlyComputedProps?
+}
+
+class OnlyComputedProps: Object {
+    @Persisted(originProperty: "link") var backlinks: LinkingObjects<LinkToOnlyComputed>
+}