Take snapshot prior to mongo update then mark any changes during beforeUpdate and update events as dirty.

grails-data-mongodb/core/src/main/groovy/org/grails/datastore/mapping/mongo/engine/MongoCodecEntityPersister.groovy

@@ -240,7 +240,26 @@ class MongoCodecEntityPersister extends ThirdPartyCacheEntityPersister<Object> {
                 mongoCodecSession.addPendingUpdate(new PendingUpdateAdapter(entity, id, obj, entityAccess) {
                     @Override
                     void run() {
+                        // Take snapshot of all property values BEFORE the PreUpdate event fires
+                        Map<String, Object> beforeUpdateSnapshot = [:]
+                        if (obj instanceof DirtyCheckable) {
+                            for (PersistentProperty prop : entity.persistentProperties) {
+                                beforeUpdateSnapshot[prop.name] = entityAccess.getProperty(prop.name)
+                            }
+                        }
                         if (!cancelUpdate(entity, entityAccess)) {
+                            // Compare with snapshot and mark modified properties dirty
+                            if (obj instanceof DirtyCheckable) {
+                                DirtyCheckable dirtyCheckable = (DirtyCheckable) obj
+                                for (PersistentProperty prop : entity.persistentProperties) {
+                                    Object oldValue = beforeUpdateSnapshot[prop.name]
+                                    Object newValue = entityAccess.getProperty(prop.name)
+                                    boolean valueChanged = oldValue != newValue && (oldValue == null || !oldValue.equals(newValue))
+                                    if (valueChanged) {
+                                        dirtyCheckable.markDirty(prop.name, newValue, oldValue)
+                                    }
+                                }
+                            }
                             updateCaches(entity, obj, id)
                             addCascadeOperation(new PendingOperationAdapter(entity, id, obj) {
                                 @Override

grails-data-mongodb/core/src/main/groovy/org/grails/datastore/mapping/mongo/engine/codecs/PersistentEntityCodec.groovy

@@ -64,7 +64,6 @@ import org.grails.datastore.mapping.engine.internal.MappingUtils
 import org.grails.datastore.mapping.model.EmbeddedPersistentEntity
 import org.grails.datastore.mapping.model.PersistentEntity
 import org.grails.datastore.mapping.model.PersistentProperty
-import org.grails.datastore.mapping.model.config.GormProperties
 import org.grails.datastore.mapping.model.types.Association
 import org.grails.datastore.mapping.model.types.Embedded
 import org.grails.datastore.mapping.model.types.EmbeddedCollection
@@ -249,12 +248,6 @@ class PersistentEntityCodec extends BsonPersistentEntityCodec {
                 }
 
             }
-            else {
-                // schedule lastUpdated if necessary
-                if (entity.getPropertyByName(GormProperties.LAST_UPDATED) != null) {
-                    dirtyProperties.add(GormProperties.LAST_UPDATED)
-                }
-            }
 
             for (propertyName in dirtyProperties) {
                 def prop = entity.getPropertyByName(propertyName)

grails-data-mongodb/core/src/test/groovy/org/grails/datastore/gorm/mongo/BeforeUpdatePropertyPersistenceSpec.groovy

@@ -18,8 +18,6 @@
  */
 package org.grails.datastore.gorm.mongo
 
-import spock.lang.PendingFeature
-
 import grails.persistence.Entity
 import org.apache.grails.data.mongo.core.GrailsDataMongoTckManager
 import org.apache.grails.data.testing.tck.base.GrailsDataTckSpec
@@ -37,7 +35,7 @@ class BeforeUpdatePropertyPersistenceSpec extends GrailsDataTckSpec<GrailsDataMo
         manager.domainClasses.addAll([UserWithBeforeUpdate, UserWithBeforeUpdateAndAutoTimestamp])
     }
 
-    @PendingFeature
+    @Issue('GRAILS-15139')
     void "Test that properties set in beforeUpdate are persisted"() {
         given: "A user is created"
         def user = new UserWithBeforeUpdate(name: "Fred")
@@ -92,7 +90,6 @@ class BeforeUpdatePropertyPersistenceSpec extends GrailsDataTckSpec<GrailsDataMo
         user.random.length() == 5
     }
 
-    @PendingFeature
     void "Test that multiple updates continue to trigger beforeUpdate"() {
         given: "A user is created"
         def user = new UserWithBeforeUpdate(name: "Fred")
@@ -119,8 +116,7 @@ class BeforeUpdatePropertyPersistenceSpec extends GrailsDataTckSpec<GrailsDataMo
         user.random.length() == 5
     }
 
-    @PendingFeature
-    @Issue('GPMONGODB-XXX')
+    @Issue('GRAILS-15120')
     void "Test that properties set in beforeUpdate with AutoTimestamp are persisted"() {
         given: "A user with auto timestamp is created"
         def user = new UserWithBeforeUpdateAndAutoTimestamp(name: "Fred")