在SaveChanges上使用ObjectContext进行实体框架审核

Question

For audit logging purposes, I need to get values of all the columns, including FK entities and relational entities that have been modifed for one of the table in the database. Database is basically for a website where user can upload resources (files, online document, picture etc.), I've a table called Material which has multiple many-2-many nd one-2-one relations like Material - Audience , Material - Category , 'Material-Uploader', 'Material-Permission Material -Tags etc. I want to log all the changes happening to a Material . For example if someone removes a Tag from a Material, then I need to log:

• [User12 - 12/12/12] - Happy tag got removed from Crappy material.

So far I got this: I can get all the ObjectStateEntries which are modified, added, deleted by using:

context.ObjectStateManager.GetObjectStateEntries(EntityState.Added | EntityState.Deleted | EntityState.Modified)

Now, I can check whether this ObjectStateEntry is RelationShip or not using:

if (e.IsRelationship) {
    HandleRelationshipEntry(e);
}
else {
    HandleEntry(e);
}

Within HandleEntry method (Entry is not relationship entry), I can check the type of Entry , in my case it is Material , so I'm doing:

// We care about only Material which are modifed
if (e.State != EntityState.Modified || !(e.Entity is Material))
    return;

Once, I know Entry is of type Material Entry , I can get all the columns that have changed for Material table using:

e.CurrentValues[ARCHIVE_COLUMN].ToString() != e.OriginalValues[ARCHIVE_COLUMN].ToString()

At this point, I can log all the non FK changes of Material table. But if column is FK to some other entity , I cannot resolve that FK value to corresponding Entity . I could just know that CategoryID has been changed from 42 to 76 but I cannot resolve name of the Category itself. I tried approach like casting DBDataRecord and CurrentValueRecord to EntityKey but it is just NULL . Is there any way to resolve these FKs to Entities using ObjectStateManager ?

My full code for the reference:

private class SingleMaterialLogger { MaterialAuditData auditData = new MaterialAuditData(); public void HandleEntity(ObjectStateEntry e, ObjectContext context) { HandlePrimaryTypeChanges(e); HandleComplexTypeChanges(e, context); }

        private void HandleComplexTypeChanges(ObjectStateEntry e, ObjectContext c) {
            // Owner, Category, Contact
            ChangeValueHelper(e, CONTACT_COLUMN, (k1, k2) => {
                // get old value
                User old = c.GetObjectByKey(k1) as User;
                User current = c.GetObjectByKey(k2) as User;
            });
        }

        public void HandlePrimaryTypeChanges(ObjectStateEntry e) {
            // Name, Description, ArchiveDate, Status
            // Again no reflection is used - So change them if column name changes
            ChangeValueHelper<string>(e, NAME_COLUMN, (change) => auditData.Name = change);
            ChangeValueHelper<string>(e, NAME_COLUMN, (change) => auditData.Description = change);
            // TODO - Fix change value helper
            if (e.CurrentValues[ARCHIVE_COLUMN].ToString() != e.OriginalValues[ARCHIVE_COLUMN].ToString()) {
                auditData.ArchiveDate = new Change<DateTime?>(e.OriginalValues[ARCHIVE_COLUMN] as DateTime?, e.CurrentValues[ARCHIVE_COLUMN] as DateTime?);
            }
        }

        private void ChangeValueHelper(ObjectStateEntry e, string columnName, Action<EntityKey, EntityKey> func) {
            if (e.CurrentValues[columnName].ToString() != e.OriginalValues[columnName].ToString()) {
                func(e.OriginalValues[columnName] as EntityKey, e.CurrentValues[columnName] as EntityKey);
            }
        }
        private void ChangeValueHelper<T>(ObjectStateEntry e, string columnName, Action<Change<T>> func) where T : class {
            if(e.CurrentValues[columnName].ToString() != e.OriginalValues[columnName].ToString()) {
                func(new Change<T>(e.OriginalValues[columnName] as T, e.OriginalValues[columnName] as T));
            }
        }
    }



    Dictionary<EntityKey, SingleMaterialLogger> singleMaterialLoggerMap = new Dictionary<EntityKey, SingleMaterialLogger>();
    private ObjectContext context;

    public MaterialAuditLogger(ObjectContext context) {
        this.context = context;
    }

    public void AuditMaterialChanges() {
        // Grab everything thats being added/deleted/modified
        foreach(var e in context.ObjectStateManager.GetObjectStateEntries(EntityState.Added | EntityState.Deleted | EntityState.Modified)) {
            if (e.IsRelationship) {
                HandleRelationshipEntity(e);
            }
            else {
                HandleEntity(e);
            }
        }
    }

    private void HandleEntity(ObjectStateEntry e) {
        // We care about only Material which are modifed
        if (e.State != EntityState.Modified || !(e.Entity is Material))
            return;

        var logger = SingleLogger(e.EntityKey);
        logger.HandleEntity(e, context);
    }

    private void HandleRelationshipEntity(ObjectStateEntry e) {
        // relations whose entity keys contains
    }

    private SingleMaterialLogger SingleLogger(EntityKey key) {
        if(singleMaterialLoggerMap.ContainsKey(key))
            return singleMaterialLoggerMap[key];
        SingleMaterialLogger logger = new SingleMaterialLogger();
        singleMaterialLoggerMap[key] = logger;
        return logger;
    }

Answer 1

I've run into the same problem.

It's not difficult to pull any entity type with an id value:

DbContext.Set(entityType).Find(id)

However this assumes that you have identified the entity type from the relevant navigation property in the first place. That requires some smarts, basically duplicating the EF logic by using reflection to look at property names and [ForeignKey()] attributes etc.

Some options are:

1) Add smarts to work out the FK model property from the FK ID property. Then do a lookup of the FK model on the fly during the audit log creation process, and store down the .ToString() value in the audit log.

This assumes:

• You have a general utility in your DataContext/Repository to lookup any model type on the fly (eg. DbContext.Set(entityType).Find(id))

• You are confident that the .ToString() implementation on all your FK models will work reliably because of one of the following:

  • They never rely on further navigation properties which may cause a run-time error

  • You can be confident the further navigation properties were properly Include()'ed in your model lookup

  • You have lazy loading enabled (which I strongly advise against.... but.. it would help here)

• You have thought through the transaction implications (if you are using transactions beyond what EF does)

2) Store down the FK ID in the audit log. Then, when viewing the audit log, do a lookup of the FK model on the fly and render ToString() on screen.

We went with this option in our project and it works fine.

However your auditing requirements may be stricter. For example, if someone changes the name/description on the FK model, then this will appear to modify the old audit logs.