Add entity audit documentation (#547)

* Add entity audit documentation

docs/Features/entity-auditing.md

@@ -0,0 +1,77 @@
+# Entities auditing
+
+JHipster.Net implement basic Auditing on all the entities you save in the database.
+
+## Audit properties
+
+This blueprint introduce a new base class named ```AuditedEntityBase``` that every generated entity will inherit from when we need to add the audit functionality. The properties use to audit entities are :
+
+1. CreateBy : The user who created the initial entry.
+2. CreatedDate : The datetime of creation of initial entry.
+3. LastModifiedBy : The user who modified the entry last.
+4. LastModifiedDate : The datetime the entry was last modified.
+
+```csharp
+public abstract class AuditedEntityBase
+{
+    public string CreatedBy { get; set; }
+    public DateTime CreatedDate { get; set; }
+    public string LastModifiedBy { get; set; }
+    public DateTime LastModifiedDate { get; set; }
+}
+```
+## Audit of generated Entities 
+
+For example, if we have a ```Task``` entity and we want to add audit functionality we would inherit from our ```AuditedEntityBase``` like that:
+```csharp
+public class Task : AuditedEntityBase
+{
+    public string Title { get; set; }
+    public string Description { get; set; }
+}
+```
+
+Our ```Task``` class will have all the audit properties.
+
+## Automatically set properties audit
+
+To automatically set the audit properties, we override the ```SaveChangesAsync``` method in our ```ApplicationDatabaseContext``` class:
+
+```csharp
+public override async Task<int> SaveChangesAsync(CancellationToken cancellationToken = default(CancellationToken))
+{
+    var entries = ChangeTracker
+        .Entries()
+        .Where(e => e.Entity is IAuditedEntityBase && (
+            e.State == EntityState.Added
+            || e.State == EntityState.Modified));
+
+    string modifiedOrCreatedBy = _httpContextAccessor?.HttpContext?.User?.Identity?.Name ?? "System";
+
+    foreach (var entityEntry in entries)
+    {
+        if (entityEntry.State == EntityState.Added)
+        {
+            ((IAuditedEntityBase)entityEntry.Entity).CreatedDate = DateTime.Now;
+            ((IAuditedEntityBase)entityEntry.Entity).CreatedBy = modifiedOrCreatedBy;
+        }
+        else
+        {
+            Entry((IAuditedEntityBase)entityEntry.Entity).Property(p => p.CreatedDate).IsModified = false;
+            Entry((IAuditedEntityBase)entityEntry.Entity).Property(p => p.CreatedBy).IsModified = false;
+        }
+        ((IAuditedEntityBase)entityEntry.Entity).LastModifiedDate = DateTime.Now;
+        ((IAuditedEntityBase)entityEntry.Entity).LastModifiedBy = modifiedOrCreatedBy;
+    }
+    return await base.SaveChangesAsync(cancellationToken);
+}
+```
+
+In our implementation, we use the ```HttpContextAccessor``` to get the ```user name``` of current user. To have ```HttpContextAccessor``` available we just inject it into our ```ApplicationDatabaseContext class```.
+```csharp
+private readonly IHttpContextAccessor _httpContextAccessor;
+public ApplicationDatabaseContext(DbContextOptions<ApplicationDatabaseContext> options, IHttpContextAccessor httpContextAccessor) : base(options)
+{
+    _httpContextAccessor = httpContextAccessor;
+}
+```

docs/index.rst

@@ -32,6 +32,7 @@ You can also find lot of information in https://www.jhipster.tech
   Features/monitoring
   Features/security
   Features/dependencies-management
+  Features/entity-auditing
 
 .. toctree::
   :maxdepth: 3