Added Defining Custom Filters section.

docs/en/Data-Filtering.md

@@ -8,7 +8,7 @@ ABP defines some filters out of the box.
 
 ### ISoftDelete
 
-Used to mark an entity as deleted instead of actually deleting it. Implement the `ISoftDelete` interface to make your entity "soft delete".
+Used to mark an [entity](Entities.md) as deleted instead of actually deleting it. Implement the `ISoftDelete` interface to make your entity "soft delete".
 
 Example:
 
@@ -119,4 +119,104 @@ Configure<AbpDataFilterOptions>(options =>
 });
 ````
 
-> Carefully change defaults for global filters, especially if you are using a pre-built module which might be developed assuming the soft delete filter is turned on by default. But you can do it for your own defined filters safely.
+> Carefully change defaults for global filters, especially if you are using a pre-built module which might be developed assuming the soft delete filter is turned on by default. But you can do it for your own defined filters safely.
+
+## Defining Custom Filters
+
+Defining and implementing a new filter highly depends on the database provider. ABP implements all pre-defined filters for all database providers.
+
+When you need it, start by defining an interface (like `ISoftDelete` and `IMultiTenant`) for your filter and implement it for your entities.
+
+Example:
+
+````csharp
+public interface IIsActive
+{
+    bool IsActive { get; }
+}
+````
+
+Such an `IIsActive` interface can be used to filter active/passive data and can be easily implemented by any [entity](Entities.md):
+
+````csharp
+public class Book : AggregateRoot<Guid>, IIsActive
+{
+    public string Name { get; set; }
+
+    public bool IsActive { get; set; } //Defined by IIsActive
+}
+````
+
+### EntityFramework Core
+
+ABP uses [EF Core's Global Query Filters](https://docs.microsoft.com/en-us/ef/core/querying/filters) system for the [EF Core Integration](Entity-Framework-Core.md). So, it is well integrated to EF Core and works as expected even if you directly work with `DbContext`.
+
+Best way to implement a custom filter is to override `CreateFilterExpression` method for your `DbContext`. Example:
+
+````csharp
+protected bool IsActiveFilterEnabled => DataFilter?.IsEnabled<IIsActive>() ?? false;
+
+protected override Expression<Func<TEntity, bool>> CreateFilterExpression<TEntity>()
+{
+    var expression = base.CreateFilterExpression<TEntity>();
+
+    if (typeof(IIsActive).IsAssignableFrom(typeof(TEntity)))
+    {
+        Expression<Func<TEntity, bool>> isActiveFilter =
+            e => !IsActiveFilterEnabled || EF.Property<bool>(e, "IsActive");
+        expression = expression == null 
+            ? isActiveFilter 
+            : CombineExpressions(expression, isActiveFilter);
+    }
+
+    return expression;
+}
+````
+
+* Added a `IsActiveFilterEnabled` property to check if `IIsActive` is enabled or not. It internally uses the `IDataFilter` service introduced before.
+* Overrided the `CreateFilterExpression` method, checked if given entity implements the `IIsActive` interface and combines the expressions if necessary.
+
+### MongoDB
+
+ABP implements data filters directly in the [repository](Repositories.md) level for the [MongoDB Integration](MongoDB.md). So, it works only if you use the repositories properly. Otherwise, you should manually filter the data.
+
+Currently, the best way to implement a data filter for the MongoDB integration is to override the `AddGlobalFilters` in the repository derived from the `MongoDbRepository` class. Example: 
+
+````csharp
+public class BookRepository : MongoDbRepository<BookStoreMongoDbContext, Book, Guid>
+{
+    protected override void AddGlobalFilters(List<FilterDefinition<Book>> filters)
+    {
+        if (DataFilter.IsEnabled<IIsActive>())
+        {
+            filters.Add(Builders<Book>.Filter.Eq(e => ((IIsActive)e).IsActive, true));
+        }
+    }
+}
+````
+
+This example implements it only for the `Book` entity. If you want to implement for all entities (those implement the `IIsActive` interface), create your own custom MongoDB repository base class and override the `AddGlobalFilters` as shown below:
+
+````csharp
+public abstract class MyMongoRepository<TMongoDbContext, TEntity, TKey> : MongoDbRepository<TMongoDbContext, TEntity, TKey>
+    where TMongoDbContext : IAbpMongoDbContext
+    where TEntity : class, IEntity<TKey>
+{
+    protected MyMongoRepository(IMongoDbContextProvider<TMongoDbContext> dbContextProvider)
+           : base(dbContextProvider)
+    {
+
+    }
+
+    protected override void AddGlobalFilters(List<FilterDefinition<TEntity>> filters)
+    {
+        if (typeof(IIsActive).IsAssignableFrom(typeof(TEntity)) 
+            && DataFilter.IsEnabled<IIsActive>())
+        {
+            filters.Add(Builders<TEntity>.Filter.Eq(e => ((IIsActive)e).IsActive, true));
+        }
+    }
+}
+````
+
+> See "Set Default Repository Classes" section of the [MongoDb Integration document](MongoDB.md) to learn how to replace default repository base with your custom class.