Documented IRepository.GetQueryableAsync() change in the blog post.

docs/en/Blog-Posts/2021-01-14 v4_2_Preview/POST.md

@@ -32,7 +32,79 @@ See the [ABP CLI documentation](https://docs.abp.io/en/abp/latest/CLI) for all t
 
 ## What's new with the ABP Framework 4.2
 
-TODO
+## IRepository.GetQueryableAsync()
+
+> **This version comes with an important change about using `IQueryable` features over the [repositories](https://docs.abp.io/en/abp/4.2/Repositories). It is suggested to read this section carefully and apply in your applications.**
+
+`IRepository` interface inherits `IQueryable`, so you can directly use the standard LINQ extension methods, like `Where`, `OrderBy`, `First`, `Sum`... etc.
+
+**Example: Using LINQ directly over the repository object**
+
+````csharp
+public class BookAppService : ApplicationService, IBookAppService
+{
+    private readonly IRepository<Book, Guid> _bookRepository;
+
+    public BookAppService(IRepository<Book, Guid> bookRepository)
+    {
+        _bookRepository = bookRepository;
+    }
+
+    public async Task DoItInOldWayAsync()
+    {
+        //Apply any standard LINQ extension method
+        var query = _bookRepository
+            .Where(x => x.Price > 10)
+            .OrderBy(x => x.Name);
+
+        //Execute the query asynchronously
+        var books = await AsyncExecuter.ToListAsync(query);
+    }
+}
+````
+
+*See [the documentation](https://docs.abp.io/en/abp/4.2/Repositories#iqueryable-async-operations) if you wonder what is the `AsyncExecuter`.*
+
+Beginning from the version 4.2, the recommended way is using `IRepository.GetQueryableAsync()` to obtain an `IQueryable`, then use the LINQ extension methods over it.
+
+**Example: Using the new GetQueryableAsync method**
+
+````csharp
+public async Task DoItInNewWayAsync()
+{
+    //Use GetQueryableAsync to obtain the IQueryable<Book> first
+    var queryable = await _bookRepository.GetQueryableAsync();
+
+    //Then apply any standard LINQ extension method
+    var query = queryable
+        .Where(x => x.Price > 10)
+        .OrderBy(x => x.Name);
+
+    //Finally, execute the query asynchronously
+    var books = await AsyncExecuter.ToListAsync(query);
+}
+````
+
+ABP may start a database transaction when you get an `IQueryable` (If current [Unit Of Work](https://docs.abp.io/en/abp/latest/Unit-Of-Work) is transactional). In this new way, it is possible to **start the database transaction in an asynchronous way**. Previously, we could not get the advantage of asynchronous while starting the transactions.
+
+> **The new way has a significant performance and scalability gain. The old usage (directly using LINQ over the repositories) will be removed in the next major version.** You have a lot of time for the change, but we recommend to immediately take the action since the old usage has a big scalability problem.
+
+#### About IRepository Async Extension Methods
+
+Using IRepository Async Extension Methods has no such a problem. The examples below are pretty fine:
+
+````csharp
+var countAll = await _personRepository
+    .CountAsync();
+
+var count = await _personRepository
+    .CountAsync(x => x.Name.StartsWith("A"));
+
+var book1984 = await _bookRepository
+    .FirstOrDefaultAsync(x => x.Name == "John");   
+````
+
+See the [repository documentation](https://docs.abp.io/en/abp/4.2/Repositories#iqueryable-async-operations) to understand the relation between `IQueryable` and asynchronous operations.
 
 ## What's new with the ABP Commercial 4.2