Added Entity Extensions section to the document: Customizing-Application-Modules-Extending-Entities.md

docs/en/Customizing-Application-Modules-Extending-Entities.md

@@ -4,7 +4,7 @@ In some cases, you may want to add some additional properties (and database fiel
 
 ## Extra Properties
 
-[Extra properties](Entities.md) is a way of storing some additional data on an entity without changing it. The entity should implement the `IHasExtraProperties` interface to allow it. All the aggregate root entities defined in the pre-built modules implement the `IHasExtraProperties` interface, so you can store extra properties on these entities.
+[Extra properties](Entities.md) is a way of storing some additional data on an entity without changing it. The entity should implement the `IHasExtraProperties` interface to allow it. All the aggregate root entities defined in the pre-built modules implement the `IHasExtraProperties` interface, so you can store extra properties on these objects.
 
 Example:
 
@@ -25,7 +25,35 @@ Extra properties are stored as a single `JSON` formatted string value in the dat
 
 See the [entities document](Entities.md) for more about the extra properties system.
 
-> It is possible to perform a **business logic** based on the value of an extra property. You can **override** a service method and get or set the value as shown above. Overriding services will be discussed below.
+> It is possible to perform a **business logic** based on the value of an extra property. You can [override a service method](Customizing-Application-Modules-Overriding-Services.md), then get or set the value as shown above.
+
+## Entity Extensions (EF Core)
+
+As mentioned above, all extra properties of an entity are stored as a single JSON object in the database table. This is not so natural especially when you want to;
+
+* Create **indexes** and **foreign keys** for an extra property.
+* Write **SQL** or **LINQ** using the extra property (search table by the property value, for example).
+* Creating your **own entity** maps to the same table, but defines an extra property as a **regular property** in the entity (see the [EF Core migration document](Entity-Framework-Core-Migrations.md) for more).
+
+To overcome the difficulties described above, ABP Framework entity extension system for the Entity Framework Core that allows you to use the same extra properties API defined above, but store a desired property as a separate field in the database table.
+
+Assume that you want to add a `SocialSecurityNumber` to the `IdentityUser` entity of the [Identity Module](Modules/Identity.md). You can use the `EntityExtensionManager` static class:
+
+````csharp
+EntityExtensionManager.AddProperty<IdentityUser, string>(
+    "SocialSecurityNumber",
+    b => { b.HasMaxLength(32); }
+);
+````
+
+* You provide the `IdentityUser` as the entity name, `string` as the type of the new property, `SocialSecurityNumber` as the property name (also, the field name in the database table).
+* You also need to provide an action that defines the database mapping properties using the [EF Core Fluent API](https://docs.microsoft.com/en-us/ef/core/modeling/entity-properties).
+
+> This code part must be executed before the related `DbContext` used. The [application startup template](Startup-Templates/Application.md) defines a static class named `YourProjectNameEntityExtensions`. You can define your extensions in this class to ensure that it is executed in the proper time. Otherwise, you should handle it yourself.
+
+Once you define an entity extension, you then need to use the standard [Add-Migration](https://docs.microsoft.com/en-us/ef/core/miscellaneous/cli/powershell#add-migration) and [Update-Database](https://docs.microsoft.com/en-us/ef/core/miscellaneous/cli/powershell#update-database) commands of the EF Core to create a code first migration class and update your database.
+
+You can then use the same extra properties system defined in the previous section to manipulate the property over the entity.
 
 ## Creating a New Entity Maps to the Same Database Table/Collection
 

docs/en/Entities.md

@@ -375,7 +375,7 @@ The way to store this dictionary in the database depends on the database provide
 
 * For [Entity Framework Core](Entity-Framework-Core.md), here are two type of configurations;
   * By default, it is stored in a single `ExtraProperties` field as a `JSON` string (that means all extra properties stored in a single database table field). Serializing to `JSON` and deserializing from the `JSON` are automatically done by the ABP Framework using the [value conversions](https://docs.microsoft.com/en-us/ef/core/modeling/value-conversions) system of the EF Core.
-  * If you want, you can use the `EntityExtensionManager` to define a separate table field for a desired extra property. Properties those are not configured through the `EntityExtensionManager` will continue to use a single `JSON` field as described above. This feature is especially useful when you are using a pre-built [application module](Modules/Index.md) and want to [extend its entities](Customizing-Application-Modules-Extending-Entities.md).
+  * If you want, you can use the `EntityExtensionManager` to define a separate table field for a desired extra property. Properties those are not configured through the `EntityExtensionManager` will continue to use a single `JSON` field as described above. This feature is especially useful when you are using a pre-built [application module](Modules/Index.md) and want to [extend its entities](Customizing-Application-Modules-Extending-Entities.md). See the [EF Core integration document](Entity-Framework-Core.md) to learn how to use the `EntityExtensionManager`.
 * For [MongoDB](MongoDB.md), it is stored as a **regular field**, since MongoDB naturally supports this kind of [extra elements](https://mongodb.github.io/mongo-csharp-driver/1.11/serialization/#supporting-extra-elements) system.
 
 ### Discussion for the Extra Properties