Added OrganizationUnit Management documentation

docs/en/Modules/Index.md

@@ -17,7 +17,7 @@ There are some **free and open source** application modules developed and mainta
 * **Blogging**: Used to create fancy blogs. ABP's [own blog](https://blog.abp.io/) already using this module.
 * [**Docs**](Docs.md): Used to create technical documentation pages. ABP's [own documentation](https://docs.abp.io) already using this module.
 * **Feature Management**: Used to persist and manage the [features](../Features.md).
-* **Identity**: Manages roles, users and their permissions, based on the Microsoft Identity library.
+* **Identity**: Manages [organization units](OrganizationUnit-Management.md), roles, users and their permissions, based on the Microsoft Identity library.
 * **IdentityServer**: Integrates to IdentityServer4.
 * **Permission Management**: Used to persist permissions.
 * **[Setting Management](Setting-Management.md)**: Used to persist and manage the [settings](../Settings.md).

docs/en/Modules/OrganizationUnit-Management.md

@@ -0,0 +1,125 @@
+# Organization Unit Management
+
+Organization units (OU) is a part of **Identity Module** and can be used to **hierarchically group users and entities**. 
+
+### OrganizationUnit Entity
+
+An OU is represented by the **OrganizationUnit** entity. The fundamental properties of this entity are:
+
+- **TenantId**: Tenant's Id of this OU. Can be null for host OUs.
+- **ParentId**: Parent OU's Id. Can be null if this is a root OU.
+- **Code**: A hierarchical string code that is unique for a tenant.
+- **DisplayName**: Shown name of the OU.
+
+The OrganizationUnit entity's primary key (Id) is a **Guid** type and it derives from the [**FullAuditedAggregateRoot**](../Entities##aggregateroot-class) class which provides audit information with **IsDeleted** property (OUs are not deleted from the database, they are just marked as deleted).
+
+#### Organization Tree
+
+Since an OU can have a parent, all OUs of a tenant are in a **tree** structure. There are some rules for this tree;
+
+- There can be more than one root (where the ParentId is null).
+- Maximum depth of the tree is defined as a constant as OrganizationUnit.**MaxDepth**. The value is **16**.
+- There is a limit for the first-level children count of an OU (because of the fixed OU Code unit length explained below).
+
+#### OU Code
+
+OU code is automatically generated and maintained by the OrganizationUnit Manager. It's a string that looks something like this:
+
+"**00001.00042.00005**"
+
+This code can be used to easily query the database for all the children of an OU (recursively). There are some rules for this code:
+
+- It must be **unique** for a [tenant](../Multi-Tenancy.md).
+- All the children of the same OU have codes that **start with the parent OU's code**.
+- It's **fixed length** and based on the level of the OU in the tree, as shown in the sample.
+- While the OU code is unique, it can be **changeable** if you move an OU.
+- We must reference an OU by Id, not Code.
+
+### OrganizationUnit Manager
+
+The **OrganizationUnitManager** class can be [injected](../Dependency-Injection.md) and used to manage OUs. Common use cases are:
+
+- Create, Update or Delete an OU
+- Move an OU in the OU tree.
+- Getting information about the OU tree and its items.
+
+#### Multi-Tenancy
+
+The OrganizationUnitManager is designed to work for a **single tenant** at a time. It works for the **current tenant** by default.
+
+### Common Use Cases
+
+There are some common use cases for OUs. You can find the source code of the samples [here](https://github.com/abpframework/abp-samples/tree/master/OrganizationUnitSample).
+
+#### Creating An Entity That Belongs To An Organization Unit
+
+The most obvious usage of OUs is to assign an entity to an OU. Sample entity:
+
+```csharp
+public class Product : AuditedAggregateRoot<Guid>, IMultiTenant
+{
+    public virtual Guid OrganizationUnitId { get; private set; }
+    public virtual Guid? TenantId { get; }
+    public virtual string Name { get; private set; }
+    public virtual float Price { get; private set; }
+}
+```
+
+You need to create **OrganizationUnitId** property to assign this entity to an OU. Depending on requirement, this property can be nullable. You can now relate a Product to an OU and query the products of a specific OU.
+
+You can use **IMultiTenant** interface if you want to distinguish products of different tenants in a multi-tenant application (see the [Multi-Tenancy document](https://aspnetboilerplate.com/Pages/Documents/Multi-Tenancy#data-filters) for more info). If your application is not multi-tenant, you don't need this interface and property.
+
+#### Getting Entities In An Organization Unit
+
+To get the Products of an OU,  you can implement a simple domain service; [Product Manager](https://github.com/abpframework/abp-samples/blob/master/OrganizationUnitSample/src/OrganizationUnitSample.Domain/Products/ProductManager.cs) in this case to get the filtered data:
+
+```csharp
+public class ProductManager : IDomainService
+{
+    private readonly IProductRepository<Product> _productRepository;
+
+    public ProductManager(IProductRepository productRepository)
+    {
+        _productRepository = productRepository;
+    }
+
+    public List<Product> GetProductsInOu(OrganizationUnit organizationUnit)
+    {
+        return (await _productRepository.GetListAsync()).Where(q => q.OrganizationUnitId == 			organizationUnit.Id).ToList();
+    }                
+}
+```
+
+**For better practice**, you should consider querying it on domain layer for performance and scalability. To do so, add a method to your repository interface:
+
+```csharp
+public interface IProductRepository : IRepository<Product, Guid>
+{
+    public Task<List<Product>> GetProductsOfOrganizationUnitAsync(Guid organizationUnitId);
+}
+```
+
+Then implement it on your ORM layer (which is EntityFrameworkCore in this [sample](https://github.com/abpframework/abp-samples/blob/master/OrganizationUnitSample/src/OrganizationUnitSample.EntityFrameworkCore/Products/ProductRepository.cs)):
+
+```csharp
+public Task<List<Product>> GetProductsOfOrganizationUnitAsync(Guid organizationUnitId)
+{
+    return DbSet.Where(p => p.OrganizationUnitId == organizationUnitId).ToListAsync();
+}
+```
+
+Afterwards, you can modify your domain service like below:
+
+```csharp
+public List<Product> GetProductsInOu(OrganizationUnit organizationUnit)
+{
+	return await _productRepository.GetProductsOfOrganizationUnitAsync(organizationUnit.Id);
+}
+```
+
+### Settings
+
+You can find **MaxUserMembershipCount** settings under SettingManagement:
+
+- **MaxUserMembershipCount**: Maximum allowed membership count for a user.
+  Default value is **int.MaxValue** which allows a user to be a member of unlimited OUs at the same time.