How to create a criterion, which returns an intersection of two many-to-many sets

Description:
There are three persistent classes: Client, Region and News. Clients-Regions and News-Regions are many-to-many associations between these objects. That is, both Client and News have a Regions collection. I need to select News for a given Client. That is, the intersection of Client.Regions and News.Regions sets is not empty. For example:
Client: John Doe
Client Regions: Nevada, Arizona.
News #1: Celebration in Grand Canyon
News Regions: Arizona, Colorado
News #2: Earthquake in California
News Regions: California, Oregon

That is, news #1 must be selected for John Doe.
Here is the implementation of my classes:

C#
using DevExpress.Xpo;  

public class News : XPObject {  
    public string Title;  

    [Association("News-Regions")]  
    public XPCollection<Region> Regions {  
        get { return GetCollection<Region>("Regions"); }  
    }  
}  

public class Region : XPObject {  
    public string Name;  

    [Association("Clients-Regions")]  
    public XPCollection<Client> Clients {  
        get { return GetCollection<Client>("Clients"); }  
    }  

    [Association("News-Regions")]  
    public XPCollection<News> News {  
        get { return GetCollection<News>("News"); }  
    }  
}  

public class Client : XPObject {  
    public string FullName;  

    [Association("Clients-Regions")]  
    public XPCollection<Region> Regions {  
        get { return GetCollection<Region>("Regions"); }  
    }  
}  

Visual Basic
Imports DevExpress.Xpo  

Public Class News  
    Inherits XPObject  

    Public Title As String  

    <Association("News-Regions")> _  
    Public ReadOnly Property Regions() As XPCollection(Of Region)  
        Get  
            Return GetCollection(Of Region)("Regions")  
        End Get  
    End Property  
End Class  

Public Class Region  
    Inherits XPObject  

    Public Name As String  

    <Association("Clients-Regions")> _  
    Public ReadOnly Property Clients() As XPCollection(Of Client)  
        Get  
            Return GetCollection(Of Client)("Clients")  
        End Get  
    End Property  

    <Association("News-Regions")> _  
    Public ReadOnly Property News() As XPCollection(Of News)  
        Get  
            Return GetCollection(Of News)("News")  
        End Get  
    End Property  
End Class  

Public Class Client  
    Inherits XPObject  

    Public FullName As String  

    <Association("Clients-Regions")> _  
    Public ReadOnly Property Regions() As XPCollection(Of Region)  
        Get  
            Return GetCollection(Of Region)("Regions")  
        End Get  
    End Property  
End Class  

Answer:
It's possible to create such a filter via two nested Contains filters. Since you need to get News, create an XPCollection with the News element type (XPCollection<News>). The News class has a Regions collection, which contains objects of type Region, which in turn has a Clients collection with Client type objects. As such, the filter string will look like this:

C#
CriteriaOperator filter = CriteriaOperator.Parse("Regions[Clients[?]]", client);  

Visual Basic
Dim filter As CriteriaOperator = CriteriaOperator.Parse("Regions[Clients[?]]", client)  

The above expression can be also represented by two nested ContainsOperator objects:

C#
CriteriaOperator filter = new ContainsOperator("Regions", new ContainsOperator("Clients", new BinaryOperator("This", client)));  

Visual Basic
Dim filter As CriteriaOperator = New ContainsOperator("Regions", New ContainsOperator("Clients", New BinaryOperator("This", client)))  

Here is the same filter written in Simplified Criteria Syntax:

C#
using DevExpress.Xpo;  
using DevExpress.Data.Filtering;  

using(UnitOfWork uow = new UnitOfWork())  
{  
    Client client = uow.FindObject<Client>(new OperandProperty("FullName") == "John Doe");  

    CriteriaOperator filter = new OperandProperty("Regions")[new OperandProperty("Clients")[new OperandProperty("This") == new OperandValue(client)]];  
    XPCollection<News> newsForJohnDoe = new XPCollection<News>(uow, filter);  
    ...  
}  

Visual Basic
Imports DevExpress.Xpo  
Imports DevExpress.Data.Filtering  

Using uow As UnitOfWork = New UnitOfWork()  
    Dim client As Client = uow.FindObject(Of Client)(New OperandProperty("FullName") = "John Doe")  

    Dim filter As CriteriaOperator = New OperandProperty("Regions").Item(New OperandProperty("Clients").Item(New OperandProperty("This") = New OperandValue(client)))  
    Dim newsForJohnDoe As XPCollection(Of News) = New XPCollection(Of News)(uow, filter)  
    ...  
End Using  

Although filter expressions written in Simplified Criteria Syntax are usually longer and appear more difficult to understand than the same filter written as a string for the parser, we still recommend that you prefer Simplified Criteria Syntax to CriteriaOperator.Parse, because the former is checked at a compilation time, while the latter is validated (parsed) only at runtime.
There is a test project attached to this article. You'll find the download link below, in the Attachment section.
See Also:
Build Criteria - Cheat Sheet
Simplified Criteria Syntax
Criteria Operators
How to: Build Complex Criteria
How to filter a collection by an associated objects' properties
What criteria should I use to get objects for a collection property which is empty?
How to use the ContainsOperator for objects in a many-to-many relationship