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KB Article A863
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What criteria should I use to get objects for a collection property which is empty?

Description:
I have a many-to-many relation which is implemented in one class:

C#
public class HelpTopic : XPObject {  
    public string Name;  
    [Association("Parent-Children", typeof(HelpTopic))]  
    public XPCollection Parents {  
        get { return GetCollection("Parents"); }  
    }  
    [Association("Parent-Children", typeof(HelpTopic))]  
    public XPCollection Children {  
        get { return GetCollection("Children"); }  
    }  
}

How can I get only the root HelpTopics?

Answer:
You can use one of the following criteria:

C#
XPCollection<HelpTopic> rootTopics = new XPCollection<HelpTopic>(session,  
    new NotOperator(new ContainsOperator("Parents", null)));  

XPCollection<HelpTopic> rootTopics = new XPCollection<HelpTopic>(session,  
    new NotOperator(new ContainsOperator("Parents",  
        new NotOperator(new NullOperator("Oid")))));

See also:
How to set query criteria for a collection which is a collection property of a persistent object
How to construct complex queries
How to filter a collection by an associated objects' properties
How to create a criterion, which returns an intersection of two many-to-many sets

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