This post explains how the QueryExpression class can be used in conjunction with IOrganizationService.RetrieveMultiple() in CRM 2013 to retrieve related records for an entity in an N:N relationship. This is something that I could not find much documentation on, and it is not very straightforward – at least for someone starting out in CRM dev.

Scenario

I have two custom entities that have an N:N relationship between them: Course (logical name ng_course) and Subject (logical name ng_subject). The relationship is named ng_course_subject. The query that I need to write is to retrieve all Subjects assigned to a given Course.

Understanding QueryExpression and LinkEntity

The QueryExpression class is where you define your query. You can learn more about this class here: http://msdn.microsoft.com/en-us/library/microsoft.xrm.sdk.query.queryexpression(v=crm.6).aspx.

The key property of this class that enables us to retrieve the related records is the LinkEntities property, which is a collection of LinkEntity objects. Essentially LinkEntity