Customize the queries for subgrids of Dynamics 365 form

 Subgrids are great but the limitation of needing a direct relationship between the sub entity and the main entity makes it very restrictive. There are many different use cases, if instead of using a pre-configured view that only allows linking to the active Record via a predefined relationship, we could instead bind a custom FetchXML do a subgrid and this custom FetchXML would allow for one token called #RecordID# to reference the currently open record on the form by returning the GUID. 

In Classic UI people used the subgrid.control.SetParameter("fetchxml") function but this is now broken in Unified Interface. In Unified Interface it appears a Retrieve Multiple Plugin is the only way to achieve this which is extremely complicated for something as simple as binding a query to a subgrid. 

 Here is the only way to currently do this: 

https://sank8sinha.wordpress.com/2020/01/07/adding-filtered-views-in-uci-in-dynamics-365-crm-finally...

 Reference:

1. https://powerusers.microsoft.com/t5/Power-Apps-Ideas/Binding-FetchXML-to-Subgrid-with-reference-to-RecordID-to-filter/idi-p/675538
2. https://cloudblogs.microsoft.com/dynamics365/no-audience/2012/04/16/deep-queries-for-subgrids/?source=crm