see on GitHub

Session: How to Get Entity ID

Entities does not have to contain an ID property or field. In the case of such an entity, and a need for knowing under what ID it is stored on the server, the GetDocumentId method was created.

Syntax

string GetDocumentId(object entity);
Parameters
entity object Instance of an entity for which an ID will be returned
Return Value
string Returns the ID for a specified entity. The method may return null if entity is null, isn't tracked, or the ID will be generated on the server.

Example

public class Comment
{
	public string Author { get; set; }

	public string Message { get; set; }
}

string commentId = session
	.Advanced
	.GetDocumentId(comment); // e.g. comments/1-A