In short. I want to be able to issue a `.Set` and reference the existing document(/entity) for fields.

I posted a question on [StackOverflow|https://stackoverflow.com/questions/56217554/mongodb-c-sharp-driver-update-an-entity-field-with-data-from-another-entity-fie] which goes like this:
My model looks something like this:
 data: {
field1 :"1"
},

history: [{
field1 :"0"
}]
{{}}

Now I want to create an update definition which takes the current object in "data" and inserts it into the "history" array.

I know how to do this with absolute values..:
 var updateDef = Builders<Entity>.Update.Push(x => x.History, historyObject);
updateDef = updateDef.Set(x => x.Data, newDataModel);
The problem is that I want the "historyObject" to be automatically gotten from the current document's "data" field.