This commit fixes an issue where a defined custom target type conversion for the id field was not properly considered when writing a document reference. Previously an eg. String was not being converted into an ObjectId correctly causing lookup queries to return empty results.
Converting the id property value on write solves the issue.
Includes a minor polish in the mapping centralizing pointer creation within the DocumentPointerFactory.
Closes: #3782
Original pull request: #3785.
bf86f39b2d