There was a discussion / question in the mailinglist [1] about eager-loaded has_many associations hitting the database again. I explained in the last reply why I think this is a bug and attached an example.

Digging into the previous eager-loading issues, I found Issue MONGOID-2177 [2], which seems to stumble across the same issue - what if the eager loaded value is nil (for has_one though)? Looking at the pattern that the implementation was based on [3], it says nothing about discarding nil / Null values. I think it is a major bug to discard nils as "non-fetched", or better not save that the eager-load resulted in a nil. The affected classes should be marked with a special "Null" class acknowledging that the eager-load did result in no value.

I saw that you made the decision to not implement the bugfix from MONGOID-2177. I fail to understand the reason. I cannot see any downsides to having a nil and a Null-class that just returns a nil / [ ] if the eager-load did not return anything.

For now I have to create my own cache for everything that is not guaranteed to have the eager-loaded values or I run into a massive n+1 Problem. I seem to miss the point of the Identity map then.

I would appreciate if you could shed some light on why dropping nils is so important.

Thank you,
Markus

[1] https://groups.google.com/d/topic/mongoid/g5U1ZYw5GhI/discussion
[2] https://github.com/mongoid/mongoid/issues/2177
[3] http://martinfowler.com/eaaCatalog/identityMap.html