reconstruct:

bulk.insert({ field1: 'abc', field2: { _id: 'someid', value: 'someval' } });

Where field2 is of type ObjectId, and references another table, where 'someid' is the OID of some entry with a single key 'value' with value 'someval' . The collection model corresponds to that.

Expected result:
A new entry in the collection, with field2 as an OID .

Actual result:
A new entry was created in the collection, but the field2 was created as an object with 2 fields, '_id' and 'value' .