Database
[{"doc-id":2,"interfaces":[{"interface-role":"ON","port-nb":1},{"interface-role":"OFF","port-nb":2},{"interface-role":"ON","port-nb":3},{"interface-role":"OFF","port-nb":3}]}]
Query
db.collection.aggregate([{$match:{"doc-id":2}},{"$addFields":{"interfaces":{"$filter":{"input":"$interfaces","as":"i","cond":{$or:[{$eq:[null,"ON"]},{$eq:["$$i.interface-role","ON"]}]}}}}}])