[/** with content.id*/{"_id":1,/** I know it will be ObjectId but for simplicity here I'll set it as int*/"content":{"id":1,"title":"some title"}},/** without content.id*/{"_id":2,"content":{"title":"some title"}},/** with content.id equal to null*/{"_id":3,"content":{"id":null,"title":"some title"}},/** with empty content object*/{"_id":4,"content":{}},/** with content as an empty array*/{"_id":5,"content":[]},/** with content equal to null*/{"_id":6,"content":null},/** without content*/{"_id":7}]

db.collection.find({$expr:{$eq:[{$ifNull:["$content.id",[]]},[]]}})