// Setup
db.a.drop()
db.a.aggregate([ { "$densify" : { "field" : "obj.obj.obj.date", "range" : { "step" : 9, "unit" : "month", "bounds" : [ ISODate("2019-01-31T21:53:45.323Z"), ISODate("2020-06-02T10:40:15.842Z") ] } } }, { "$sort" : { "obj.obj.obj.date" : 1, "_id" : 1 } }, { "$limit" : 1000 } ])
// Result:
// { "obj" : { "obj" : { "obj" : { "date" : ISODate("2019-01-31T21:53:45.323Z") } } } }
// { "obj" : { "obj" : { "obj" : { "date" : ISODate("2019-10-31T21:53:45.323Z") } } } }
// -----
// Adding a document in the range produces additional documents at the start of the densify range
db.a.insert({obj: {obj: {obj: {date: new Date("2019-10-01")}}}})
db.a.aggregate([ { "$densify" : { "field" : "obj.obj.obj.date", "range" : { "step" : 9, "unit" : "month", "bounds" : [ ISODate("2019-01-31T21:53:45.323Z"), ISODate("2020-06-02T10:40:15.842Z") ] } } }, { "$sort" : { "obj.obj.obj.date" : 1, "_id" : 1 } }, { "$limit" : 1000 } ])
// Result:
// { "obj" : { "obj" : { "obj" : { "date" : ISODate("2019-01-30T21:53:45.323Z") } } } }
// { "obj" : { "obj" : { "obj" : { "date" : ISODate("2019-01-31T21:53:45.323Z") } } } }
// { "_id" : ObjectId("6772948fbfb88c160d07a21e"), "obj" : { "obj" : { "obj" : { "date" : ISODate("2019-10-01T00:00:00Z") } } } }
// { "obj" : { "obj" : { "obj" : { "date" : ISODate("2019-10-31T21:53:45.323Z") } } } }
// ------
// Worse yet the number of erroneous documents is linear in the number of intermediary documents
db.a.insert({obj: {obj: {obj: {date: new Date("2019-10-01")}}}})
db.a.aggregate([ { "$densify" : { "field" : "obj.obj.obj.date", "range" : { "step" : 9, "unit" : "month", "bounds" : [ ISODate("2019-01-31T21:53:45.323Z"), ISODate("2020-06-02T10:40:15.842Z") ] } } }, { "$sort" : { "obj.obj.obj.date" : 1, "_id" : 1 } }, { "$limit" : 1000 } ])
// Result:
// { "obj" : { "obj" : { "obj" : { "date" : ISODate("2019-01-30T21:53:45.323Z") } } } }
// { "obj" : { "obj" : { "obj" : { "date" : ISODate("2019-01-30T21:53:45.323Z") } } } }
// { "obj" : { "obj" : { "obj" : { "date" : ISODate("2019-01-31T21:53:45.323Z") } } } }
// { "_id" : ObjectId("6772948fbfb88c160d07a21e"), "obj" : { "obj" : { "obj" : { "date" : ISODate("2019-10-01T00:00:00Z") } } } }
// { "_id" : ObjectId("677294cbbfb88c160d07a220"), "obj" : { "obj" : { "obj" : { "date" : ISODate("2019-10-01T00:00:00Z") } } } }
// { "obj" : { "obj" : { "obj" : { "date" : ISODate("2019-10-31T21:53:45.323Z") } } } }