mongodb - how it processes $lt on subdocuments - mongodb

Question

I have a collection where one of the fields is a subdocument. I am confused how mongodb supports the $lt, $gt query operators on the complete subdocument.
sample:
db.test.insert({a:1, subdocA:{x:4, y:7, z:10}, b:10})
db.test.insert({a:9, subdocA:{x:2, y:70, z:5}, b:9})
db.test.insert({a:4, subdocA:{x:8, y:2, z:45}, b:19})
In the above collection, I see that mongodb supports a query like:
db.test.find({subdocA:{$lt:{x:6, y:5, z:25}})
In fact it also supports similar queries with $gt operator. It also supports sort({subdocA:1}) on the query.
I would like to know the "logic" it uses to compare the subdocuments and thereby process the $lt, $gt operators.
I see mongodb documentation about how exact matches are processed with subdocuments. But I don't see any documentation on how $lt, $gt are handled with subdocuments.
Thanks.

Answer 1

You have to specify the operator for each field, naming the field with a dot (.) to reach inside the embeeded document. The documentation about $gt hints at this.
So to query a subdocument on z lower than 20, you actually search for subdocA.z being lower than 20, like this :
> db.test.find({'subdocA.z':{$lt:20}}, {_id:0})
{ "a" : 1, "subdocA" : { "x" : 4, "y" : 7, "z" : 10 }, "b" : 10 }
{ "a" : 9, "subdocA" : { "x" : 2, "y" : 70, "z" : 5 }, "b" : 9 }
You can add other criteria in the same way, here with subdocA.x lower than 3 :
> db.test.find({'subdocA.z':{$lt:20}, 'subdocA.x':{$lt:3}}, {_id:0})
{ "a" : 9, "subdocA" : { "x" : 2, "y" : 70, "z" : 5 }, "b" : 9 }
Finally, you can mix and match fields from the "base" document :
> db.test.find({'subdocA.z':{$lt:20}, 'a':{$gt:3}}, {_id:0})
{ "a" : 9, "subdocA" : { "x" : 2, "y" : 70, "z" : 5 }, "b" : 9 }