Q:

How to query a sub sub document in MongoDB?

Here is an example of the document I'm working on:
{
            "_id" : ObjectId("5b35019563726438989381d3"),
            "ref" : "123",
            "nom" : "pc",
            "Revendeurs" : [
                    {
                            "revendeur" : "Tdiscount",
                            "selecteur_prix" : ".price",
                            "hist_prix" : [
                                    {
                                            "date" : ISODate("2018-06-28T00:00:00Z"),
                                            "prix" : 200
                                    },
                                    {
                                            "date" : ISODate("2018-06-27T00:00:00Z"),
                                            "prix" : 100.8}
                            ]
                    }
            ]
    }

I want to query the max of 'prix' field.
I need also to  list all the 'prix' by 'revendeur' but 
Mongodb is returning only empty result.
When executing this query :
db.Tunicompare.aggregate([
    { $unwind: '$Revendeurs' }, 
    { $group: { _id: null, max: { $max: '$Revendeurs.hist_prix.prix' } } },
    { $project: { max: 1, _id:0 } }
])

I get this result (instead of getting only 200 as a maximum)
{ "max" : [ 200, 100.8 ] }

A:

If you want to find the max value throughout both outer & inner lists, do something like this(Globally max value):
db.Tunicompare.aggregate([
    { $unwind: '$Revendeurs' }, 
    { $unwind: '$Revendeurs.hist_prix' },
    { $group: { _id: null, max: { $max: '$Revendeurs.hist_prix.prix' } } },
    { $project: { max: 1, _id:0 } }
])

Output
/* 1 */
{
    "max" : 200.0
}

