MongoDB Type Order

Question

I have this collection in MongoDB. It contains
values of different types under the val key.
Also, note that I am sorting it by val ascending.

[test] 2014-02-20 08:53:11.857 >>> db.account.find().sort({val:1});
{
        "_id" : ObjectId("5304d25786dd4b348bcc2b2e"),
        "username" : "usr10",
        "password" : "123",
        "val" : [ ]
}
{
        "_id" : ObjectId("5304d29986dd4b348bcc2b2f"),
        "username" : "usr20",
        "password" : "456",
        "val" : null
}
{
        "_id" : ObjectId("5304e31686dd4b348bcc2b37"),
        "username" : "usr80",
        "password" : "555",
        "val" : 1
}
{
        "_id" : ObjectId("5304d50a86dd4b348bcc2b32"),
        "username" : "usr50",
        "password" : "555",
        "val" : [
                40
        ]
}
{
        "_id" : ObjectId("5304d4c886dd4b348bcc2b31"),
        "username" : "usr40",
        "password" : "777",
        "val" : 200
}
{
        "_id" : ObjectId("5304d2a186dd4b348bcc2b30"),
        "username" : "usr30",
        "password" : "888",
        "val" : {

        }
}
{
        "_id" : ObjectId("5304d97786dd4b348bcc2b33"),
        "username" : "usr50",
        "password" : "555",
        "val" : {
                "ok" : 1
        }
}
{
        "_id" : ObjectId("5304e2dc86dd4b348bcc2b36"),
        "username" : "usr80",
        "password" : "555",
        "val" : true
}
{
        "_id" : ObjectId("5304e22f86dd4b348bcc2b34"),
        "username" : "usr60",
        "password" : "555",
        "val" : ISODate("2014-02-19T16:56:15.787Z")
}
{
        "_id" : ObjectId("5304e2c786dd4b348bcc2b35"),
        "username" : "usr70",
        "password" : "555",
        "val" : /abc/
}
[test] 2014-02-20 08:53:19.357 >>>

I am reading a book which says the following.

MongoDB has a hierarchy as to how types compare. Sometimes you will have
a single key with multiple types: for instance, integers and booleans, or strings
and nulls. If you do a sort on a key with a mix of types, there is a predefined
order that they will be sorted in. From least to greatest value, this ordering
is as follows:
1. Minimum value
2. null
3. Numbers (integers, longs, doubles)
4. Strings
5. Object/document
6. Array
7. Binary data
8. Object ID
9. Boolean
10. Date
11. Timestamp
12. Regular expression
13. Maximum value

So why is my sorting order different? For example,
when I sort (see above) I see these strange things:

1) I have no idea what 'minimum value' and 'maximum value' mean.

2) An array comes before a number. And an empty
array comes even before null.

3) The number 1 comes before an array

4) The array [40] comes between numbers 1 and 200.

Could someone just explain this result in some details?
Many thanks in advance.

Answer 1

Your book says the same as the official documentation. But this also does not explain the obscure sorting order of the two arrays. At least the two types Minimum value and Maximum value are explained. They are internal.

Answer 2

The type order is only used when there isn't another supported way of ordering elements. Array fields have their own sorting behavior where the minimum value of their elements are used on an ascending sort, and the maximum value on a descending sort. The type of that minimum or maximum value is then used to order the docs with fields of that type.

So [40] comes after 1, but before 200 because the minimum value of that array is 40.

The empty array has no value at all, which is why it ends up with the doc where the value is null. If I reverse the sort, they stay in the same order which implies that MongoDB considers them equal.