Due to the nature of MongoDB, it's perfectly acceptable to have different data type objects in the same field. This may happen by accident or on purpose (that is, null and actual values in a field)

The sorting order of different types of data is as follows from highest to lowest:

1. MaxKey (internal type).
2. Regular expression.
3. Timestamp.
4. Date.
5. Boolean.
6. ObjectId.
7. BinData.
8. Array.
9. Object.
10. Symbol, string.
11. Numbers (ints, longs, doubles).
12. Null.
13. MinKey (internal type).

Non-existent fields get sorted as if they have null in the respective field. Comparing arrays is a bit more complex. An ascending order of comparison (or <) will compare the smallest element of each array. A descending order of comparison (or >) will compare the largest element of each array.

For example, see the following scenario:

> db.types.find()
{ "_id" : ObjectId("5908d58455454e2de6519c49"), "a" : [ 1, 2, 3 ] }
{ "_id" : ObjectId("5908d59d55454e2de6519c4a"), "a" : [ 2, 5 ] }

In ascending order, this is as follows:

> db.types.find().sort({a:1})
{ "_id" : ObjectId("5908d58455454e2de6519c49"), "a" : [ 1, 2, 3 ] }
{ "_id" : ObjectId("5908d59d55454e2de6519c4a"), "a" : [ 2, 5 ] }

Whereas in descending order it is as follows:

> db.types.find().sort({a:-1})
{ "_id" : ObjectId("5908d59d55454e2de6519c4a"), "a" : [ 2, 5 ] }
{ "_id" : ObjectId("5908d58455454e2de6519c49"), "a" : [ 1, 2, 3 ] }

The same applies when comparing an array with a single number value, as illustrated in the following example.

Inserting a new document with an integer value of 4:

> db.types.insert({"a":4})
WriteResult({ "nInserted" : 1 })

Descending sort:

> db.types.find().sort({a:-1})
{ "_id" : ObjectId("5908d59d55454e2de6519c4a"), "a" : [ 2, 5 ] }
{ "_id" : ObjectId("5908d73c55454e2de6519c4c"), "a" : 4 }
{ "_id" : ObjectId("5908d58455454e2de6519c49"), "a" : [ 1, 2, 3 ] }

Ascending sort:

> db.types.find().sort({a:1})
{ "_id" : ObjectId("5908d58455454e2de6519c49"), "a" : [ 1, 2, 3 ] }
{ "_id" : ObjectId("5908d59d55454e2de6519c4a"), "a" : [ b, 5 ] }
{ "_id" : ObjectId("5908d73c55454e2de6519c4c"), "a" : 4 }

In each case, we highlighted the values being compared in bold.