Comparison/Sort Order
When comparing values of different BSON types in sort operations, MongoDB uses the following comparison order, from lowest to highest:
MinKey (internal type)
Null
Numbers (ints, longs, doubles, decimals)
Symbol, String
Object
Array
BinData
ObjectId
Boolean
Date
Timestamp
Regular Expression
MaxKey (internal type)
Note
$range query and aggregation operators perform comparisons only on fields where the BSON type matches the query value's type. MongoDB supports limited cross-BSON comparison through Type Bracketing.
Numeric Types
MongoDB treats some types as equivalent for comparison purposes. For instance, numeric types undergo conversion before comparison.
Strings
Binary Comparison
By default, MongoDB uses the simple binary comparison to compare strings.
Collation
New in version 3.4.
Collation allows users to specify language-specific rules for string comparison, such as rules for lettercase and accent marks.
Collation specification has the following syntax:
{ locale: <string>, caseLevel: <boolean>, caseFirst: <string>, strength: <int>, numericOrdering: <boolean>, alternate: <string>, maxVariable: <string>, backwards: <boolean> }
When specifying collation, the locale field is mandatory; all
other collation fields are optional. For descriptions of the fields,
see Collation Document.
If no collation is specified for the collection or for the operations, MongoDB uses the simple binary comparison used in prior versions for string comparisons.
Arrays
With arrays, a less-than comparison or an ascending sort compares the
smallest element of arrays, and a greater-than comparison or a
descending sort compares the largest element of the arrays. As such,
when comparing a field whose value is a single-element array (e.g. [
1 ]) with non-array fields (e.g. 2), the comparison is between
1 and 2. A comparison of an empty array (e.g. [ ]) treats
the empty array as less than null or a missing field.
Objects
MongoDB's comparison of BSON objects uses the following order:
Recursively compare key-value pairs in the order that they appear within the BSON object.
Compare the field types. MongoDB uses the following comparison order for field types, from lowest to highest:
MinKey (internal type)
Null
Numbers (ints, longs, doubles, decimals)
Symbol, String
Object
Array
BinData
ObjectId
Boolean
Date
Timestamp
Regular Expression
MaxKey (internal type)
If the field types are equal, compare the key field names.
If the key field names are equal, compare the field values.
If the field values are equal, compare the next key/value pair (return to step 1). An object without further pairs is less than an object with further pairs.
Dates and Timestamps
Date objects sort before Timestamp objects.
Non-existent Fields
The comparison treats a non-existent field as if it were an empty BSON
Object. As such, a sort on the a field in documents { } and {
a: null } would treat the documents as equivalent in sort order.
BinData
MongoDB sorts BinData in the following order:
First, the length or size of the data.
Then, by the BSON one-byte subtype.
Finally, by the data, performing a byte-by-byte comparison.