What is the Compare Order for BSON Types

MongoDB allows objects in the same collection which have values which may differ in type. When comparing values from different types, a convention is utilized as to which value is less than the other. This (somewhat arbitary but well defined) ordering is listed below.

Note that some types are treated as equivalent for comparison purposes -- specifically numeric types which undergo conversion before comparison.

See also the BSON specification.

Null
Numbers (ints, longs, doubles)
Symbol, String
Object
Array
BinData
ObjectID
Boolean
Date, Timestamp
Regular Expression

Example (using the mongo shell ):

> t = db.mycoll;
> t.insert({x:3});
> t.insert( {x : 2.9} );
> t.insert( {x : new Date()} );
> t.insert( {x : true } )
> t.find().sort({x:1})
{ "_id" : ObjectId("4b03155dce8de6586fb002c7"), "x" : 2.9 }
{ "_id" : ObjectId("4b03154cce8de6586fb002c6"), "x" : 3 }
{ "_id" : ObjectId("4b031566ce8de6586fb002c9"), "x" : true }
{ "_id" : ObjectId("4b031563ce8de6586fb002c8"), "x" : "Tue Nov 17 2009 16:28:03 GMT-0500 (EST)" }

MinKey and MaxKey

In addition to the above types MongoDB internally uses a special type for MinKey and MaxKey which are less than, and greater than all other possible BSON element values, respectively.

From the mongo Javascript Shell

For example we can continue our example from above adding two objects which have x key values of MinKey and MaxKey respectively:

> t.insert( { x : MaxKey } )
> t.insert( { x : MinKey } )
> t.find().sort({x:1})
{ "_id" : ObjectId("4b04094b7c65b846e2090112"), "x" : { $minKey : 1 } }
{ "_id" : ObjectId("4b03155dce8de6586fb002c7"), "x" : 2.9 }
{ "_id" : ObjectId("4b03154cce8de6586fb002c6"), "x" : 3 }
{ "_id" : ObjectId("4b031566ce8de6586fb002c9"), "x" : true }
{ "_id" : ObjectId("4b031563ce8de6586fb002c8"), "x" : "Tue Nov 17 2009 16:28:03 GMT-0500 (EST)" }
{ "_id" : ObjectId("4b0409487c65b846e2090111"), "x" : { $maxKey : 1 } }

From C++

See also the Tailable Cursors page for an example of using MinKey from C++. See also minKey and maxKey definitions in jsobj.h.

BSON Type Comparison in Queries

Comparing two distinct BSON types in a query always returns false.

> t.insert( { x : 1 } )
> t.insert( { x: 'abc' } )
> t.find().sort( { x : 1 } )
{ "_id" : ObjectId("4e51589a029fa59b62174306"), "x" : 10 }
{ "_id" : ObjectId("4e24c0ddb497b6f1d53beca1"), "x" : "abc" }
> db.t.find({x: {$gt:''}})
{ "_id" : ObjectId("4e24c0ddb497b6f1d53beca1"), "x" : "abc" }
> db.t.find({x: {$lt:''}})

What is the Compare Order for BSON Types

MinKey and MaxKey

From the mongo Javascript Shell

From C++

BSON Type Comparison in Queries

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