In MongoDB, the $lt aggregation pipeline operator compares two values and returns either true or false, depending on whether or not the first value is less than the second value.

Example

Suppose we have a collection called data with the following documents:

{ "_id" : 1, "a" : 250, "b" : 250 }
{ "_id" : 2, "a" : 300, "b" : 250 }
{ "_id" : 3, "a" : 250, "b" : 300 }

We can use the $lt operator to compare the a and b fields:

db.data.aggregate(
   [
     { $match: { _id: { $in: [ 1, 2, 3 ] } } },
     {
       $project:
          {
            _id: 0,
            a: 1,
            b: 1,
            result: { $lt: [ "$a", "$b" ] }
          }
     }
   ]
)

Result:

{ "a" : 250, "b" : 250, "result" : false }
{ "a" : 300, "b" : 250, "result" : false }
{ "a" : 250, "b" : 300, "result" : true }

In the first document, the a and b fields are equivalent, which results in a return value of false (because a is not less than b).

In the second document a is greater than b, which results in a return value of false (because a is not less than b).

In the third document, a is less than b and therefore the $lt operator returns true.

Comparing Types

The $lt operator compares both value and type using the specified BSON comparison order for values of different types.

Suppose our collection contains the following documents:

{ "_id" : 4, "a" : 250, "b" : "250" }
{ "_id" : 5, "a" : "250", "b" : 250 }
{ "_id" : 6, "a" : 250, "b" : NumberDecimal("250") }
{ "_id" : 7, "a" : NumberDecimal("250"), "b" : 250 }
{ "_id" : 8, "a" : NumberDecimal("250"), "b" : NumberDecimal("250.00") }
{ "_id" : 9, "a" : NumberDecimal("250.00"), "b" : NumberDecimal("250") }
{ "_id" : 10, "a" : "2022-01-03T23:30:15.100Z", "b" : ISODate("2021-01-03T23:30:15.100Z") }
{ "_id" : 11, "a" : ISODate("2021-01-03T23:30:15.100Z"), "b" : "2021-01-03T23:30:15.100Z" }

Here’s what happens when we apply $lt to the a and b fields of those documents:

db.data.aggregate(
   [
     { $match: { _id: { $in: [ 4, 5, 6, 7, 8, 9, 10, 11 ] } } },
     {
       $project:
          {
            a: 1,
            b: 1,
            result: { $lt: [ "$a", "$b" ] }
          }
     }
   ]
).pretty()

Result:

{ "_id" : 4, "a" : 250, "b" : "250", "result" : true }
{ "_id" : 5, "a" : "250", "b" : 250, "result" : false }
{ "_id" : 6, "a" : 250, "b" : NumberDecimal("250"), "result" : false }
{ "_id" : 7, "a" : NumberDecimal("250"), "b" : 250, "result" : false }
{
	"_id" : 8,
	"a" : NumberDecimal("250"),
	"b" : NumberDecimal("250.00"),
	"result" : false
}
{
	"_id" : 9,
	"a" : NumberDecimal("250.00"),
	"b" : NumberDecimal("250"),
	"result" : false
}
{
	"_id" : 10,
	"a" : "2022-01-03T23:30:15.100Z",
	"b" : ISODate("2021-01-03T23:30:15.100Z"),
	"result" : true
}
{
	"_id" : 11,
	"a" : ISODate("2021-01-03T23:30:15.100Z"),
	"b" : "2021-01-03T23:30:15.100Z",
	"result" : false
}

Null Values

$lt comparisons can be made against null.

Suppose we add the following documents to our collection:

{ "_id" : 12, "a" : 250, "b" : null }
{ "_id" : 13, "a" : null, "b" : 250 }
{ "_id" : 14, "a" : null, "b" : null }

Let’s apply $lt to those documents:

db.data.aggregate(
   [
     { $match: { _id: { $in: [ 12, 13, 14 ] } } },
     {
       $project:
          {
            _id: 0,
            a: 1,
            b: 1,
            result: { $lt: [ "$a", "$b" ] }
          }
     }
   ]
)

Result:

{ "a" : 250, "b" : null, "result" : false }
{ "a" : null, "b" : 250, "result" : true }
{ "a" : null, "b" : null, "result" : false }

Missing Fields

If one of the fields that you’re trying to compare is missing, $lt returns false if the second field is missing, and true if the first is missing.

Suppose we add the following documents to our collection:

{ "_id" : 15, "a" : 250 }
{ "_id" : 16, "b" : 250 }

Let’s apply $lt to those documents:

db.data.aggregate(
   [
     { $match: { _id: { $in: [ 15, 16 ] } } },
     {
       $project:
          {
            _id: 0,
            a: 1,
            b: 1,
            result: { $lt: [ "$a", "$b" ] }
          }
     }
   ]
)

Result:

{ "a" : 250, "result" : false }
{ "b" : 250, "result" : true }

Infinity

Comparing Infinity to Infinity returns false.

Comparing -Infinity to -Infinity returns false.

Comparing Infinity to -Infinity returns false.

Comparing -Infinity to Infinity returns true.

Suppose we add the following documents to our collection:

{ "_id" : 17, "a" : Infinity, "b" : Infinity }
{ "_id" : 18, "a" : -Infinity, "b" : -Infinity }
{ "_id" : 19, "a" : Infinity, "b" : -Infinity }
{ "_id" : 20, "a" : -Infinity, "b" : Infinity }

Let’s apply $lt to those documents:

db.data.aggregate(
   [
     { $match: { _id: { $in: [ 17, 18, 19, 20 ] } } },
     {
       $project:
          {
            _id: 0,
            a: 1,
            b: 1,
            result: { $lt: [ "$a", "$b" ] }
          }
     }
   ]
)

Result:

{ "a" : Infinity, "b" : Infinity, "result" : false }
{ "a" : -Infinity, "b" : -Infinity, "result" : false }
{ "a" : Infinity, "b" : -Infinity, "result" : false }
{ "a" : -Infinity, "b" : Infinity, "result" : true }