In MongoDB, the $eq aggregation pipeline operator compares two values and returns either true or false, depending on whether the two values are equivalent or not.

Example

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

{ "_id" : 1, "a" : 250, "b" : 250 }
{ "_id" : 2, "a" : 250, "b" : 100 }

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

db.data.aggregate(
   [
     { $match: { _id: { $in: [ 1, 2 ] } } },
     {
       $project:
          {
            a: 1,
            b: 1,
            isEquivalent: { $eq: [ "$a", "$b" ] }
          }
     }
   ]
)

Result:

{ "_id" : 1, "a" : 250, "b" : 250, "isEquivalent" : true }
{ "_id" : 2, "a" : 250, "b" : 100, "isEquivalent" : false }

In the first document, the a and b fields are equivalent, which results in a return value of true. But in the second document they are not equivalent, which results in a return value of false.

Comparing Types

The $eq 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" : 3, "a" : 250, "b" : "250" }
{ "_id" : 4, "a" : 250, "b" : NumberDecimal("250") }
{ "_id" : 5, "a" : NumberDecimal("250"), "b" : NumberDecimal("250.00") }
{ "_id" : 6, "a" : "2021-01-03T23:30:15.100Z", "b" : ISODate("2021-01-03T23:30:15.100Z") }

We can apply $eq to the a and b fields of those documents:

db.data.aggregate(
   [
     { $match: { _id: { $in: [ 3, 4, 5, 6 ] } } },
     {
       $project:
          {
            a: 1,
            b: 1,
            isEquivalent: { $eq: [ "$a", "$b" ] }
          }
     }
   ]
).pretty()

Result:

{ "_id" : 3, "a" : 250, "b" : "250", "isEquivalent" : false }
{ "_id" : 4, "a" : 250, "b" : NumberDecimal("250"), "isEquivalent" : true }
{
	"_id" : 5,
	"a" : NumberDecimal("250"),
	"b" : NumberDecimal("250.00"),
	"isEquivalent" : true
}
{
	"_id" : 6,
	"a" : "2021-01-03T23:30:15.100Z",
	"b" : ISODate("2021-01-03T23:30:15.100Z"),
	"isEquivalent" : false
}

In document 3, both a and b have a value of 250, but if you look closely at b, it’s a string (it’s surrounded by double quotes). Therefore, the first document returns false.

Document 4 returns true because both fields contain 250 and both are numbers. This is true even though a is a double and b is a decimal.

Document 5 returns true because both are decimals and both are the same value (even though one explicitly uses decimal places and the other doesn’t).

Document 6 returns false because, even though the date/time value is exactly the same, they use different types to express that date (a uses a date/time string and b uses a Date object).

Null Values

Comparing a value to null returns false, unless they’re both null.

Suppose we add the following documents to our collection:

{ "_id" : 7, "a" : 250, "b" : null }
{ "_id" : 8, "a" : null, "b" : null }

Let’s apply $eq to those documents:

db.data.aggregate(
   [
     { $match: { _id: { $in: [ 7, 8 ] } } },
     {
       $project:
          {
            a: 1,
            b: 1,
            isEquivalent: { $eq: [ "$a", "$b" ] }
          }
     }
   ]
)

Result:

{ "_id" : 7, "a" : 250, "b" : null, "isEquivalent" : false }
{ "_id" : 8, "a" : null, "b" : null, "isEquivalent" : true }

Missing Fields

If one of the fields that you’re trying to compare is missing, $eq returns false.

Suppose we add the following document to our collection:

{ "_id" : 9, "a" : 250 }

Let’s apply $eq to that document:

db.data.aggregate(
   [
     { $match: { _id: { $in: [ 9 ] } } },
     {
       $project:
          {
            a: 1,
            b: 1,
            isEquivalent: { $eq: [ "$a", "$b" ] }
          }
     }
   ]
)

Result:

{ "_id" : 9, "a" : 250, "isEquivalent" : false }

Infinity

Comparing Infinity to Infinity returns true.

Comparing -Infinity to -Infinity returns true.

Comparing Infinity to -Infinity returns false.

Suppose we add the following documents to our collection:

{ "_id" : 10, "a" : Infinity, "b" : Infinity }
{ "_id" : 11, "a" : -Infinity, "b" : -Infinity }
{ "_id" : 12, "a" : Infinity, "b" : -Infinity }

Let’s apply $eq to those documents:

db.data.aggregate(
   [
     { $match: { _id: { $in: [ 10, 11, 12 ] } } },
     {
       $project:
          {
            a: 1,
            b: 1,
            isEquivalent: { $eq: [ "$a", "$b" ] }
          }
     }
   ]
)

Result:

{ "_id" : 10, "a" : Infinity, "b" : Infinity, "isEquivalent" : true }
{ "_id" : 11, "a" : -Infinity, "b" : -Infinity, "isEquivalent" : true }
{ "_id" : 12, "a" : Infinity, "b" : -Infinity, "isEquivalent" : false }