You can use the $substr function in MongoDB to extract a substring from a string.

This function uses the following basic syntax:

db.myCollection.aggregate([
  { $project: {substring: { $substr: [ "$fullstring", 0, 4 ] }}}
])

This particular example extracts the four characters from the field titled “fullString” starting from position 0.

The following example shows how to use this syntax in practice with a collection sales with the following documents:

db.sales.insertOne({yearMonth: 201702, amount: 40})
db.sales.insertOne({yearMonth: 201802, amount: 32})
db.sales.insertOne({yearMonth: 201806, amount: 19})
db.sales.insertOne({yearMonth: 201910, amount: 29})
db.sales.insertOne({yearMonth: 201907, amount: 35})

Example: How to Use the $susbtr Function in MongoDB

We can use the following code to extract the first four characters from the “yearMonth” field and display it in a new field titled “year”:

db.sales.aggregate([
  { $project: {year: { $substr: [ "$yearMonth", 0, 4 ] }}}
])

This code produces the following output:

{ _id: ObjectId("620145544cb04b772fd7a929"), year: '2017' }
{ _id: ObjectId("620145544cb04b772fd7a92a"), year: '2018' }
{ _id: ObjectId("620145544cb04b772fd7a92b"), year: '2018' }
{ _id: ObjectId("620145544cb04b772fd7a92c"), year: '2019' }
{ _id: ObjectId("620145544cb04b772fd7a92d"), year: '2019' } 

Notice that the first four characters from the “monthYear” field in  each document are displayed in a new field titled “year.”

It’s important to note that this code only displays the substring.

To actually add a new field to the collection that contains this substring, we must use the $merge function as follows:

db.sales.aggregate([
  { $project: {year: { $substr: [ "$yearMonth", 0, 4 ] }}},
  { $merge: "sales" }
])

Here’s what the updated collection now looks like:

{ _id: ObjectId("620145544cb04b772fd7a929"),
  yearMonth: 201702,
  amount: 40,
  year: '2017' }
{ _id: ObjectId("620145544cb04b772fd7a92a"),
  yearMonth: 201802,
  amount: 32,
  year: '2018' }
{ _id: ObjectId("620145544cb04b772fd7a92b"),
  yearMonth: 201806,
  amount: 19,
  year: '2018' }
{ _id: ObjectId("620145544cb04b772fd7a92c"),
  yearMonth: 201910,
  amount: 29,
  year: '2019' }
{ _id: ObjectId("620145544cb04b772fd7a92d"),
  yearMonth: 201907,
  amount: 35,
  year: '2019' } 

Notice that the new field titled “year” has been added to each document in the collection and it displays the first four characters from the “yearMonth” field.

Note: You can find the complete documentation for the $substr function here.

Additional Resources

The following tutorials explain how to perform other common operations in MongoDB:

MongoDB: How to Check if Field Contains a String
MongoDB: How to Concatenate Strings from Two Fields
MongoDB: How to Replace Strings