Exercise 2: Converting string values in one of the lookup fields to integers

In this exercise, you’ll add a stage to the aggregation pipeline after the $replaceRoot stage. The new stage will convert the data in the population field from string values to integers. As part of this process, you’ll first remove the commas from the string values so you do not receive an error when trying to convert the values to integers.

To convert the string field to integers

1. On the Pipeline tab of the Aggregation Editor, right-click the $replaceRoot pipeline stage, and then click Add New Stage After Selected Stage.
2. On the new tab, select the $set option from the Operator drop-down list. The $set operator lets you add new fields to the pipeline or overwrite existing ones. 
3. In the editor window, replace the placeholder text and curly braces with the following code:

{
  population: { $toInt: { $replaceAll: { input: { $first: "$population" }, find: ",", replacement: "" } } }
}

The $set expression overwrites the existing population field and updates the data in two ways. The first is by using the $replaceAll operator to remove the commas from the string values so they can be converted to integers. The operator takes three arguments:

• The input argument uses the $first operator to specify that the field values should be based on the first element in the population array.
• The find argument tells the $replaceAll operator to locate all commas in the string value.
• The replacement argument indicates that the commas should be replaced with an empty string, thus removing each comma from the string values.

The $set expression then uses the $toInt operator to convert the output from the $replaceAll operator to integers. Integers can be useful when performing calculations or sorting documents, as you’ll see in the following exercises.

4. In the Stage Input pane, click the Execute button. Studio 3T runs the pipeline for all stages up to but not including the $set stage.
5. In the Stage Output pane, click the Execute button. This will execute the pipeline up to and including the $set stage. The following figure shows part of the results, as they appear in Tree View.

The population data is now included as an Int32 field in the primary document, rather than being a string element in an array or string field in an array’s embedded document.

6. Click Save on the Aggregation Editor toolbar to save your changes.
7. Leave the Aggregation tab open and the existing statement in place for the next exercise. You’ll be building on this statement by adding another pipeline stage.