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Exercise 4: Writing pipeline results to a new collection

MongoDB 301: Aggregation Working with Arrays in the Aggregation Pipeline Exercise 4: Writing pipeline results to a new collection

In this exercise, you’ll add a stage to the aggregation pipeline that uses the $merge aggregate operator to save the pipeline results to a new collection.

To write the pipeline results to the new collection

  1. On the Pipeline tab of the Aggregation Editor, right-click the $sortByCount pipeline stage, and then click Add New Stage After Selected Stage.
  2. On the new tab, select the $merge option from the Operator drop-down list. The $merge operator writes the stage’s output to the specified collection.
  3. In the editor window, replace the placeholder text and curly braces with the following code:
{
  into: { db: "reference", coll: "interests" },
  whenMatched: "replace"
}

The operator expression includes two fields (arguments):

  • The into field is mandatory and specifies the output collection, which in this case is the interests collection in the reference database.
  • The whenMatched field determines the operator’s behavior when the source document matches the target document. In this case, the replace value is specified, indicating that duplicate documents should be replaced.
  1. Run the Stage Input and Stage Output tabs. The tabs should return identical results. The following figure shows those results as they appear in Table View.

Be aware that when you run the stage output, you are carrying out the actual operation, which means the pipeline output will be added to the target collection. If the collection doesn’t exist, it will be created. If the collection does exist, the operation will overwrite matching documents in that collection because the whenMatched option is set to replace.

  1. Go to the Pipeline tab and click the Execute button on the Aggregation Editor toolbar. Studio 3T runs the aggregate statement and returns the results to the Pipeline output pane at the bottom of the tab, as shown in the following figure.
  1. As expected, the pipeline returns the list of interests and provides the total number of documents in each group. The operation will once again write the pipeline output to the target collection, replacing any matching documents.
  2. Click Save on the Aggregation Editor toolbar to save your changes.
  3. In the Connection Tree, right-click the top-level connection node, and then click Refresh All.
  4. Expand the reference database node and, if necessary, expand the Collections node.
  5. Double-click the interests collection node to open the interests collection in a new tab, which is shown in the following figure.
  1. Close the Aggregation tab and the interests tab and then close Studio 3T.

Quizzes
Working with Arrays in the Aggregation Pipeline: Test your skills
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  • Course Home Expand All
    Building a Basic Aggregation
    4 Topics | 1 Quiz
    Exercise 1: Filtering the documents in the aggregation pipeline
    Exercise 2: Grouping the documents in the aggregation pipeline
    Exercise 3: Sorting the documents in the aggregation pipeline
    Exercise 4: Adding processing options to the aggregation
    Building a Basic Aggregation: Test your skills
    Introducing the Aggregation Editor
    4 Topics | 1 Quiz
    Exercise 1: Importing an aggregate statement into the Aggregation Editor
    Exercise 2: Replace a field in the aggregation pipeline
    Exercise 3: Reorder the fields in the aggregation pipeline
    Exercise 4: Changing the sort order in the aggregation pipeline
    Introducing the Aggregation Editor: Test your skills
    Working with Arrays in the Aggregation Pipeline
    5 Topics | 1 Quiz
    Exercise 1: Using expression operators to filter input documents
    Exercise 2: Unwinding an array to create individual documents
    Exercise 3: Grouping array values and generating a document count for each group
    Exercise 4: Writing pipeline results to a new collection
    Working with Arrays in the Aggregation Pipeline: Test your skills
    MongoDB 301 Mid-Course Feedback
    Adding Lookup Data to the Aggregation Pipeline
    4 Topics | 1 Quiz
    Exercise 1: Adding lookup data to the aggregation pipeline
    Exercise 2: Converting string values in one of the lookup fields to integers
    Exercise 3: Adding a computed ratio field based on the converted lookup field
    Exercise 4: Limiting the number of returned documents
    Adding Lookup Data to the Aggregation Pipeline: Test your skills
    Working with Reschema for MongoDB
    4 Topics | 1 Quiz
    Exercise 1: Setting up a reschema unit that includes lookup data
    Exercise 2: Defining a target collection in the reschema unit
    Exercise 3: Adding and scheduling a task to create the target collection
    Exercise 4: Running an aggregate statement against the target collection
    Working with Reschema for MongoDB: Test your skills
    Reporting with Studio 3T Aggregations
    3 Topics | 1 Quiz
    Exercise 1: Creating a view based on an aggregation query
    Exercise 2: Exporting a collection as a .csv file for use by a third-party tool
    Exercise 3: Visualizing collection data in MongoDB Charts
    Reporting with Studio 3T Aggregations: Test your skills
    Course Extras
    Return to MongoDB 301: Aggregation
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