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MongoDB $project Example | The MongoDB Aggregation Pipeline

The $project stage is extremely useful for showing only the fields you need. In this article, we walk through an example of the $project stage in tandem with $lookup.

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Kathryn Vargas
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1–2 minutes

The $project stage

Sometimes we need to do a bit of data clean-up in our collection.

Picking up from Stage 1 – our $lookup example – we definitely don’t need all the fields about the customer or affordable housing options.

The JSON document from the first part of the aggregation query example

JSON document (feat. data we don’t need)

The $project stage is extremely useful for filtering a document to show only the fields we need:

  • first
  • last
  • address.city
  • address.state
  • address.zip_code.affordable_housing_options.Property Type
  • address.zip_code.affordable_housing_options.Property Name
  • address.zip_code.affordable_housing_options.Units
  • address.zip_code.affordable_housing_options.Zip Code
Highlighting the fields we'll filter using $project

$project syntax

The $project syntax is quite straightforward. We need to indicate 1 or true as the value to include the field in our projection.

<field> : <1 or true>

The nature of an aggregation pipeline is to have multiple stages, so you will likely use $project in tandem with other operators like $find, $match, or $lookup.

$project in Aggregation Editor

To illustrate how $project works, let’s build off of our $lookup stage example.

Let’s add a new stage by clicking on Add stage.

Choose $project in the Operator dropdown menu.

Click Add stage and then select the $project operator

Let’s paste the query:

{
    "first" : 1.0, 
    "last" : 1.0, 
    "address.city" : 1.0, 
    "address.state" : 1.0,
    "address.zip_code.affordable_housing_options.Property Type" : 1.0, 
    "address.zip_code.affordable_housing_options.Property Name" : 1.0, 
    "address.zip_code.affordable_housing_options.Units" : 1.0,
    "address.zip_code.affordable_housing_options.Zip Code" : 1.0, 
}

On the Stage input/output tab, click on the Run (play) button under Stage2: Output to run just this stage.

Or execute the entire pipeline by clicking on the Run button in the toolbar.

The resulting documents only show the fields we wanted to project

The pipeline and stage outputs should now only show the fields we want.

$project in mongo shell

To view the aggregation queries in mongo shell code, click on Query Code.

mongo shell code with lookup and project stages

You can then open this query directly in IntelliShell by clicking on the Open in IntelliShell button.

Alternatively, you can paste the following query in IntelliShell, which should get you the same results:

db.getCollection("customers").aggregate(
    [
        { 
            "$lookup" : {
                "from" : "housing", 
                "localField" : "address.zip_code", 
                "foreignField" : "Zip Code", 
                "as" : "address.zip_code.affordable_housing_options"
            }
        }, 
        { 
            "$project" : {
                "first" : 1.0, 
                "last" : 1.0, 
                "address.city" : 1.0, 
                "address.state" : 1.0, 
                "address.zip_code.affordable_housing_options.Property Type" : 1.0, 
                "address.zip_code.affordable_housing_options.Property Name" : 1.0, 
                "address.zip_code.affordable_housing_options.Units" : 1.0, 
                "address.zip_code.affordable_housing_options.Zip Code" : 1.0
            }
        }
    ], 
    { 
        "allowDiskUse" : false
    }
);

Now, let’s add a third stage and save these output documents to a new collection using the $out stage.

Kathryn Vargas