One-to-One Relations, Reference approach

Joining

Situation: Among many possible relationships, this is a one-to-one relation between two tables using the “Reference” instead of the “Embedded Document” approach.

Here i’m joining a bounties collection with the customers collection through the customer_id:

db.bounties.aggregate([
                        {$lookup: {from: "customers", 
                                   localField: "customer_id",
                                   foreignField: "customer_id", 
                                   as:"customerName"}
                        }
                      ])

Joining and Displaying Specific Fields

Situation: After joining two collections, you don’t want to print everything out, only a specific set of key-value pairs.

Here I’m joining the bounties and customers collection and only want to display the title of the bounty and the customerName of the customer. (note: To omit the “_id”, set it to 0.)

db.bounties.aggregate([{
    $lookup: {
        from: "customers", 
        localField: "customer_id", 
        foreignField: "customer_id", 
        as: "customerName"
    }
}, {
    $unwind: "$customerName"
}, {$project: {
        "_id": 0,
        "title": 1, 
        "customerName.customerName": 1
    } 
}
]);

Pipelines in Mongo

The “pipeline” here is $lookup -> $unwind -> $project.

Use $lookup to create the join. Then $unwind to focus on the “result” of the $lookup (“as:…"). Finally, $project to specify the specific key-value pairs you want.

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