Aggregation Pipeline Intro

Delete the existing pets collection (by clicking the delete button in mongodb cloud) and re-insert all the data with the below command:

// reset pets:

db.pets.insertMany([

{
    _id: "1",
    pet_name: "juice",
    breed: "maltese",
    age: 11,
    sex: "female",
    personality: "friendly",
    type: "dog"
},
{
    _id: "2",
    pet_name: "squack",
    breed: "parakeet",
    age: 4,
    sex: "male",
    personality: "loud",
    type: "bird"
},
{
    _id: "3",
    pet_name: "bobby",
    breed: "pigeon",
    age: 2,
    sex: "female",
    personality: "energetic",
    type: "bird"
},
{
    _id: "4",
    pet_name: "whalina",
    breed: "blue",
    age: 23,
    sex: "female",
    weight: 187000,
    type: "whale"
},
{
    _id: "5",
    pet_name: "Hershey",
    breed: "boxer",
    age: 3,
    sex: "male",
    personality: "playful",
    type: "dog"
},
{
    _id: "6",
    pet_name: "squeek",
    breed: "siamese",
    age: 15,
    sex: "male",
    weight: 2,
    type: "cat"
},
{
    _id: "7",
    pet_name: "kipper",
    breed: "greyhound",
    age: 5,
    sex: "male",
    neutered: true,
    type: "dog"
},
{
    _id: "8",
    pet_name: "noki",
    breed: "short-haired persian",
    age: 3,
    sex: "female",
    type: "cat"
},
{
    _id: "9",
    pet_name: "pooki",
    age: 3,
    sex: "female",
},
{
    _id: "10",
    pet_name: "snooki",
    age: 5,
    sex: "male",
},
{
    _id: "11",
    pet_name: "wooki",
    age: 4,
    sex: "male",
}
])

The below “code block” is not meant to be run inside MongoDB Atlas. Below we’re walking through the creation of our query one step at a time first.

//// find all documents where age is greater than or equal to 10.

// step 1: query for age, like normal.

age: { $gte: 10 }


// step 2: put into match:

$match: {
    age: { $gte: 10 }
}


// step 3.
// In aggregations there can be multiple stages, which all need to be in a separate object.
// this is an entire stage.
{
    $match: {
        age: { $gte: 10 }
    }
}

// step 4. aggregations are a list of stages.
[
    {
        $match: {
            age: { $gte: 10 }
        }
    }
]


step 5.
 // This is the final query from the steps above. We can run this.

db.pets.aggregate(
    [
        {
            $match: {
                age: { $gte: 10 }
            }
        }
    ]
)

// stage 1. find all documents where the age is greater than or equal to 10.
// stage 2. take results from stage 1, and add the # of results and store the total sum of documents in "countyodude"

db.pets.aggregate(
    [
        {
            $match: {
                age: { $gte: 10 }
            }
        },
        {
            $group: {
                _id: null,
                countyodude: { $sum: 1},
            }
        }
    ]
)

/// stage 1. Find all documents where age is greater than or equal to 10.
/// stage 2. take results from stage 1, store sum of results into countyodude, AND calculate the average age of results and store that value
// inside "avgdude"

db.pets.aggregate(
    [
        {
            $match: {
                age: { $gte: 10 }
            }
        },
        {
            $group: {
                _id: null,
                countyodude: { $sum: 1},
                avgdude: { $avg: "$age" }
            }
        }
    ]
)

Aggregate with a match + sum of documents

/// Get the total number of documents in pets, and average all the ages of all the documents.
db.pets.aggregate(
    [
        {
            $group: {
                _id: null,
                countyodude: { $sum: 1},
                avgdude: { $avg: "$age" }
            }
        }
    ]
)

/// Remove Unnecessary fields With $unset:

db.pets.aggregate(
    [
        {
            $match: {
                age: { $gte: 10 }
            }
        },
        {
            $unset: ["age", "sex"]
        }
    ]
)