How to find most common value for specific categories in MongoDB? - mongodb

Question

I have a dataset in MongoDB that looks like this:
{ "name": "Tom's", "category": "coffee shop" },
{ "name": "Red Lobster", "category": "restaurant" },
{ "name": "Tom's", "category": "coffee shop" },
{ "name": "Starbucks", "category": "coffee shop" },
{ "name": "Central Park", "category": "park" },
{ "name": "Office", "category": "office" },
{ "name": "Red Lobster", "category": "restaurant" },
{ "name": "Home", "category": "home" },
{ ... } // and so on
How can I find the most common value for specific categories? For example, the most common occurring value for coffee shop and restaurant should be Tom's and Red Lobster, respectively.
My current $aggregate query only seems to list the most common occurring value among ALL of the dataset:
db.collection.aggregate(
{ "$group": { "_id": { "name": "$name" }, "count": { "$sum":1 } }},
{ "$group": { "_id": "$_id.name", "count": { "$sum": "$count" } }},
{ "$sort": { "count":-1 }}
)

Answer 1

You can try the below query.
$group on category and name to get the count for each category and name combination.
$sort the input documents by category and count desc.
$group on category with $first to pick the document with most occurrences.
db.collection_name.aggregate([
{
"$group": {
"_id": {
"category": "$category",
"name": "$name"
},
"count": {
"$sum": 1
}
}
},
{
"$sort": {
"_id.category": 1,
"count": -1
}
},
{
"$group": {
"_id": {
"category": "$_id.category"
},
"name": {
"$first": "$_id.name"
},
"count": {
"$first": "$count"
}
}
}
])