Update multiple documents with aggregated values in MongoDB - mongodb

Question

Problem 1
I have a collection named recipe in which all docs have a array field ingredients. I want to count those array items and write them into a new field ingredient_count.
Problem 2
There is also a collection named ingredient. The docs have a count field which is the total number of uses in all recipes.
My Current Approach
My solution right now is a script that aggregates over the collection and updates all documents one by one:
// PROBLEM 1: update recipe documents
db.recipe.aggregate(
[
{
$project: {
numberOfIngredients: { $size: "$ingredients" }
}
}
]
).forEach(function(recipe) {
db.recipe.updateOne(
{ _id: recipe._id },
{ $set: { incredient_count: recipe.numberOfIngredients } }
)
});
// PROBLEM 2: update ingredient documents
db.ingredient.find().snapshot().forEach(function(ingredient) {
db.ingredient.updateOne(
{ _id: ingredient._id },
{ $set: { count: db.recipe.count({ ingredients: { $in: [ingredient.name] } })) } }
)
});
This is terribly slow. Any idea how to do this more efficiently?

Answer 1

For both problem it's possible to only perform aggregation that output to new collections that would replace existing one :
Problem1
The aggregation contains one $project for counting ingredients with the list of field to keep :
db.recipe.aggregate([{
$project: {
ingredients: 1,
numberOfIngredients: { $size: "$ingredients" }
}
}, {
$out: "recipeNew"
}])
that give you :
{ "_id" : ObjectId("58155bc09c924e717c5c4240"), "ingredients" : [......], "numberOfIngredients" : 5 }
{ "_id" : ObjectId("58155bc19c924e717c5c4241"), "ingredients" : [......], "numberOfIngredients" : 3 }
The result of the aggregation is written to a new collection recipeNew that can replace the existing recipe collection
Problem2
The aggregation contains :
1 $unwind to remove ingredients array
1 $group to sum occurence of each ingredients & group by ingredients _id
1 $lookup that join ingredients collection to the current aggregation to retrieve all fields for specified ingredients
1 $unwind to remove the array of imported ingredients items
1 $project to select fields to keep
1 $out to output the result to a new collection
Query is :
db.recipe.aggregate([{
$unwind: "$ingredients"
}, {
$group: { _id: "$ingredients", IngredientsNumber: { $sum: 1 } }
}, {
$lookup: {
from: "ingredients",
localField: "_id",
foreignField: "_id",
as: "ingredientsDB"
}
}, {
$unwind: { path: "$ingredientsDB", preserveNullAndEmptyArrays: true }
}, {
$project: {
ingredientsNumber: "$IngredientsNumber",
name: "$ingredientsDB.name"
}
}, {
$out: "ingredientsTemp"
}])
That gives :
{ "_id" : ObjectId("5812caaeb4829937f4599b54"), "ingredientsNumber" : 2, "name" : "ingredients5" }
{ "_id" : ObjectId("5812caaeb4829937f4599b53"), "ingredientsNumber" : 1, "name" : "ingredients4" }
{ "_id" : ObjectId("5812caaeb4829937f4599b52"), "ingredientsNumber" : 2, "name" : "ingredients3" }
{ "_id" : ObjectId("5812caaeb4829937f4599b51"), "ingredientsNumber" : 1, "name" : "ingredients2" }
{ "_id" : ObjectId("5812caaeb4829937f4599b50"), "ingredientsNumber" : 2, "name" : "ingredients1" }
The cons of this solution :
It uses $project so you need to specify the fields to keep
you will get a new ingredientsTemp collection containing only ingredients that are actually present in recipes so one additionnal aggregation with a $lookup should be necessary to join the existing one with the one you got from that aggregation :
The following will join the existing ingredients collection with the one we have created :
db.ingredients.aggregate([{
$lookup: {
from: "ingredientsTemp",
localField: "_id",
foreignField: "_id",
as: "ingredientsDB"
}
}, {
$unwind: { path: "$ingredientsDB", preserveNullAndEmptyArrays: true }
}, {
$project: {
name: "$name",
ingredientsNumber: "$ingredientsDB.ingredientsNumber"
}
}])
Then you would have :
{ "_id" : ObjectId("5812caaeb4829937f4599b50"), "name" : "ingredients1", "ingredientsNumber" : 2 }
{ "_id" : ObjectId("5812caaeb4829937f4599b51"), "name" : "ingredients2", "ingredientsNumber" : 1 }
{ "_id" : ObjectId("5812caaeb4829937f4599b52"), "name" : "ingredients3", "ingredientsNumber" : 2 }
{ "_id" : ObjectId("5812caaeb4829937f4599b53"), "name" : "ingredients4", "ingredientsNumber" : 1 }
{ "_id" : ObjectId("5812caaeb4829937f4599b54"), "name" : "ingredients5", "ingredientsNumber" : 2 }
{ "_id" : ObjectId("5812caaeb4829937f4599b57"), "name" : "ingredients6" }
The goods :
It uses only aggregation so it should be quicker